Radio receiver for use in a radio tracking system and a method of operation thereof

• US 5,650,769 A
• Filed: 02/24/1995
• Issued: 07/22/1997
• Est. Priority Date: 02/24/1995
• Status: Expired due to Term

• Current Assignee: NTP Incorporated
• Original Assignee: NTP Incorporated
• Inventors: Campana, Thomas J. Jr.
• Primary Examiner(s): MULLEN, THOMAS J
• Application Number:
  US08/394,267
• Time in Patent Office: 879 Days
• Field of Search: 340/573, 340/574, 340/539, 340/825.14, 340/825.2, 340/825.34, 340/825.49, 342/42, 342/44, 342/147, 455/53.1, 455/54.1, 455/100, 371/47.1, 375/346, 379/37-38
• US Class Current: 340/573.4
• CPC Class Codes: G08B 21/0222 Message structure or message content, e.g. message protocol
• CPC Class Codes: G08B 21/0227 System arrangements with a plurality of child units
• CPC Class Codes: G08B 21/023 Power management, e.g. system sleep and wake up provisions
• CPC Class Codes: G08B 21/0263 System arrangements wherein the object is to detect the direction in which child or item is located

Abstract

A radio tracking and ranging system is disclosed which has at least one mobile radio frequency transmitter and a radio frequency receiver. The radio frequency receiver has a range control which sets a maximum range that each mobile radio frequency transmitter may be moved from the radio frequency receiver without generating an alert to the user of the radio frequency receiver. Each radio frequency transmitter has a "panic" switch which permits a user of the radio frequency transmitter to transmit an alert or alarm to the user of the radio frequency receiver. After receipt of an alert, the user of the radio frequency receiver switches the radio frequency receiver into a tracking mode for locating a direction from which a maximum signal strength is received which is indicative of the relative bearing of the radio frequency transmitter which is being tracked from the radio frequency receiver. Signal processing techniques are used to eliminate the effects of fading and interference in detecting an identification code periodically broadcast by each radio frequency transmitter and further, to eliminate the effects of fading and interference on RSSI signals which are used for determining the range of each radio frequency transmitter relative to the radio frequency receiver and tracking of each radio frequency transmitter.

Claims

1. A method for determining where at least one radio frequency transmitter is located with respect to a set range measured from a radio frequency receiver with each radio frequency transmitter periodically transmitting an identification code which identifies each radio frequency transmitter with a radio frequency carrier modulated with a subcarrier with the subcarrier being modulated with the identification code comprising:

in response to receiving each radio frequency carrier determining if an identification code of one of the at least one radio frequency transmitter is contained therein, calculating an integral of a received signal strength indicator of each radio frequency carrier determined to contain an identification code of one of the at least one radio frequency transmitter, computing an average of a plurality of the calculated integrals which is updated to include newly calculated integrals only when each newly calculated integral differs from the computed average of the calculated integrals by less than a function of the average of the calculated integrals so as to exclude from the computation of the average of the calculated integrals newly calculated integrals which differ from the average of the calculated integrals by more than the function, comparing the average of the calculated integrals to a numerical value representing the set range and generating an alert when the comparison reveals that at least one of the at least one radio frequency transmitter is outside the set range.

2. A method in accordance with claim 1 further comprising:

using an omnidirectional antenna to receive the radio frequency carrier transmitted by each radio frequency transmitter which is used to compute the average of the calculated integrals; and

after generation of the alert using a directional antenna to receive transmissions of the radio frequency carrier containing the identification code identifying the radio frequency transmitter from which the identification codes were transmitted which caused the generation of the alert and in response to the radio frequency receiver being moved by a user the radio frequency receiver displaying a magnitude of an integral of each successively received signal strength indicator generated in response to reception of transmissions of the radio frequency carrier containing the identification code of the radio frequency transmitter from which the identification codes were transmitted which caused the generation of the alert to permit a user of the radio frequency receiver to locate a direction, from which a radio frequency carrier containing the identification code of the radio frequency transmitter from which the identification codes were transmitted to generate the alert is received, producing a maximum magnitude of the integral of each successively received signal strength indicator relative to the radio frequency receiver whereby a direction of the radio frequency transmitter which is outside of the set range is determined by the user of the radio frequency receiver relative to the radio frequency receiver.

3. A method in accordance with claim 1 further comprising:

using an omnidirectional antenna to receive the radio frequency carrier transmitted by one of the at least one radio frequency transmitter which contains the identification code of the one radio frequency transmitter and an alert which signals to a user of the radio frequency receiver that the user of the one radio frequency transmitter is transmitting a change in status of the user; and

in response to reception of the alert from one of the at least one radio frequency transmitter using a directional antenna to receive transmissions of the radio frequency carrier containing the identification code identifying the radio frequency transmitter from which the alert was transmitted and in response to the radio frequency receiver being moved by a user the radio frequency receiver displaying a magnitude of an integral of each successively received signal strength indicator generated in response to reception of transmissions of the radio frequency carrier containing the identification code of the radio frequency transmitter from which the alert was received to permit a user of the radio frequency receiver to locate a direction, from which a radio frequency carrier is received from the radio frequency transmitter transmitting the alert, producing a maximum magnitude of the integral of each successively received signal strength indicator relative to the radio frequency receiver whereby a direction of the radio frequency transmitter which transmitted the alert is determined by the user of the radio frequency receiver relative to the radio frequency receiver.

4. A method in accordance with claim 3 further comprising:

the subcarrier of each received radio frequency carrier is modulated with at least one identification frame group, each identification frame group comprising a plurality of frames with at least one of the plurality of frames of the identification frame group containing bits encoding the identification code of each radio frequency transmitter, a plurality of bits of error correction code in each frame, synchronization information for synchronizing a clock of the radio frequency receiver, and a command field for encoding the alert transmitted to the radio frequency receiver from the user of the radio frequency transmitter transmitting the alert.

5. A method in accordance with claim 1 wherein:

the subcarrier of each received radio frequency carrier has cycles which are modulated with bits encoding the identification code of the radio frequency transmitter which transmitted the received radio frequency carrier with each cycle of the subcarrier being modulated with bits at a plurality of separated angular positions; and

for each radio frequency carrier received from each radio frequency transmitter processing detected individual cycles of the subcarrier to calculate an integral of at least one selected modulated part of each of the individual cycles, numerically comparing each of the calculated integrals with a plurality of stored numerical ranges which ranges each represent one of a plurality of possible numerical values that the selected part may encode to identify a stored range numerically including the calculated integral, substituting for the at least one selected part of each of the cycles the one of the plurality of numerical values representative of the identified stored range including the calculated integral with each numerical value encoding one bit of the identification code of the radio frequency transmitter and decoding the plurality of numerical values to produce the identification code of the radio frequency transmitter.

6. A method in accordance with claim 5 wherein:

the processing of the detected individual cycles of the subcarrier includes calculating the integral by taking a plurality of samples of each selected modulated part of each of the individual cycles with each sample having a numerical value and each sample is compared with a range of numerical values representing a valid sample which should be included within the calculation of the integral and when the comparison reveals that the sample value is outside the range of numerical values, the compared sample value is replaced with a value which is a function of the sample values adjacent the sample value which is replaced.

7. A method in accordance with claim 6 wherein:

the compared sample value is replaced with a value which is an average of at least one sample value which precedes the compared sample value and at least one sample value which succeeds the compare sample value.

8. A method in accordance with claim 2 wherein:

the subcarrier of each received radio frequency carrier has cycles which are modulated with bits encoding the identification code of the radio frequency transmitter which transmitted the received radio frequency carrier with each cycle of the subcarrier being modulated with bits at a plurality of separated angular positions; and

for each radio frequency carrier received from each radio frequency transmitter processing detected individual cycles of the subcarrier to calculate an integral of at least one selected modulated part of each of the individual cycles, numerically comparing each of the calculated integrals with a plurality of stored numerical ranges which ranges each represent one of a plurality of possible numerical values that the selected part may encode to identify a stored range numerically including the calculated integral, substituting for the at least one selected part of each of the cycles the one of the plurality of numerical values representative of the identified stored range including the calculated integral with each numerical value encoding one bit of the identification code of the radio frequency transmitter and decoding the plurality of numerical values to produce the identification code of the radio frequency transmitter.

9. A method in accordance with claim 8 wherein:

the processing of the detected individual cycles of the subcarrier includes calculating the integral by taking a plurality of samples of each selected modulated part of each of the individual cycles with each sample having a numerical value and each sample is compared with a range of numerical values representing a valid sample which should be included within the calculation of the integral and when the comparison reveals that the sample value is outside the range of numerical values, the compared sample value is replaced with a value which is a function of the sample values adjacent the sample value which is replaced.

10. A method in accordance with claim 9 wherein:

the compared sample value is replaced with a value which is an average of at least one sample value which precedes the compared sample value and at least one sample value which succeeds the compared sample value.

