VLF navigation system

• US 3,936,828 A
• Filed: 11/19/1973
• Issued: 02/03/1976
• Est. Priority Date: 12/22/1972
• Status: Expired due to Term

• Current Assignee: Rockwell International Corporation
• Original Assignee: Communications Components Corporation
• Inventors: Wright, Jess C., Muesse, Allen R.
• Primary Examiner(s): Tubbesing, T. H.
• Assistant Examiner(s): Berger, Richard E.
• Application Number:
  US05/417,384
• Time in Patent Office: 806 Days
• Field of Search: 343/105 R, 343/112 D, 235/150.27, 235/150.272
• US Class Current: 342/394
• CPC Class Codes: G01S 1/02 using radio waves G01S19/00 takes precedence
• CPC Class Codes: G01S 1/30 the synchronised signals being continuous waves or intermittent trains of continuous waves, the intermittency not being for the purpose of determining direction or position line and the transit times being compared by measuring the phase difference

Abstract

A VLF navigation system suitable for highly accurate worldwide aircraft navigation utilizing existing VLF transmitting stations on a "dead reckoning" basis with initial and intermediate fixes. The system is enabled to utilize all existing VLF transmitting stations regardless of station frequency by novel receiver phase-locked loop means which normalizes all station frequencies to phase coherent independent data signals of a common working frequency. In one form of the invention, utilizing hyperbolic coordinates, both RF stations of each station pair utilized for computation are such VLF transmitting stations; while in another form of the invention, utilizing Rho-Rho coordinates, one RF station of each station pair is an on-board frequency standard normalized to the common working frequency, and the other station of each pair is one of the VLF transmitting stations.

Computer means, which may be part of the "on-board" equipment, selects station pairs for optimum station geometry and range, and by comparing the phase differences between the phase coherent data signals for respective station pairs, determines a Δ time and hence a Δ position in suitable coordinates from the initial or intermediate calibration point. Novel multiplexing and counting means, and logic circuitry associated therewith, enable a single counting means to be rapidly sequentially employed by the computer to make the time difference comparison for a series of station pairs.

The system is extremely accurate for a number of reasons, including but not limited to the inherent frequency stability of VLF transmitting stations (which are generally atomic clock controlled); the inherent minimal distortion of VLF electromagnetic signals over very long ranges; the fact that errors are not cumulative in the system; and a station pair averaging capability which decreases random errors and permits rejection of the defective signals.

Claims

1. The navigation method which comprises simultaneously obtaining a pair of RF signals of different frequency originating at geographically spaced RF stations, simultaneously normalizing the said RF signals into respective first and second coherent signals of a common frequency, measuring the phase relationship between said normalized signals at a first geographical location, measuring the phase relationship between said normalized signals at a second geographical location, and determining the degree of change in said relationship to determine Δ

position information.

2. The method of claim 1, wherein at least one of said RF signals is a VLF signal transmitted from a remote station.

3. The method of claim 1, wherein one of said RF signals is an on-board time standard obtained from an on-board station.

4. The method of claim 1, wherein said normalized signals have a phase coherent relationship.

5. The method of claim 4, wherein said phase coherent relationship is established by having both of said RF signals referenced to atomic time standards.

6. The method of claim 4, wherein said phase coherent relationship is a phase synchronous relationship.

7. The method of claim 1, which utilizes a Rho-Rho system of coordinates.

8. The method of claim 1, which utilizes a hyperbolic system of coordinates.

9. The method of claim 1, wherein said normalized signals comprise first and second digital waveforms, and said phase shift is measured by providing a string of high frequency electrical clock pulses, gating said clock pulses on by a leading edge of said first waveform, gating said clock pulses off by the corresponding leading edge of said second waveform, and counting the gated pulses.

10. The method of claim 1, wherein at least one of said RF signals is normalized by providing an oscillator signal for said one RF signal which has a frequency that is a multiple of said one RF signal frequency, said oscillator frequency being divisible by an integer to said common frequency, phase locking said oscillator signal to said one RF signal, and dividing said oscillator signal by said integer to produce the normalized signal.

