Jamming detection and blanking for GPS receivers

• US 6,448,925 B1
• Filed: 02/04/1999
• Issued: 09/10/2002
• Est. Priority Date: 02/04/1999
• Status: Expired due to Term

• Current Assignee: CSR Technology Incorporated (Qualcomm, Inc.)
• Original Assignee: Conexant Systems Incorporated (Synaptics Incorporated)
• Inventors: Shridhara, Kudhrethaya A.
• Primary Examiner(s): Pihulic, Daniel T.
• Application Number:
  US09/244,354
• Time in Patent Office: 1,314 Days
• Field of Search: 342/357.14, 342/357.12, 342/357.06, 342/357.03, 342/357.02, 342/16, 342/14, 342/362, 701/214
• US Class Current: 342/357.59
• CPC Class Codes: G01S 19/21 Interference related issues ; Issues related to cross-correlation, spoofing or other methods of denial of service
• CPC Class Codes: G01S 19/49 whereby the further system is an inertial position system, e.g. loosely-coupled
• CPC Class Codes: G01S 5/011 Identifying the radio environment
• CPC Class Codes: H04K 3/224 with countermeasures at transmission and/or reception of the jammed signal, e.g. stopping operation of transmitter or receiver, nulling or enhancing transmitted power in direction of or at frequency of jammer
• CPC Class Codes: H04K 3/228 Elimination in the received signal of jamming or of data corrupted by jamming interference suppression in receivers H04B1/10
• CPC Class Codes: H04K 3/90 related to allowing or preventing navigation or positioning, e.g. GPS

Abstract

GPS satellites broadcast GPS signals with low power transmitters less than 50 watts. Although the GPS broadcast signals are encoded using spread spectrum technology which decreases the effect noise has on the reception of the signal, jamming signals can adversely affect GPS receiver performance. Jamming signals can cause position and velocity indication within the system to be erroneous. These errors can build up within the system and the errors may take a significant time to dissipate even after the jamming signal has gone. These errors can manifest themselves as errors in velocity and errors in position on the order of tens of miles. Methods of detection of the presence of the jamming signal including observing the output of a correlator chain in which the PN code being correlated is delayed in each link in the chain are presented in this disclosure. The present disclosure also proposes countermeasures for dealing with the jamming signal. These methods include shutting off the receiver, and computing the position by alternate methods, such as dead reckoning, during periods in which the jamming signal persists.

Claims

1. A method of minimizing the error within a GPS receiver in the presence of a jamming signal the method comprising:

receiving a GPS signal in conjunction with a jamming signal;

detecting the jamming signal using at least one detection scheme selected from the group of detection schemes consisting of;

(a) detecting a temperature change within the GPS receiver, determining an expected clock drift from the temperature change detected within the GPS receiver, determining an actual clock drift of the GPS receiver within the GPS receiver, and observing a difference between the expected clock drift and the actual clock drift of the GPS receiver whereby the presence of the jamming signal is detected;

(b) monitoring a signal to noise ratio for the received GPS signal, and detecting a sudden drop in the signal to noise ratio whereby the presence of the jamming signal is detected;

(c) monitoring a signal strength value of the received GPS signal at the GPS frequency, and detecting a sudden increase in the signal strength value of the received GPS signal at the GPS frequency whereby the presence of the jamming signal is detected;

(d) correlating the received GPS signal with a first PN code in a first correlator, wherein the first correlator output indicates that the first PN code is present in the received GPS signal, correlating the received GPS signal with a delayed version of the first PN code in a second correlator wherein the delay is sufficient so that the second correlator output indicates that the delayed PN code is not present in the received GPS signal, and monitoring the output of the first correlator for a drop in output value which coincides with an increase in the output of the second correlator whereby the presence of the jamming signal is detected;

(e) selecting a PN code not contained within the received GPS signal, correlating the received GPS signal with the selected PN code, and detecting a rise in the correlated output whereby the presence of the jamming signal is detected; and

(f) selecting a plurality of PN codes not contained within the received GPS signal, correlating the received GPS signal in a plurality of correlators with the plurality of selected PN codes, and detecting a rise in the correlated output of the plurality of correlators, whereby the presence of the jamming signal is detected; and

applying a countermeasure whereby the error induced into the GPS receiver by the jamming signal is reduced.