11. A method in accordance with claim 3 wherein:

the subcarrier of each received radio frequency carrier has cycles which are modulated with bits encoding the identification code of the radio frequency transmitter which transmitted the received radio frequency carrier with each cycle of the subcarrier being modulated with bits at a plurality of separated angular positions; and

for each radio frequency carrier received from each radio frequency transmitter processing detected individual cycles of the subcarrier to calculate an integral of at least one selected modulated part of each of the individual cycles, numerically comparing each of the calculated integrals with a plurality of stored numerical ranges which ranges each represent one of a plurality of possible numerical values that the selected part may encode to identify a stored range numerically including the calculated integral, substituting for the at least one selected part of each of the cycles the one of the plurality of numerical values representative of the identified stored range including the calculated integral with each numerical value encoding one bit of the identification code of the radio frequency transmitter and decoding the plurality of numerical values to produce the identification code of the radio frequency transmitter.

12. A method in accordance with claim 11 wherein:

the processing of the detected individual cycles of the subcarrier includes calculating the integral by taking a plurality of samples of each selected modulated part of each of the individual cycles with each sample having a numerical value and each sample is compared with a range of numerical values representing a valid sample which should be included within the calculation of the integral and when the comparison reveals that the sample value is outside the range of numerical values, the compared sample value is replaced with a value which is a function of the sample values adjacent the sample value which is replaced.

13. A method in accordance with claim 12 wherein:

the compared sample value is replaced with a value which is an average of at least one sample value which precedes the compared sample value and at least one sample value which succeeds the compared sample value.

14. A method in accordance with claim 4 wherein:

the subcarrier of each received radio frequency carrier has cycles which are modulated with bits encoding the identification code of the radio frequency transmitter which transmitted the received radio frequency carrier with each cycle of the subcarrier being modulated with bits at a plurality of separated angular positions; and

for each radio frequency carrier received from each radio frequency transmitter processing detected individual cycles of the subcarrier to calculate an integral of at least one selected modulated part of each of the individual cycles, numerically comparing each of the calculated integrals with a plurality of stored numerical ranges which ranges each represent one of a plurality of possible numerical values that the selected part may encode to identify a stored range numerically including the calculated integral, substituting for the at least one selected part of each of the cycles the one of the plurality of numerical values representative of the identified stored range including the calculated integral with each numerical value encoding one bit of the identification code of the radio frequency transmitter and decoding the plurality of numerical values to produce the identification code of the radio frequency transmitter.

15. A method in accordance with claim 14 wherein:

the processing of the detected individual cycles of the subcarrier includes calculating the integral by taking a plurality of samples of each selected modulated part of each of the individual cycles with each sample having a numerical value and each sample is compared with a range of numerical values representing a valid sample which should be included within the calculation of the integral and when the comparison reveals that the sample value is outside the range of numerical values, the compared sample value is replaced with a value which is a function of the sample values adjacent the sample value which is replaced.

16. A method in accordance with claim 15 wherein:

the compared sample value is replaced with a value which is an average of at least one sample value which precedes the compared sample value and at least one sample value which succeeds the compared sample value.

17. A method in accordance with claim 1 wherein:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least one radio frequency transmitter, produces each received signal strength indicator.

18. A method in accordance with claim 17 wherein:

the amplifier is an intermediate frequency amplifier.

19. A method in accordance with claim 2 wherein:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least one radio frequency transmitter, produces each received signal strength indicator.

20. A method in accordance with claim 19 wherein:

the amplifier is an intermediate frequency amplifier.

21. A method in accordance with claim 3 wherein:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least one radio frequency transmitter, produces each received signal strength indicator.

22. A method in accordance with claim 21 wherein:

the amplifier is an intermediate frequency amplifier.

23. A method in accordance with claim 4 wherein:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least one radio frequency transmitter, produces each received signal strength indicator.

24. A method in accordance with claim 23 wherein:

the amplifier is an intermediate frequency amplifier.

25. A method in accordance with claim 5 wherein:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least one radio frequency transmitter, produces each received signal strength indicator.

26. A method in accordance with claim 25 wherein:

the amplifier is an intermediate frequency amplifier.

27. A method in accordance with claim 6 wherein:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least one radio frequency transmitter, produces each received signal strength indicator.

28. A method in accordance with claim 27 wherein:

the amplifier is an intermediate frequency amplifier.

29. A method in accordance with claim 7 wherein:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least one radio frequency transmitter, produces each received signal strength indicator.

30. A method in accordance with claim 29 wherein:

the amplifier is an intermediate frequency amplifier.

31. A method for determining where at least one radio frequency transmitter is located with respect to a set range measured from a radio frequency receiver with each radio frequency transmitter periodically transmitting an identification code which identifies each radio frequency transmitter with a radio frequency carrier modulated with a subcarrier with the subcarrier being modulated with the identification code comprising:

in response to receiving each radio frequency carrier determining if an identification code of one of the at least one radio frequency transmitter is contained therein, calculating an integral of a received signal strength indicator of each radio frequency carrier determined to contain an identification code of one of the at least one radio frequency transmitter, computing an average of a plurality of the calculated integrals which is updated to include newly calculated integrals, comparing the average of the calculated integrals to a numerical value representing the set range and generating an alert when the comparison reveals that at least one of the at least one radio frequency transmitter is outside the set range.

32. A method in accordance with claim 31 further comprising:

using an omnidirectional antenna to receive the radio frequency carrier transmitted by each radio frequency transmitter which is used to compute the average of the calculated integrals; and

after generation of the alert using a directional antenna to receive transmissions of the radio frequency carrier containing the identification code identifying the radio frequency transmitter from which the identification codes were transmitted which caused the generation of the alert and in response to the radio frequency receiver being moved by a user the radio frequency receiver displaying a magnitude of an integral of each successively received signal strength indicator generated in response to reception of transmissions of the radio frequency carrier containing the identification code of the radio frequency transmitter from which the identification codes were transmitted which caused the generation of the alert to permit a user of the radio frequency receiver to locate a direction, from which a radio frequency carrier containing the identification code of the radio frequency transmitter from which the identification codes were transmitted to generate the alert is received, producing a maximum magnitude of the integral of each successively received signal strength indicator relative to the radio frequency receiver whereby a direction of the radio frequency transmitter which is outside of the set range is determined by the user of the radio frequency receiver relative to the radio frequency receiver.

33. A method in accordance with claim 31 further comprising:

using an omnidirectional antenna to receive the radio frequency carrier transmitted by one of the at least one radio frequency transmitter which contains the identification code of the one radio frequency transmitter and an alert which signals to a user of the radio frequency receiver that the user of the one radio frequency transmitter is transmitting a change in status of the user; and

in response to reception of the alert from one of the at least one radio frequency transmitter using a directional antenna to receive transmissions of the radio frequency carrier containing the identification code identifying the radio frequency transmitter from which the alert was transmitted and in response to the radio frequency receiver being moved by a user the radio frequency receiver displaying a magnitude of an integral of each successively received signal strength indicator generated in response to reception of transmissions of the radio frequency carrier containing the identification code of the radio frequency transmitter from which the alert was received to permit a user of the radio frequency receiver to locate a direction, from which a radio frequency carrier is received from the radio frequency transmitter transmitting the alert, producing a maximum magnitude of the integral of each successively received signal strength indicator relative to the radio frequency receiver whereby a direction of the radio frequency transmitter which transmitted the alert is determined by the user of the radio frequency receiver relative to the radio frequency receiver.

34. A method in accordance with claim 33 further comprising:

the subcarrier of each received radio frequency carrier is modulated with at least one identification frame group, each identification frame group comprising a plurality of frames with at least one of the plurality of frames of the identification frame group containing bits encoding the identification code of each radio frequency transmitter, a plurality of bits of error correction code in each frame, synchronization information for synchronizing a clock of the radio frequency receiver, and a command field for encoding the alert transmitted to the radio frequency receiver from the user of the radio frequency transmitter transmitting the alert.

35. A method in accordance with claim 31 wherein:

the subcarrier of each received radio frequency carrier has cycles which are modulated with bits encoding the identification code of the radio frequency transmitter which transmitted the received radio frequency carrier with each cycle of the subcarrier being modulated with bits at a plurality of separated angular positions; and

for each radio frequency carrier received from each radio frequency transmitter processing detected individual cycles of the subcarrier to calculate an integral of at least one selected modulated part of each of the individual cycles, numerically comparing each of the calculated integrals with a plurality of stored numerical ranges which ranges each represent one of a plurality of possible numerical values that the selected part may encode to identify a stored range numerically including the calculated integral, substituting for the at least one selected part of each of the cycles the one of the plurality of numerical values representative of the identified stored range including the calculated integral with each numerical value encoding one bit of the identification code of the radio frequency transmitter and decoding the plurality of numerical values to produce the identification code of the radio frequency transmitter.

36. A method in accordance with claim 35 wherein:

the processing of the detected individual cycles of the subcarrier includes calculating the integral by taking a plurality of samples of each selected modulated part of each of the individual cycles with each sample having a numerical value and each sample is compared with a range of numerical values representing a valid sample which should be included within the calculation of the integral and when the comparison reveals that the sample value is outside the range of numerical values, the compared sample value is replaced with a value which is a function of the sample values adjacent the sample value which is replaced.