11. The method of claim 1, wherein at least two of said pairs of RF signals are obtained from respective RF station pairs providing at least two independent items of Δ

position information.

12. The method of claim 11, wherein all of said RF signals are normalized to said common frequency.

13. The method of claim 12, wherein said phase shifts between the normalized signal pairs for all station pairs are measured successively by the same measuring means.

14. The method of claim 13, wherein said normalized signal pairs each comprise first and second digital waveforms, and said phase shifts are measured by providing a string of high frequency electrical clock pulses, gating said clock pulses on by a leading edge of said first waveform of a first said normalized signal pair and off by the corresponding leading edge of said second waveform of that normalized signal pair, and then gating said clock pulses on by a leading edge of the first of a second said normalized signal pair and off by the corresponding leading edge of said second waveform of that normalized second signal pair.

15. The method of claim 11, wherein at least four of said pairs of RF signals are obtained from respective RF station pairs, and wherein a plurality of separate Δ

positions are calculated and averaged from said station pairs.

16. The method of claim 15, wherein any Δ

position calculation unreasonably departing from the average Δ

position is rejected.

17. The method of claim 11, wherein a diurnal shift correction is applied.

18. The method of claim 1, wherein said common frequency is within the range of from about 10 to about 1000 Hz.

19. The method of claim 1, wherein said common frequency is about 100 Hz.

20. The navigation method of claim 1, wherein both of said RF signals are intercepted from RF electromagnetic signals transmitted by geographically spaced RF transmitting stations.

21. The method of claim 20, wherein said RF signals are VLF signals.

22. The method of claim 20, wherein said RF signals are normalized by providing an oscillator signal for each of said RF signals which has a frequency that is a multiple of its respective RF signal frequency, said oscillator frequencies each being divisible by a respective integer to said common frequency, phase locking each of said oscillator signals to its respective said RF signal, and dividing said oscillator signals by their respective said integers to produce said normalized signals.

23. The method of claim 22, wherein said phase relationship is a phase coherent phase relationship, and synchronizing said phase coherent relationship by initiating said dividing of said oscillator signals simultaneously.

24. The method of claim 23, wherein said dividing of each of said oscillator signals is done by respective programmable dividers each having a plurality of divider sections, and wherein said phase synchronous relationship is established by initiating the division simultaneously in all of said programmable divider sections.

25. An RF navigation system which comprises means for obtaining simultaneously a pair of RF signals of different frequency originating at respective geographically spaced RF points, at least one of said signal obtaining means comprising a receiver tuned to receive a respective one of said RF signals, frequency normalizing means for normalizing said RF signals into respective normalized signals of a common frequency, measuring means connected to said normalizing means for measuring the phase relationship between said normalized signals on a first occasion and on a second and subsequent occasion;

and phase change measuring means for determining Δ

position information.

26. A system as defined in claim 25, wherein said RF signal to which said receiver is tuned is a VLF signal.

27. A system as defined in claim 25, which includes an on-board time standard that produces one of said RF signals at an on-board station.

28. A system as defined in claim 25, wherein said receiver includes a phase-locked loop for its respective said RF signal.

29. A system as defined in claim 28, wherein said frequency normalizing means associated with said receiver comprises an oscillator in said loop having a frequency that is a multiple of the respective RF signal frequency, said oscillator frequency being divisible by an integer to said common frequency, and divider means connected to said oscillator and arranged to divide the oscillator signal by said integer to produce the respective said normalized signal.

30. A system as defined in claim 29, wherein said oscillator is a temperature compensated voltage controlled crystal oscillator.

31. A system as defined in claim 29, wherein said divider means comprises a programmable divider.

32. A system as defined in claim 29, which includes second divider means in said loop which is arranged to divide said oscillator frequency down to the respective RF signal frequency to provide a digital reference signal for locking the loop.