37. A method in accordance with claim 36 wherein:

the compared sample value is replaced with a value which is an average of at least one sample value which precedes the compared sample value and at least one sample value which succeeds the compared sample value.

38. A method in accordance with claim 32 wherein:

the subcarrier of each received radio frequency carrier has cycles which are modulated with bits encoding the identification code of the radio frequency transmitter which transmitted the received radio frequency carrier with each cycle of the subcarrier being modulated with bits at a plurality of separated angular positions; and

for each radio frequency carrier received from each radio frequency transmitter processing detected individual cycles of the subcarrier to calculate an integral of at least one selected modulated part of each of the individual cycles, numerically comparing each of the calculated integrals with a plurality of stored numerical ranges which ranges each represent one of a plurality of possible numerical values that the selected part may encode to identify a stored range numerically including the calculated integral, substituting for the at least one selected part of each of the cycles the one of the plurality of numerical values representative of the identified stored range including the calculated integral with each numerical value encoding one bit of the identification code of the radio frequency transmitter and decoding the plurality of numerical values to produce the identification code of the radio frequency transmitter.

39. A method in accordance with claim 38 wherein:

the processing of the detected individual cycles of the subcarrier includes calculating the integral by taking a plurality of samples of each selected modulated part of each of the individual cycles with each sample having a numerical value and each sample is compared with a range of numerical values representing a valid sample which should be included within the calculation of the integral and when the comparison reveals that the sample value is outside the range of numerical values, the compared sample value is replaced with a value which is a function of the sample values adjacent the sample value which is replaced.

40. A method in accordance with claim 39 wherein:

the compared sample value is replaced with a value which is an average of at least one sample value which precedes the compared sample value and at least one sample value which succeeds the compared sample value.

41. A method in accordance with claim 33 wherein:

the subcarrier of each received radio frequency carrier has cycles which are modulated with bits encoding the identification code of the radio frequency transmitter which transmitted the received radio frequency carrier with each cycle of the subcarrier being modulated with bits at a plurality of separated angular positions; and

for each radio frequency carrier received from each radio frequency transmitter processing detected individual cycles of the subcarrier to calculate an integral of at least one selected modulated part of each of the individual cycles, numerically comparing each of the calculated integrals with a plurality of stored numerical ranges which ranges each represent one of a plurality of possible numerical values that the selected part may encode to identify a stored range numerically including the calculated integral, substituting for the at least one selected part of each of the cycles the one of the plurality of numerical values representative of the identified stored range including the calculated integral with each numerical value encoding one bit of the identification code of the radio frequency transmitter and decoding the plurality of numerical values to produce the identification code of the radio frequency transmitter.

42. A method in accordance with claim 41 wherein:

the processing of the detected individual cycles of the subcarrier includes calculating the integral by taking a plurality of samples of each selected modulated part of each of the individual cycles with each sample having a numerical value and each sample is compared with a range of numerical values representing a valid sample which should be included within the calculation of the integral and when the comparison reveals that the sample value is outside the range of numerical values, the compared sample value is replaced with a value which is a function of the sample values adjacent the sample value which is replaced.

43. A method in accordance with claim 42 wherein:

the compared sample value is replaced with a value which is an average of at least one sample value which precedes the compared sample value and at least one sample value which succeeds the compared sample value.

44. A method in accordance with claim 34 wherein:

the subcarrier of each received radio frequency carrier has cycles which are modulated with bits encoding the identification code of the radio frequency transmitter which transmitted the received radio frequency carrier with each cycle of the subcarrier being modulated with bits at a plurality of separated angular positions; and

for each radio frequency carrier received from each radio frequency transmitter processing detected individual cycles of the subcarrier to calculate an integral of at least one selected modulated part of each of the individual cycles, numerically comparing each of the calculated integrals with a plurality of stored numerical ranges which ranges each represent one of a plurality of possible numerical values that the selected part may encode to identify a stored range numerically including the calculated integral, substituting for the at least one selected part of each of the cycles the one of the plurality of numerical values representative of the identified stored range including the calculated integral with each numerical value encoding one bit of the identification code of the radio frequency transmitter and decoding the plurality of numerical values to produce the identification code of the radio frequency transmitter.

45. A method in accordance with claim 44 wherein:

the processing of the detected individual cycles of the subcarrier includes calculating the integral by taking a plurality of samples of each selected modulated part of each of the individual cycles with each sample having a numerical value and each sample is compared with a range of numerical values representing a valid sample which should be included within the calculation of the integral and when the comparison reveals that the sample value is outside the range of numerical values, the compared sample value is replaced with a value which is a function of the sample values adjacent the sample value which is replaced.

46. A method in accordance with claim 45 wherein:

the compared sample value is replaced with a value which is an average of at least one sample value which precedes the compared sample value and at least one sample value which succeeds the compared sample value.

47. A method in accordance with claim 31 wherein:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least one radio frequency transmitter, produces each received signal strength indicator.

48. A method in accordance with claim 47 wherein:

the amplifier is an intermediate frequency amplifier.

49. A method in accordance with claim 32 wherein:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least one radio frequency transmitter, produces each received signal strength indicator.

50. A method in accordance with claim 49 wherein:

the amplifier is an intermediate frequency amplifier.

51. A method in accordance with claim 33 wherein:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least one radio frequency transmitter, produces each received signal strength indicator.

52. A method in accordance with claim 51 wherein:

the amplifier is an intermediate frequency amplifier.

53. A method in accordance with claim 34 wherein:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least one radio frequency transmitter, produces each received signal strength indicator.

54. A method in accordance with claim 53 wherein:

the amplifier is an intermediate frequency amplifier.

55. A method in accordance with claim 35 wherein:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least one radio frequency transmitter, produces each received signal strength indicator.

56. A method in accordance with claim 55 wherein:

the amplifier is an intermediate frequency amplifier.

57. A method in accordance with claim 36 wherein:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least one radio frequency transmitter, produces each received signal strength indicator.

58. A method in accordance with claim 57 wherein:

the amplifier is an intermediate frequency amplifier.

59. A method in accordance with claim 37 wherein:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least one radio frequency transmitter, produces each received signal strength indicator.

60. A method in accordance with claim 59 wherein:

the amplifier is an intermediate frequency amplifier.

61. A method for determining where at least one radio frequency transmitter is located with respect to a set range measured from a radio frequency receiver with each radio frequency transmitter periodically transmitting an identification code which identifies each radio frequency transmitter with a radio frequency carrier modulated with a subcarrier with the subcarrier being modulated with the identification code comprising:

in response to receiving each radio frequency carrier determining if an identification code of one of the at least one radio frequency transmitter is contained therein, producing a received signal strength indicator of each radio frequency carrier determined to contain an identification code of one of the at least one radio frequency transmitter, computing an average of a plurality of the received signal strength indicators, comparing the average of the received signal strength indicators to a value representing the set range and generating an alert when the comparison reveals that at least one of the at least one radio frequency transmitter is outside the set range.

62. A method in accordance with claim 61 further comprising:

using an omnidirectional antenna to receive the radio frequency carrier transmitted by each radio frequency transmitter which is used to calculate the average of the calculated integrals; and

after generation of the alert using a directional antenna to receive transmissions of the radio frequency carrier containing the identification code identifying the radio frequency transmitter from which the identification codes were transmitted which caused the generation of the alert and in response to the radio frequency receiver being moved by a user displaying a magnitude of an integral of each successively received signal strength indicator generated in response to reception of transmissions of the radio frequency carrier containing the identification code of the radio frequency transmitter from which the identification codes were transmitted which caused the generation of the alert to permit a user of the radio frequency receiver to locate a direction, from which a radio frequency carrier containing the identification code of the radio frequency transmitter from which the identification codes were transmitted to generate the alert is received, producing a maximum magnitude of the integral of each successively received signal strength indicator relative to the radio frequency receiver whereby a direction of the radio frequency transmitter which is outside of the set range is determined by the user of the radio frequency receiver relative to the radio frequency receiver.

63. A method in accordance with claim 61 further comprising:

using an omnidirectional antenna to receive the radio frequency carrier transmitted by one of the at least one radio frequency transmitter which contains the identification code of the one radio frequency transmitter and an alert which signals to a user of the radio frequency receiver that the user of the one radio frequency transmitter is transmitting a change in status of the user contains an alert; and

in response to reception of the alert from one of the at least one radio frequency transmitter using a directional antenna to receive transmissions of the radio frequency carrier containing the identification code identifying the radio frequency transmitter from which the alert was transmitted and in response to the radio frequency receiver being moved by a user displaying a magnitude of an integral of each successively received signal strength indicator generated in response to reception of the radio frequency carrier from the radio frequency transmitter from which the alert was received to permit a user of the radio frequency receiver to locate a direction, from which a radio frequency carrier is received from the radio frequency transmitter transmitting the alert, producing a maximum magnitude of the integral of each successively received signal strength indicator relative to the radio frequency receiver whereby a direction of the radio frequency transmitter which transmitted the alert is determined by the user of radio frequency receiver relative to the radio frequency receiver.

64. A method in accordance with claim 63 further comprising:

the subcarrier of each received radio frequency carrier is modulated with at least one identification frame group, each identification frame group comprising a plurality of frames with at least one of the plurality of frames of the identification frame group containing bits encoding the identification code of each radio frequency transmitter, a plurality of bits of error correction code in each frame, synchronization information for synchronizing a clock of the radio frequency receiver, and a command field for encoding the alert transmitted to the radio frequency receiver from the user of the radio frequency transmitter transmitting the alert.

65. A method in accordance with claim 61 wherein:

the subcarrier of each received radio frequency carrier has cycles which are modulated with bits encoding the identification code of the radio frequency transmitter which transmitted the received radio frequency carrier with each cycle of the subcarrier being modulated with bits at a plurality of separated angular positions; and

for each radio frequency carrier received from each radio frequency transmitter processing detected individual cycles of the subcarrier to calculate an integral of at least one selected modulated part of each of the individual cycles, numerically comparing each of the calculated integrals with a plurality of stored numerical ranges which ranges each represent one of a plurality of possible numerical values that the selected part may encode to identify a stored range numerically including the calculated integral, substituting for the at least one selected part of each of the cycles the one of the plurality of numerical values representative of the identified stored range including the calculated integral with each numerical value encoding one bit of the identification code of the radio frequency transmitter and decoding the plurality of numerical values to produce the identification code of the radio frequency transmitter.

66. A method in accordance with claim 65 wherein:

the processing of the detected individual cycles of the subcarrier includes calculating the integral by taking a plurality of samples of each selected modulated part of each of the individual cycles with each sample having a numerical value and each sample is compared with a range of numerical values representing a valid sample which should be included within the calculation of the integral and when the comparison reveals that the sample value is outside the range of numerical values, the compared sample value is replaced with a value which is a function of the sample values adjacent the sample value which is replaced.

67. A method in accordance with claim 66 wherein:

the compared sample value is replaced with a value which is an average of at least one sample value which precedes the compared sample value and at least one sample value which succeeds the compared sample value.

68. A method in accordance with claim 61 wherein:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least one radio frequency transmitter, produces each received signal strength indicator.

69. A method in accordance with claim 68 wherein:

the amplifier is an intermediate frequency amplifier.

70. A method in accordance with claim 62 wherein:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least one radio frequency transmitter, produces each received signal strength indicator.

71. A method in accordance with claim 70 wherein:

the amplifier is an intermediate frequency amplifier.

72. A method in accordance with claim 63 wherein:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least one radio frequency transmitter, produces each received signal strength indicator.

73. A method in accordance with claim 72 wherein:

the amplifier is an intermediate frequency amplifier.

74. A method in accordance with claim 64 wherein:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least one radio frequency transmitter, produces each received signal strength indicator.

75. A method in accordance with claim 74 wherein:

the amplifier is an intermediate frequency amplifier.

76. A method in accordance with claim 65 wherein:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least one radio frequency transmitter, produces each received signal strength indicator.

77. A method in accordance with claim 76 wherein:

the amplifier is an intermediate frequency amplifier.

78. A method in accordance with claim 66 wherein:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least one radio frequency transmitter, produces each received signal strength indicator.

79. A method in accordance with claim 78 wherein:

the amplifier is an intermediate frequency amplifier.

80. A method in accordance with claim 67 wherein:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least one radio frequency transmitter, produces each received signal strength indicator.

81. A method in accordance with claim 80 wherein:

the amplifier is an intermediate frequency amplifier.

82. A radio frequency receiver for determining where at least one radio frequency transmitter is located with respect to a set range measured from the radio frequency receiver with each radio frequency transmitter periodically transmitting an identification code from each radio frequency transmitter which identifies each radio frequency transmitter with a radio frequency carrier modulated with a subcarrier with the subcarrier being modulated with the identification code comprising:

a processor contained in the radio frequency receiver, the processor in response to reception of each radio frequency carrier determines if an identification code of one of the at least one radio frequency transmitter is contained therein, calculates an integral of a received signal strength indicator of each radio frequency carrier determined to contain an identification code of one of the at least one radio frequency transmitter, computes an average of a plurality of the calculated integrals which is updated to include newly calculated integrals only when each newly calculated integral differs from the computed average of the calculated integrals by less than a function of the average of the calculated integrals so as to exclude from the computation of the average of the calculated integrals newly calculated integrals which differ from the average of the calculated integrals by more than the function, compares the average of the calculated integrals to a value representing the set range and generates an alert when the comparison reveals that at least one radio frequency transmitter is outside the set range.

83. A radio frequency in accordance with claim 82 further comprising:

an omnidirectional antenna, the omnidirectional antenna receiving the radio frequency carrier transmitted by each radio frequency transmitter which is used to calculate the average of the calculated integrals; and

a directional antenna, the directional antenna after generation of the alert receiving transmissions of the radio frequency carrier containing the identification code identifying the radio frequency transmitter from which the identification codes were transmitted which caused the generation of the alert and in response to the radio frequency receiver being moved by a user the processor controls display of a magnitude of an integral of each successively received signal strength indicator generated in response to reception of transmissions of the radio frequency carrier containing the identification code of the radio frequency transmitter from which the identification codes were transmitted which caused the generation of the alert to permit a user of the radio frequency receiver to locate a direction, from which a radio frequency carrier containing the identification code of the radio frequency transmitter from which the identification codes were transmitted to generate the alert is received, producing a maximum magnitude of the integral of each successively received signal strength relative to the radio frequency receiver whereby a direction of the radio frequency transmitter which is outside of the set range is determined by the user of the radio frequency receiver relative to the radio frequency receiver.

84. A radio frequency receiver in accordance with claim 82 further comprising:

an omnidirectional antenna, the omnidirectional antenna receiving the radio frequency carrier transmitted by one of the at least one radio frequency transmitter which contains the identification code of the one radio frequency transmitter and an alert which signals to a user of the radio frequency receiver that the user of the one radio frequency transmitter is transmitting a change in status of the user; and

the directional antenna in response to reception of the alert from one of the at least one radio frequency transmitter receives transmissions of the radio frequency carrier containing the identification code identifying the radio frequency transmitter from which the alert was transmitted and in response to the radio frequency receiver being moved by a user the processor controls display of a magnitude of an integral of each successively received signal strength indicator generated in response to reception of the radio frequency carrier from the radio frequency transmitter from which the alert was received to permit a user of the radio frequency receiver to locate a direction, from which a radio frequency carrier is received from the radio frequency transmitter transmitting the alert, producing a maximum magnitude of the integral of each successively received signal strength indicator relative to the radio frequency receiver whereby a direction of the radio frequency transmitter which transmitted the alert is determined by the user of radio frequency receiver relative to the radio frequency receiver.

85. A radio frequency receiver in accordance with claim 84 further comprising:

the subcarrier of each received radio frequency carrier is modulated with at least one identification frame group, each identification frame group comprising a plurality of frames with at least one of the plurality of frames of the identification frame group containing bits encoding the identification code of each radio frequency transmitter, a plurality of bits of error correction code in each frame, synchronization information for synchronizing a clock of the radio frequency receiver, and a command field for encoding the alert transmitted to the radio frequency receiver from the user of the radio frequency transmitter transmitting the alert.

86. A radio frequency receiver in accordance with claim 82 wherein:

the subcarrier of each received radio frequency carrier has cycles which are modulated with bits encoding the identification code of the radio frequency transmitter which transmitted the received radio frequency carrier with each cycle of the subcarrier being modulated with bits at a plurality of separated angular positions; and

for each radio frequency carrier received from each radio frequency transmitter the processor processes detected individual cycles of the subcarrier to calculate an integral of at least one selected modulated part of each of the individual cycles, numerically compares each of the calculated integrals with a plurality of stored numerical ranges which ranges each represent one of a plurality of possible numerical values that the selected part may encode to identify a stored range numerically including the calculated integral, substitutes for the at least one selected part of each of the cycles the one of the plurality of numerical values representative of the identified stored range including the calculated integral with each numerical value encoding one bit of the identification code of the radio frequency transmitter and decodes the plurality of numerical values to produce the identification code of the radio frequency transmitter.

87. A radio frequency receiver in accordance with claim 86 wherein:

the processing of the detected individual cycles of the subcarrier by the processor includes calculating the integral by taking a plurality of samples of each selected modulated part of each of the individual cycles with each sample having a numerical value and each sample is compared with a range of numerical values representing a valid sample which should be included within the calculation of the integral and when the comparison reveals that the sample value is outside the range of numerical values, the compared sample value is replaced with a value which is a function of the sample values adjacent the sample value which is replaced.

88. A radio frequency receiver in accordance with claim 86 wherein:

the compared sample value is replaced with a value which is an average of at least one sample value which precedes the compared sample value and at least one sample value which succeeds the compared sample value.