33. A system as defined in claim 32, wherein said second divider means in said loop comprises a programmable divider.

34. A system as defined in claim 32, which includes phase detector means in said loop having an output connected to the respective oscillator to hold the oscillator frequency to its respective multiple of its respective RF signal frequency, said phase detector means having a first input connected to receive said RF signal as an analog data signal and a second input connected to said second divider means to receive said digital reference signal.

35. A system as defined in claim 34, which includes second phase detector means in said loop having an output arranged to have a "signal present" electrical signal thereon, said second phase detector means having a first input connected to receive the respective said RF signal as an analog data signal and a second input connected to said second divider means to receive said digital reference signal.

36. A system as defined in claim 25, which includes means for obtaining at least two respective pairs of said RF signals from respective RF station pairs, and wherein said phase shift measuring means is adapted to measure the phase shift for each pair of normalized signals at said second geographical location to determine at least two independent items of Δ

position information.

37. A system as defined in claim 36, wherein all of said RF signals are normalized to said common frequency.

38. A system as defined in claim 37, wherein said phase shift measuring means is arranged for successively measuring said phase shifts between the normalized signal pairs for all of said station pairs.

39. A system as defined in claim 38, wherein said normalized signal pairs each comprise first and second low frequency digital waveforms, said phase shift measuring means comprising gating circuit means, first and second input connections to said gating circuit means applying the respective said first and second waveforms to said gating circuit means, a clock connected to said gating circuit means and providing a string of high frequency electrical pulses to said gating circuit means, an output from said gating circuit means, and counting means having an input connected to said gating circuit means output for counting gated clock pulses, said gating circuit means being adapted to gate said clock pulses to said counting means by a leading edge of said first waveform and to gate off said clock pulses from said counting means by the corresponding leading edge of said second waveform, whereby the number of said clock pulses counted by said counting means will be in direct proportion to said phase difference.

40. A system as defined in claim 39, wherein said counting means comprises a BCD counter.

41. A system as defined in claim 39, which includes multiplexer means connected to the normalized signal outputs of a plurality of said receivers and connected to at least one of said gating circuit input connections, computer means connected to said multiplexer means and adapted to successively select and connect said receiver outputs to said one gating circuit input connection, and an electrical connection from said computer means to said counter for applying a take data starting pulse to said counter for starting the counter after each of said selections.

42. A system as defined in claim 41, which includes an on-board time standard that produces one of said RF signals of each of said RF signal pairs at an on-board station, whereby one of the normalized signals of each of said signal pairs is derived from said on-board time standard.

43. A system as defined in claim 42, wherein the normalized signal output of said on-board time standard is connected to the other said gating circuit input connection.

44. A system as defined in claim 43, wherein said first low frequency digital waveforms are the normalized signal outputs of said receivers, and said second low frequency digital waveforms are the normalized signal output of said on-board time standard.

45. A system as defined in claim 41, wherein said multiplexer means is connected to both of said gating circuit input connections, said computer means being adapted to successively connect start-stop pairs of said receiver outputs to said first and second gating input circuit connections.

46. A system as defined in claim 41, which includes logic circuit means in said "take data" connection responsive to the first waveform of each waveform pair to delay the counting after the respective said take data starting pulse until said leading edge of said first waveform in one of the cycles thereof is received so as to assure the counting of a valid phase difference interval between said waveforms.

47. A system as defined in claim 36, which includes computer means connected to said phase shift measuring means and programmed to calculate a Δ

position.

48. A system as defined in claim 47, which includes at least four of said receivers tuned to at least four respective said RF signals from respective RF stations, and wherein said phase shift measuring means is adapted to measure the phase shift for each pair of normalized signals at said second geographical location to determine Δ

position information from said first position for at least four station pairs, said computer means being programmed to calculate and average a plurality of separate Δ

positions.

49. The method of claim 48, wherein said computer means is programmed to reject from the average any Δ

position unreasonably departing from the average Δ

position.

50. A system as defined in claim 47, wherein said computer means is programmed to apply a diurnal shift correction to said Δ

position calculation.