89. A radio frequency receiver in accordance with claim 83 wherein:

the subcarrier of each received radio frequency carrier has cycles which are modulated with bits encoding the identification code of the radio frequency transmitter which transmitted the received radio frequency carrier with each cycle of the subcarrier being modulated with bits at a plurality of separated angular positions; and

for each radio frequency carrier received from each radio frequency transmitter the processor processes detected individual cycles of the subcarrier to calculate an integral of at least one selected modulated part of each of the individual cycles, numerically compares each of the calculated integrals with a plurality of stored numerical ranges which ranges each represent one of a plurality of possible numerical values that the selected part may encode to identify a stored range numerically including the calculated integral, substitutes for the at least one selected part of each of the cycles the one of the plurality of numerical values representative of the identified stored range including the calculated integral with each numerical value encoding one bit of the identification code of the radio frequency transmitter and decodes the plurality of numerical values to produce the identification code of the radio frequency transmitter.

90. A radio frequency receiver in accordance with claim 89 wherein:

the processing of the detected individual cycles of the subcarrier by the processor includes calculating the integral by taking a plurality of samples of each selected modulated part of each of the individual cycles with each sample having a numerical value and each sample is compared with a range of numerical values representing a valid sample which should be included within the calculation of the integral and when the comparison reveals that the sample value is outside the range of numerical values, the compared sample value is replaced with a value which is a function of the sample values adjacent the sample value which is replaced.

91. A radio frequency receiver in accordance with claim 90 wherein:

the compared sample value is replaced with a value which is an average of at least one sample value which precedes the compared sample value and at least one sample value which succeeds the compared sample value.

92. A radio frequency receiver in accordance with claim 84 wherein:

the subcarrier of each received radio frequency carrier has cycles which are modulated with bits encoding the identification code of the radio frequency transmitter which transmitted the received radio frequency carrier with each cycle of the subcarrier being modulated with bits at a plurality of separated angular positions; and

for each radio frequency carrier received from each radio frequency transmitter the processor processes detected individual cycles of the subcarrier to calculate an integral of at least one selected modulated part of each of the individual cycles, numerically compares each of the calculated integrals with a plurality of stored numerical ranges which ranges each represent one of a plurality of possible numerical values that the selected part may encode to identify a stored range numerically including the calculated integral, substitutes for the at least one selected part of each of the cycles the one of the plurality of numerical values representative of the identified stored range including the calculated integral with each numerical value encoding one bit of the identification code of the radio frequency transmitter and decodes the plurality of numerical values to produce the identification code of the radio frequency transmitter.

93. A radio frequency receiver in accordance with claim 92 wherein:

the processing of the detected individual cycles of the subcarrier by the processor includes calculating the integral by taking a plurality of samples of each selected modulated part of each of the individual cycles with each sample having a numerical value and each sample is compared with a range of numerical values representing a valid sample which should be included within the calculation of the integral and when the comparison reveals that the sample value is outside the range of numerical values, the compared sample value is replaced with a value which is a function of the sample values adjacent the sample value which is replaced.

94. A radio frequency receiver in accordance with claim 93 wherein:

the compared sample value is replaced with a value which is an average of at least one sample value which precedes the compared sample value and at least one sample value which succeeds the compared sample value.

95. A radio frequency receiver in accordance with claim 85 wherein:

the subcarrier of each received radio frequency carrier has cycles which are modulated with bits encoding the at least one identification frame group with each cycle of the subcarrier being modulated with bits at a plurality of separated angular positions; and

for each radio frequency carrier received from each radio frequency transmitter the processor processes detected individual cycles of the subcarrier to calculate an integral of at least one selected modulated part of each of the individual cycles, numerically compares each of the calculated integrals with a plurality of stored numerical ranges which ranges each represent one of a plurality of possible numerical values that the selected part may encode to identify a stored range numerically including the calculated integral, substitutes for the at least one selected part of each of the cycles the one of the plurality of numerical values representative of the identified stored range including the calculated integral with each numerical value encoding one bit of the at least one identification frame group of the radio frequency transmitter and decodes the plurality of numerical values to produce at least the identification code of the radio frequency transmitter.

96. A radio frequency receiver in accordance with claim 95 wherein:

the processing of the detected individual cycles of the subcarrier by the processor includes calculating the integral by taking a plurality of samples of each selected modulated part of each of the individual cycles with each sample having a numerical value and each sample is compared with a range of numerical values representing a valid sample which should be included within the calculation of the integral and when the comparison reveals that the sample value is outside the range of numerical values, the compared sample value is replaced with a value which is a function of the sample values adjacent the sample value which is replaced.

97. A radio frequency receiver in accordance with claim 96 wherein:

the compared sample value is replaced with a value which is an average of at least one sample value which precedes the compared sample value and at least one sample value which succeeds the compared sample value.

98. A radio receiver in accordance with claim 82 further comprising:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least on radio frequency transmitter, for producing each received signal strength indicator.

99. A radio frequency receiver in accordance with claim 98 wherein:

the amplifier is an intermediate frequency amplifier.

100. A radio receiver in accordance with claim 83 further comprising:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least on radio frequency transmitter, for producing each received signal strength indicator.

101. A radio frequency receiver in accordance with claim 100 wherein:

the amplifier is an intermediate frequency amplifier.

102. A radio receiver in accordance with claim 84 further comprising:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least on radio frequency transmitter, for producing each received signal strength indicator.

103. A radio frequency receiver in accordance with claim 102 wherein:

the amplifier is an intermediate frequency amplifier.

104. A radio receiver in accordance with claim 85 further comprising:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least on radio frequency transmitter, for producing each received signal strength indicator.

105. A radio frequency receiver in accordance with claim 104 wherein:

the amplifier is an intermediate frequency amplifier.

106. A radio receiver in accordance with claim 86 further comprising:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least on radio frequency transmitter, for producing each received signal strength indicator.

107. A radio frequency receiver in accordance with claim 106 wherein:

the amplifier is an intermediate frequency amplifier.

108. A radio receiver in accordance with claim 87 further comprising:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least on radio frequency transmitter, for producing each received signal strength indicator.

109. A radio frequency receiver in accordance with claim 108 wherein:

the amplifier is an intermediate frequency amplifier.

110. A radio receiver in accordance with claim 88 further comprising:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least on radio frequency transmitter, for producing each received signal strength indicator.

111. A radio frequency receiver in accordance with claim 110 wherein:

the amplifier is an intermediate frequency amplifier.

112. A radio frequency receiver for determining where at least one radio frequency transmitter is located with respect to a set range measured from the radio frequency receiver with each radio frequency transmitter periodically transmitting an identification code which identifies each radio frequency transmitter with a radio frequency carrier modulated with a subcarrier with the subcarrier being modulated with the identification code comprising:

a processor contained in the radio frequency receiver, the processor in response to reception of each radio frequency carrier determines if an identification code of one of the at least one radio frequency transmitter is contained therein, calculates an integral of a received signal strength indicator of each radio frequency carrier determined to contain an identification code of one of the at least one radio frequency transmitter, computes an average of the calculated integrals which is updated to include newly calculated integrals, compares the average of the calculated integrals to a numerical value representing the set range and generates an alert when the comparison reveals that at least one of the at least one radio frequency transmitter is outside the set range.

113. A radio frequency receiver in accordance with claim 112 further comprising:

an omnidirectional antenna, the omnidirectional antenna receiving the radio frequency carrier transmitted by each radio frequency transmitter which is used to calculate the average of the calculated integrals; and

a directional antenna, the directional antenna after generation of the alert receiving transmissions of the radio frequency carrier containing the identification code identifying the radio frequency transmitter from which the identification codes were transmitted which caused the generation of the alert and in response to the radio frequency receiver being moved by a user the processor controls display of a magnitude of an integral of each successively received signal strength indicator generated in response to reception of transmissions of the radio frequency carrier containing the identification code of the radio frequency transmitter from which the identification codes were transmitted which caused the generation of the alert to permit a user of the radio frequency receiver to locate a direction, from which a radio frequency carrier containing the identification code of the radio frequency transmitter from which the identification codes were transmitted to generate the alert is received, producing a maximum magnitude of the integral of each successively received signal strength relative to the radio frequency receiver whereby a direction of the radio frequency transmitter which is outside of the set range is determined by the user of the radio frequency receiver relative to the radio frequency receiver.

114. A radio frequency receiver in accordance with claim 112 further comprising:

an omnidirectional antenna, the omnidirectional antenna receiving the radio frequency carrier transmitted by one of the at least one radio frequency transmitter which contains the identification code of the one radio frequency transmitter and an alert which signals to a user of the radio frequency receiver that the user of the one radio frequency transmitter is transmitting a change in status of the user; and

a directional antenna, the directional antenna in response to reception of the alert from one of the at least one radio frequency transmitter receives transmissions of the radio frequency carrier containing the identification code identifying the radio frequency transmitter from which the alert was transmitted and in response to the radio frequency receiver being moved by a user the processor controls display of a magnitude of an integral of each successively received signal strength indicator generated in response to reception of the radio frequency carrier from the radio frequency transmitter from which the alert was received to permit a user of the radio frequency receiver to locate a direction, from which a radio frequency carrier is received from the radio frequency transmitter transmitting the alert, producing a maximum magnitude of the integral of each successively received signal strength indicator relative to the radio frequency receiver whereby a direction of the radio frequency transmitter which transmitted the alert is determined by the user of radio frequency receiver relative to the radio frequency receiver.

115. A radio frequency receiver in accordance with claim 114 further comprising:

the subcarrier of each received radio frequency carrier is modulated with at least one identification frame group, each identification frame group comprising a plurality of frames with at least one of the plurality of frames of the identification frame group containing bits encoding the identification code of each radio frequency transmitter, a plurality of bits of error correction code in each frame, synchronization information for synchronizing a clock of the radio frequency receiver, and a command field for encoding the alert transmitted to the radio frequency receiver from the user of the radio frequency transmitter transmitting the alert.

116. A radio frequency receiver in accordance with claim 112 wherein:

the subcarrier of each received radio frequency carrier has cycles which are modulated with bits encoding the identification code of the radio frequency transmitter which transmitted the received radio frequency carrier with each cycle of the subcarrier being modulated with bits at a plurality of separated angular positions; and

for each radio frequency carrier received from each radio frequency transmitter the processor processes detected individual cycles of the subcarrier to calculate an integral of at least one selected modulated part of each of the individual cycles, numerically compares each of the calculated integrals with a plurality of stored numerical ranges which ranges each represent one of a plurality of possible numerical values that the selected part may encode to identify a stored range numerically including the calculated integral, substitutes for the at least one selected part of each of the cycles the one of the plurality of numerical values representative of the identified stored range including the calculated integral with each numerical value encoding one bit of the identification code of the radio frequency transmitter and decodes the plurality of numerical values to produce the identification code of the radio frequency transmitter.

117. A radio frequency receiver in accordance with claim 116 wherein:

the processing of the detected individual cycles of the subcarrier by the processor includes calculating the integral by taking a plurality of samples of each selected modulated part of each of the individual cycles with each sample having a numerical value and each sample is compared with a range of numerical values representing a valid sample which should be included within the calculation of the integral and when the comparison reveals that the sample value is outside the range of numerical values, the compared sample value is replaced with a value which is a function of the sample values adjacent the sample value which is replaced.

118. A radio frequency receiver in accordance with claim 117 wherein:

the compared sample value is replaced with a value which is an average of at least one sample value which precedes the compared sample value and at least one sample value which succeeds the compared sample value.

119. A radio frequency receiver in accordance with claim 113 wherein:

the subcarrier of each received radio frequency carrier has cycles which are modulated with bits encoding the identification code of the radio frequency transmitter which transmitted the received radio frequency carrier with each cycle of the subcarrier being modulated with bits at a plurality of separated angular positions; and

for each radio frequency carrier received from each radio frequency transmitter the processor processes detected individual cycles of the subcarrier to calculate an integral of at least one selected modulated part of each of the individual cycles, numerically compares each of the calculated integrals with a plurality of stored numerical ranges which ranges each represent one of a plurality of possible numerical values that the selected part may encode to identify a stored range numerically including the calculated integral, substitutes for the at least one selected part of each of the cycles the one of the plurality of numerical values representative of the identified stored range including the calculated integral with each numerical value encoding one bit of the identification code of the radio frequency transmitter and decodes the plurality of numerical values to produce the identification code of the radio frequency transmitter.

120. A radio frequency receiver in accordance with claim 119 wherein:

the processing of the detected individual cycles of the subcarrier by the processor includes calculating the integral by taking a plurality of samples of each selected modulated part of each of the individual cycles with each sample having a numerical value and each sample is compared with a range of numerical values representing a valid sample which should be included within the calculation of the integral and when the comparison reveals that the sample value is outside the range of numerical values, the compared sample value is replaced with a value which is a function of the sample values adjacent the sample value which is replaced.

121. A radio frequency receiver in accordance with claim 120 wherein:

the compared sample value is replaced with a value which is an average of at least one sample value which precedes the compared sample value and at least one sample value which succeeds the compared sample value.

122. A radio frequency receiver in accordance with claim 114 wherein:

the subcarrier of each received radio frequency carrier has cycles which are modulated with bits encoding the identification code of the radio frequency transmitter which transmitted the received radio frequency carrier with each cycle of the subcarrier being modulated with bits at a plurality of separated angular positions; and

for each radio frequency carrier received from each radio frequency transmitter the processor processes detected individual cycles of the subcarrier to calculate an integral of at least one selected modulated part of each of the individual cycles, numerically compares each of the calculated integrals with a plurality of stored numerical ranges which ranges each represent one of a plurality of possible numerical values that the selected part may encode to identify a stored range numerically including the calculated integral, substitutes for the at least one selected part of each of the cycles the one of the plurality of numerical values representative of the identified stored range including the calculated integral with each numerical value encoding one bit of the identification code of the radio frequency transmitter and decodes the plurality of numerical values to produce the identification code of the radio frequency transmitter.

123. A radio frequency receiver in accordance with claim 122 wherein:

the processing of the detected individual cycles of the subcarrier by the processor includes calculating the integral by taking a plurality of samples of each selected modulated part of each of the individual cycles with each sample having a numerical value and each sample is compared with a range of numerical values representing a valid sample which should be included within the calculation of the integral and when the comparison reveals that the sample value is outside the range of numerical values, the compared sample value is replaced with a value which is a function of the sample values adjacent the sample value which is replaced.

124. A radio frequency receiver in accordance with claim 123 wherein:

the compared sample value is replaced with a value which is an average of at least one sample value which precedes the compared sample value and at least one sample value which succeeds the compared sample value.

125. A radio frequency receiver in accordance with claim 115 wherein:

the subcarrier of each received radio frequency carrier has cycles which are modulated with bits encoding the at least one identification frame group with each cycle of the subcarrier being modulated with bits at a plurality of separated angular positions; and

for each radio frequency carrier received from each radio frequency transmitter the processor processes detected individual cycles of the subcarrier to calculate an integral of at least one selected modulated part of each of the individual cycles, numerically compares each of the calculated integrals with a plurality of stored numerical ranges which ranges each represent one of a plurality of possible numerical values that the selected part may encode to identify a stored range numerically including the calculated integral, substitutes for the at least one selected part of each of the cycles the one of the plurality of numerical values representative of the identified stored range including the calculated integral with each numerical value encoding one bit of the at least one identification frame group of the radio frequency transmitter and decodes the plurality of numerical values to produce the identification code of the radio frequency transmitter.

126. A radio frequency receiver in accordance with claim 125 wherein:

the processing of the detected individual cycles of the subcarrier by the processor includes calculating the integral by taking a plurality of samples of each selected modulated part of each of the individual cycles with each sample having a numerical value and each sample is compared with a range of numerical values representing a valid sample which should be included within the calculation of the integral and when the comparison reveals that the sample value is outside the range of numerical values, the compared sample value is replaced with a value which is a function of the sample values adjacent the sample value which is replaced.

127. A radio frequency receiver in accordance with claim 126 wherein:

the compared sample value is replaced with a value which is an average of at least one sample value which precedes the compared sample value and at least one sample value which succeeds the compared sample value.

128. A radio receiver in accordance with claim 112 further comprising:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least on radio frequency transmitter, for producing each received signal strength indicator.

129. A radio frequency receiver in accordance with claim 128 wherein:

the amplifier is an intermediate frequency amplifier.

130. A radio receiver in accordance with claim 113 further comprising:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least on radio frequency transmitter, for producing each received signal strength indicator.

131. A radio frequency receiver in accordance with claim 130 wherein:

the amplifier is an intermediate frequency amplifier.

132. A radio receiver in accordance with claim 114 further comprising:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least on radio frequency transmitter, for producing each received signal strength indicator.

133. A radio frequency receiver in accordance with claim 132 wherein:

the amplifier is an intermediate frequency amplifier.

134. A radio receiver in accordance with claim 115 further comprising:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least on radio frequency transmitter, for producing each received signal strength indicator.

135. A radio frequency receiver in accordance with claim 134 wherein:

the amplifier is an intermediate frequency amplifier.

136. A radio receiver in accordance with claim 116 further comprising:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least on radio frequency transmitter, for producing each received signal strength indicator.

137. A radio frequency receiver in accordance with claim 136 wherein:

the amplifier is an intermediate frequency amplifier.

138. A radio receiver in accordance with claim 117 further comprising:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least on radio frequency transmitter, for producing each received signal strength indicator.

139. A radio frequency receiver in accordance with claim 138 wherein:

the amplifier is an intermediate frequency amplifier.

140. A radio receiver in accordance with claim 118 further comprising:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least on radio frequency transmitter, for producing each received signal strength indicator.

141. A radio frequency receiver in accordance with claim 140 wherein:

the amplifier is an intermediate frequency amplifier.