51. A system as defined in claim 47, wherein said computer means is programmed to provide present position information from said Δ

position calculation.

52. A system as defined in claim 47, wherein said computer means is programmed to provide left/right steering track information from said Δ

position calculation.

53. A system as defined to claim 47, wherein said computer means is programmed to provide distance information to a target location from said Δ

position calculation.

54. A system as defined in claim 47, wherein said computer means is programmed to provide heading information from said Δ

position calculation.

55. A system as defined in claim 47, wherein said computer means is programmed to provide speed information from said Δ

position calculation.

56. A system as defined in claim 47, wherein said computer means is programmed to provide time to a target location from said Δ

position calculation.

57. A system as defined in claim 25, wherein said common frequency is within the range of from about 10 to about 1000 Hz.

58. A system as defined in claim 25, wherein said common frequency is about 100 Hz.

59. An RF navigation system as defined in claim 25, which comprises antenna means for intercepting both of said RF signals of different frequency from RF electromagnetic signals transmitted by respective geographically spaced RF transmitting stations, both of said signal obtaining means comprising receivers connected to said antenna means and tuned to the respective RF signal frequencies.

60. A system as defined in claim 59, wherein said RF signals are VLF signals.

61. A system as defined in claim 59, wherein each of said receivers includes a phase-locked loop for its respective said RF signal.

62. A system as defined in claim 61, wherein said frequency normalizing means comprises an oscillator in each of said loops having a frequency that is a multiple of the respective RF signal frequency, said oscillator frequencies each being divisible by a respective integer to said common frequency, and divider means connected to each of said oscillators and arranged to divide the respective oscillator signal by the respective said integer to produce the respective said normalized signal.

63. A system as defined in claim 62, wherein said divider means in each loop comprises a programmable divider.

64. A system as defined in claim 63, wherein each of said programmable dividers is a milti-stage divider, and said calibration means is connected to each stage of each divider so as to simultaneously initiate operation of all divider stages.

65. A system as defined in claim 59, wherein said RF signals are VLF signals, said antenna means comprising a common antenna for both of said receivers, and including broadband preamplifier means connected between said antenna and said receivers.

66. A system as defined in claim 65, wherein said preamplifier is located proximate said antenna means.

67. A system as defined in claim 65, wherein each of said receivers includes a narrowband TRF preamplifier tuned to the respective said RF signal frequency.

68. The navigation method which comprises the steps of:

receiving at least one radio signal from a distant radio station;

developing a pair of cyclically varying electrical signals having known like frequency and differing in phase by an amount corresponding to the difference in phase between two other cyclically varying electrical signals one of which is said radio signal; and

measuring the time interval between occurance of corresponding portions of the cycles of said first-mentioned pair of cyclically varying electrical signals.

69. The invention defined in claim 68 which comprises the further step of generating a train of clock pulses and in which each of said first-mentioned pair of cyclically varying electrical signals is made to have pulse wave shape;

and

in which the difference in phase between said first-mentioned pair of cyclically varying electrical signals is measured by measuring the number of clock pulses that occur in the interval between corresponding portions of said pulse wave shapes.

70. The invention defined in claim 68 in which said pair of cyclically varying electrical signals comprise radio signals from distant, and spaced radio stations.

71. A navigation system comprising in combination:

means for developing two cyclically varying electrical signals differing in frequency and at least one of which corresponds in phase and frequency to the signal arriving from a distant radio station;

frequency normalizing means for normalizing said pair of cyclically varying electrical signals into first and second normalized signals having common frequency and being coherent in time;

means responsive to like conditions in each of said first and second normalized signals for measuring the phase difference between said first and second normalized signals as a time interval.

72. The invention defined in claim 71 in which said means for measuring the phase difference between said first and second normalized signals comprises means in the form of a clock for generating a train of pulses and means for counting the number of pulses that occur between the occurance of an event in said first normalized signal and the occurance of a corresponding event in said second normalized signal.

73. The invention defined in claim 71 in which each of said pair of cyclically varying signals is derived from a distant radio station.