142. A radio frequency receiver for determining where at least one radio frequency transmitter is located with respect to a set range measured from the radio frequency receiver with each radio frequency transmitter periodically transmitting an identification code from each radio frequency transmitter which identifies each radio frequency transmitter with a radio frequency carrier modulated with a subcarrier with the subcarrier being modulated with the identification code comprising:

a processor contained in the radio frequency receiver, the processor in response to reception of each radio frequency carrier determines if an identification code of one of the at least one radio frequency transmitter is contained therein, produces a received signal strength indicator of each radio frequency carrier determined to contain an identification code of one of the at least one radio frequency transmitter, computes an average of a plurality of the received signal strength indicators, compares the average of the received signal strength indicators to a numerical value representing the set range and generates an alert when a comparison reveals that at least one of the at least one radio frequency transmitter is outside the set range.

143. A radio frequency receiver in accordance with claim 142 further comprising:

an omnidirectional antenna, the omnidirectional antenna receiving the radio frequency carrier transmitted by each radio frequency transmitter which is used to calculate the average of the calculated integrals; and

a directional antenna, the directional antenna after generation of the alert receives transmissions of the radio frequency carrier containing the identification code identifying the radio frequency transmitter from which the identification codes were transmitted which caused the generation of the alert and in response to the radio frequency receiver being moved by a user the processor controls display of a magnitude of an integral of each successively received signal strength indicator generated in response to reception of transmissions of the radio frequency carrier containing the identification code of the radio frequency transmitter from which the identification codes were transmitted which caused the radio frequency receiver to generate the alert to permit a user of the radio frequency receiver to locate a direction, from which a radio frequency carrier containing the identification code of the radio frequency transmitter from which the identification codes were transmitted to generate the alert is received, producing a maximum magnitude of the integral of each successively received signal strength indicator relative to the radio frequency receiver whereby a direction of the radio frequency transmitter which is outside of the set range is determined by the user of the radio frequency receiver relative to the radio frequency receiver.

144. A radio frequency receiver in accordance with claim 142 further comprising:

an omnidirectional antenna, the omnidirectional antenna receiving the radio frequency carrier transmitted by one of the at least one radio frequency transmitter which contains the identification code of the one radio frequency transmitter and an alert which signals to a user of the radio frequency receiver that the user of the one radio frequency transmitter is transmitting a change in status of the user; and

a directional antenna, the directional antenna in response to reception of the alert from one of the at least one radio frequency transmitter receives transmissions of the radio frequency carrier containing the identification code identifying the radio frequency transmitter from which the alert was transmitted and in response to the radio frequency receiver being moved by a user the processor controls display of a magnitude of an integral of each successively received signal strength indicator generated in response to reception of the radio frequency carrier from the radio frequency transmitter from which the alert was received to permit a user of the radio frequency receiver to locate a direction, from which a radio frequency carrier is received from the radio frequency transmitter transmitting the alert, producing a maximum magnitude of the integral of each successively received signal strength indicator relative to the radio frequency receiver whereby a direction of the radio frequency transmitter which transmitted the alert is determined by the user of radio frequency receiver relative to the radio frequency receiver.

145. A radio frequency receiver in accordance with claim 144 further comprising:

the subcarrier of each received radio frequency carrier is modulated with at least one identification frame group, each identification frame group comprising a plurality of frames with at least one of the plurality of frames of the identification frame group containing bits encoding the identification code of each radio frequency transmitter, a plurality of bits of error correction code in each frame, synchronization information for synchronizing a clock of the radio frequency receiver, and a command field for encoding the alert transmitted to the radio frequency receiver from the user of the radio frequency transmitter transmitting the alert.

146. A radio frequency receiver in accordance with claim 142 wherein:

the subcarrier of each received radio frequency carrier has cycles which are modulated with bits encoding the identification code of the radio frequency transmitter which transmitted the received radio frequency carrier with each cycle of the subcarrier being modulated with bits at a plurality of separated angular positions; and

for each radio frequency carrier received from each radio frequency transmitter the processor processes detected individual cycles of the subcarrier to calculate an integral of at least one selected modulated part of each of the individual cycles, numerically compares each of the calculated integrals with a plurality of stored numerical ranges which ranges each represent one of a plurality of possible numerical values that the selected part may encode to identify a stored range numerically including the calculated integral, substitutes for the at least one selected part of each of the cycles the one of the plurality of numerical values representative of the identified stored range including the calculated integral with each numerical value encoding one bit of the identification code of the radio frequency transmitter and decodes the plurality of numerical values to produce the identification code of the radio frequency transmitter.

147. A radio frequency receiver in accordance with claim 146 wherein:

the processing of the detected individual cycles of the subcarrier by the processor includes calculating the integral by taking a plurality of samples of each selected modulated part of each of the individual cycles with each sample having a numerical value and each sample is compared with a range of numerical values representing a valid sample which should be included within the calculation of the integral and when the comparison reveals that the sample value is outside the range of numerical values, the compared sample value is replaced with a value which is a function of the sample values adjacent the sample value which is replaced.

148. A radio frequency receiver in accordance with claim 147 wherein:

the compared sample value is replaced with a value which is an average of at least one sample value which precedes the compared sample value and at least one sample value which succeeds the compared sample value.

149. A radio receiver in accordance with claim 142 further comprising:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least on radio frequency transmitter, for producing each received signal strength indicator.

150. A radio frequency receiver in accordance with claim 149 wherein:

the amplifier is an intermediate frequency amplifier.

151. A radio receiver in accordance with claim 143 further comprising:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least on radio frequency transmitter, for producing each received signal strength indicator.

152. A radio frequency receiver in accordance with claim 151 wherein:

the amplifier is an intermediate frequency amplifier.

153. A radio receiver in accordance with claim 144 further comprising:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least on radio frequency transmitter, for producing each received signal strength indicator.

154. A radio frequency receiver in accordance with claim 153 wherein:

the amplifier is an intermediate frequency amplifier.

155. A radio receiver in accordance with claim 145 further comprising:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least on radio frequency transmitter, for producing each received signal strength indicator.

156. A radio frequency receiver in accordance with claim 155 wherein:

the amplifier is an intermediate frequency amplifier.

157. A radio receiver in accordance with claim 146 further comprising:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least on radio frequency transmitter, for producing each received signal strength indicator.

158. A radio frequency receiver in accordance with claim 157 wherein:

the amplifier is an intermediate frequency amplifier.

159. A radio receiver in accordance with claim 147 further comprising:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least on radio frequency transmitter, for producing each received signal strength indicator.

160. A radio frequency receiver in accordance with claim 159 wherein:

the amplifier is an intermediate frequency amplifier.

161. A radio receiver in accordance with claim 148 further comprising:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least on radio frequency transmitter, for producing each received signal strength indicator.

162. A radio frequency receiver in accordance with claim 161 wherein:

the amplifier is an intermediate frequency amplifier.

163. A method for determining where at least one radio frequency transmitter is located with respect to a set range measured from a radio frequency receiver with each radio frequency transmitter periodically transmitting an identification code which identifies each radio frequency transmitter with a radio frequency carrier modulated with a subcarrier with the subcarrier being modulated with the identification code comprising:

in response to receiving each radio frequency carrier determining if an identification code of one of the at least one radio frequency transmitter is contained therein, producing a first function of a received signal strength indicator of each radio frequency carrier determined to contain an identification code of one of the at least one radio frequency transmitter, computing a second function in response to at least one first function of the received signal strength indicator, comparing the second function to a value representing the set range and generating an alert when the comparison reveals that at least one of the at least one radio frequency transmitter is outside the set range.

164. A method in accordance with claim 163 further comprising:

using an omnidirectional antenna to receive the radio frequency carrier transmitted by each radio frequency transmitter; and

after generation of the alert using a directional antenna to receive transmissions of the radio frequency carrier containing the identification code identifying the radio frequency transmitter from which the identification codes were transmitted which caused the generation of the alert and in response to the radio frequency receiver being moved by a user the radio frequency receiver displaying in response to a function of each successively received signal strength indicator generated in response to reception of transmissions of the radio frequency carrier containing the identification code of the radio frequency transmitter from which the identification codes were transmitted which caused the generation of the alert a magnitude display to permit a user of the radio frequency receiver to locate a direction, from which a radio frequency carrier containing the identification code of the radio frequency transmitter from which the identification codes were transmitted to generate the alert is received, producing a maximum magnitude display in response to the function of each successively received signal strength indicator relative to the radio frequency receiver whereby a direction of the radio frequency transmitter which is outside of the set range is determined by the user of the radio frequency receiver relative to the radio frequency receiver.

165. A method in accordance with claim 163 further comprising:

using an omnidirectional antenna to receive the radio frequency carrier transmitted by one of the at least one radio frequency transmitter which contains the identification code of the one radio frequency transmitter and an alert which signals to a user of the radio frequency receiver that the user of the one radio frequency transmitter is transmitting a change in status of the user; and

in response to reception of the alert from one of the at least one radio frequency transmitter using a directional antenna to receive transmissions of the radio frequency carrier containing the identification code identifying the radio frequency transmitter from which the alert was transmitted and in response to the radio frequency receiver being moved by a user the radio frequency receiver displaying, in response to a function of each successively received signal strength indicator generated in response to reception of transmissions of the radio frequency carrier containing the identification code of the radio frequency transmitter from which the alert was received, a magnitude display to permit a user of the radio frequency receiver to locate a direction, from which a radio frequency carrier is received from the radio frequency transmitter transmitting the alert, producing a maximum magnitude display in response to the function of each successively received signal strength indicator relative to the radio frequency receiver whereby a direction of the radio frequency transmitter which transmitted the alert is determined by the user of the radio frequency receiver relative to the radio frequency receiver.

166. A method in accordance with claim 165 further comprising:

the subcarrier of each received radio frequency carrier is modulated with at least one identification frame group, each identification frame group comprising a plurality of frames with at least one of the plurality of frames of the identification frame group containing bits encoding the identification code of each radio frequency transmitter, a plurality of bits of error correction code in each frame, synchronization information for synchronizing a clock of the radio frequency receiver, and a command field for encoding the alert transmitted to the radio frequency receiver from the user of the radio frequency transmitter transmitting the alert.

167. A method in accordance with claim 163 wherein:

the subcarrier of each received radio frequency carrier has cycles which are modulated with bits encoding the identification code of the radio frequency transmitter which transmitted the received radio frequency carrier with each cycle of the subcarrier being modulated with bits at a plurality of separated angular positions; and

for each radio frequency carrier received from each radio frequency transmitter processing detected individual cycles of the subcarrier to calculate an integral of at least one selected modulated part of each of the individual cycles, numerically comparing each of the calculated integrals with a plurality of stored numerical ranges which ranges each represent one of a plurality of possible numerical values that the selected part may encode to identify a stored range numerically including the calculated integral, substituting for the at least one selected part of each of the cycles the one of the plurality of numerical values representative of the identified stored range including the calculated integral with each numerical value encoding one bit of the identification code of the radio frequency transmitter and decoding the plurality of numerical values to produce the identification code of the radio frequency transmitter.

168. A method in accordance with claim 163 wherein:

the second function is responsive to a plurality of first functions of the received signal strength indicator.

169. A method in accordance with claim 164 wherein:

the second function is responsive to a plurality of first functions of the received signal strength indicator.

170. A method in accordance with claim 165 wherein:

the second function is responsive to a plurality of first functions of the received signal strength indicator.

171. A method in accordance with claim 166 wherein:

the second function is responsive to a plurality of first functions of the received signal strength indicator.

172. A method in accordance with claim 167 wherein:

the second function is responsive to a plurality of first functions of the received signal strength indicator.

173. A method in accordance with claim 163 wherein:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least one radio frequency transmitter, produces each received signal strength indicator.

174. A method in accordance with claim 173 wherein:

the amplifier is an intermediate frequency amplifier.

175. A method in accordance with claim 168 wherein:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least one radio frequency transmitter, produces each received signal strength indicator.

176. A method in accordance with claim 175 wherein:

the amplifier is an intermediate frequency amplifier.

177. A radio frequency receiver for determining where at least one radio frequency transmitter is located with respect to a set range measured from the radio frequency receiver with each radio frequency transmitter periodically transmitting an identification code from each radio frequency transmitter which identifies each radio frequency transmitter with a radio frequency carrier modulated with a subcarrier with the subcarrier being modulated with the identification code comprising:

a processor contained in the radio frequency receiver, the processor in response to reception of each radio frequency carrier determines if an identification code of one of the at least one radio frequency transmitter is contained therein, produces a first function of a received signal strength indicator of each radio frequency carrier determined to contain an identification code of one of the at least one radio frequency transmitter, computes a second function in response to at least one first function of the received signal strength indicator, compares the second function to a numerical value representing the set range and generates an alert when a comparison reveals that at least one of the at least one radio frequency transmitter is outside the set range.

178. A radio frequency receiver in accordance with claim 177 further comprising:

an omnidirectional antenna, the omnidirectional antenna receiving the radio frequency carrier transmitted by each radio frequency transmitter which is used to calculate the function of each received signal strength indicator; and

a directional antenna, the directional antenna after generation of the alert receives transmissions of the radio frequency carrier containing the identification code identifying the radio frequency transmitter from which the identification codes were transmitted which caused the radio frequency receiver to generate the alert and in response to the radio frequency receiver being moved by a user the processor in response to a function of each successively received signal strength indicator generated in response to reception of transmissions of the radio frequency carrier containing the identification code of the radio frequency transmitter from which the identification codes were transmitted which caused the radio frequency receiver to generate the alert controls display of a magnitude display to permit a user of the radio frequency receiver to locate a direction, from which a radio frequency carrier containing the identification code of the radio frequency transmitter from which the identification codes were transmitted to generate the alert is received, produces a maximum magnitude display in response to the function of each successively received signal strength indicator relative to the radio frequency receiver whereby a direction of the radio frequency transmitter which is outside of the set range is determined by the user of the radio frequency receiver relative to the radio frequency receiver.

179. A radio frequency receiver in accordance with claim 178 further comprising:

an omnidirectional antenna, the omnidirectional antenna receiving the radio frequency carrier transmitted by one of the at least one radio frequency transmitter which contains the identification code of the one radio frequency transmitter and an alert which signals to a user of the radio frequency receiver that the user of the one radio frequency transmitter is transmitting a change in status of the user; and

a directional antenna, the directional antenna in response to reception of the alert from one of the at least one radio frequency transmitter receives transmissions of the radio frequency carrier containing the identification code identifying the radio frequency transmitter from which the alert was transmitted and in response to the radio frequency receiver being moved by a user the processor, in response to a function of each successively received signal strength indicator generated in response to reception of the radio frequency carrier from the radio frequency transmitter from which the alert was received, controls displaying a magnitude display to permit a user of the radio frequency receiver to locate a direction, from which a radio frequency carrier is received from the radio frequency transmitter transmitting the alert, producing a maximum magnitude display in response to a function of each successively received signal strength indicator relative to the radio frequency receiver whereby a direction of the radio frequency transmitter which transmitted the alert is determined by the user of radio frequency receiver relative to the radio frequency receiver.

180. A radio frequency receiver in accordance with claim 179 further comprising:

the subcarrier of each received radio frequency carrier is modulated with at least one identification frame group, each identification frame group comprising a plurality of frames with at least one of the plurality of frames of the identification frame group containing bits encoding the identification code of each radio frequency transmitter, a plurality of bits of error correction code in each frame, synchronization information for synchronizing a clock of the radio frequency receiver, and a command field for encoding the alert transmitted to the radio frequency receiver from the user of the radio frequency transmitter transmitting the alert.

181. A radio frequency receiver in accordance with claim 178 wherein:

the subcarrier of each received radio frequency carrier has cycles which are modulated with bits encoding the identification code of the radio frequency transmitter which transmitted the received radio frequency carrier with each cycle of the subcarrier being modulated with bits at a plurality of separated angular positions; and

for each radio frequency carrier received from each radio frequency transmitter the processor processes detected individual cycles of the subcarrier to calculate an integral of at least one selected modulated part of each of the individual cycles, numerically compares each of the calculated integrals with a plurality of stored numerical ranges which ranges each represent one of a plurality of possible numerical values that the selected part may encode to identify a stored range numerically including the calculated integral, substitutes for the at least one selected part of each of the cycles the one of the plurality of numerical values representative of the identified stored range including the calculated integral with each numerical value encoding one bit of the identification code of the radio frequency transmitter and decodes the plurality of numerical values to produce the identification code of the radio frequency transmitter.

182. A radio frequency receiver in accordance with claim 177 wherein:

the second function is responsive to a plurality of first functions of the received signal strength indicator.

183. A radio frequency receiver in accordance with claim 178 wherein:

the second function is responsive to a plurality of first functions of the received signal strength indicator.

184. A radio frequency receiver in accordance with claim 179 wherein:

the second function is responsive to a plurality of first functions of the received signal strength indicator.

185. A radio frequency receiver in accordance with claim 180 wherein:

the second function is responsive to a plurality of first functions of the received signal strength indicator.

186. A radio frequency receiver in accordance with claim 181 wherein:

the second function is responsive to a plurality of first functions of the received signal strength indicator.

187. A radio frequency receiver in accordance with claim 182 wherein:

the second function is responsive to a plurality of first functions of the received signal strength indicator.

188. A radio receiver in accordance with claim 177 further comprising:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least on radio frequency transmitter, for producing each received signal strength indicator.

189. A radio frequency receiver in accordance with claim 188 wherein:

the amplifier is an intermediate frequency amplifier.

190. A radio receiver in accordance with claim 182 further comprising:

an amplifier, responsive to each radio carrier modulated with the subcarrier containing the identification of one of the at least on radio frequency transmitter, for producing each received signal strength indicator.

191. A radio frequency receiver in accordance with claim 190 wherein:

the amplifier is an intermediate frequency amplifier.