2. The method as in claim 1 wherein detecting the jamming signal comprises:

correlating the received GPS signal in a plurality of correlators with successively delayed versions of the selected PN code; and

detecting a rise in the correlated output of the plurality of correlators, whereby the presence of the jamming signal is detected.

3. The method of claim 1 wherein the selected PN code is a GPS code from a GPS satellite not being received by the GPS receiver.

4. The method of claim 3 wherein the selected GPS code is the 25th GPS code, guaranteed by the GPS authority not to be present in any GPS satellite.

5. The method as in claim 1 wherein applying countermeasures comprises:

disabling the output of the GPS receiver.

6. The method as in claim 5 wherein applying countermeasures comprises:

applying a dead reckoning method of position determination while the jamming signal is present.

7. The method as in claim 5 wherein applying countermeasures comprises:

providing an indication of the presence of the jamming signal.

8. The method as in claim 5 wherein applying countermeasures comprises:

defaulting to a backup method of position determination.

9. The method as in claim 5 wherein the GPS receiver is disabled only while the jamming signal is detected.

10. The method as in claim 6 wherein the application of the dead reckoning method includes:

storing position data determined from a plurality of received GPS signals in the GPS receiver while the jamming signal is not detected;

calculating directional data from changes in the position data while the jamming signal is not detected,

storing the directional data in the GPS receiver; and

determining the velocity at which the GPS receiver is travelling.

11. The method as in claim 10 wherein the velocity is determined by:

correlating the received GPS signal with a first PN code in a first correlator, wherein the first correlator output indicates that the first PN code is present in the received GPS signal;

calculating an expected delay amount

delaying the first PN code upon detecting a drop in the first correlator output by an actual delay amount which changes until the first correlator output reaches a desired value; and

determining the difference between the expected and actual delay amount.

12. The method as in claim 10 wherein the velocity is determined by:

correlating the received GPS signal with a first PN code in a first correlator, wherein the first correlator output indicates that the first PN code is present in the received GPS signal;

calculating an expected delay amount;

delaying the received GPS signal upon detecting a drop in the first correlator output by an actual delay amount which changes until the first correlator output reaches a desired valued; and

determining the difference between the expected and actual delay amount.

13. The method as in claim 1 wherein detecting the jamming signal further comprises:

detecting a sudden drop in a signal to noise ratio for the received GPS signal; and

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of the sudden drop in the signal to noise ratio and the observation of a difference between the expected and actual clock drift.

14. The method as in claim 13 wherein detecting the jamming signal further comprises:

detecting a sudden increase in a signal strength value of the received GPS signal at the GPS frequency; and

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of at least two conditions selected from the group of the sudden drop in the signal to noise ratio, the observation of a difference between the expected and actual clock drift and the sudden increase in the signal strength value of the received GPS signal.

15. The method as in claim 14 wherein detecting the jamming signal further comprises:

correlating the received GPS signal with a first PN code in a first correlator, wherein the first correlator output indicates that the first PN code is present in the received GPS signal;

correlating the received GPS signal with a delayed version of the first PN code in a second correlator wherein the delay is sufficient so that the second correlator output indicates that the delayed PN code is not present in the received GPS signal; and

monitoring the output of the first correlator for a drop in output value which coincides with an increase in the output of the second correlator; and

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of at least two conditions selected from the group of the sudden drop in the signal to noise ratio, the observation of a difference between the expected and actual clock drift, the sudden increase in the signal strength value of the received GPS signal and a coincident reversal in the first and second correlator outputs.

16. The method as in claim 15 wherein detecting the jamming signal further comprises:

selecting a PN code not contained within the received GPS signal;

correlating the received GPS signal with the selected PN code in a third correlator; and

detecting a rise in the correlated output of the third correlator;

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of at least two conditions selected from the group of the sudden drop in the signal to noise ratio, the observation of a difference between the expected and actual clock drift, the sudden increase in the signal strength value of the received GPS signal, a coincident reversal in the first and second correlator outputs and the rise in the third correlator output.

17. The method as in claim 15 wherein detecting the jamming signal further comprises:

selecting a PN code not contained within the received GPS signal;

correlating the received GPS signal in a plurality of third correlators with successively delayed versions of the selected PN code; and

detecting a rise in the correlated output of the plurality of third correlators;

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of at least two conditions selected from the group of the sudden drop in the signal to noise ratio, the observation of a difference between the expected and actual clock drift, the sudden increase in the signal strength value of the received GPS signal, a coincident reversal in the first and second correlator outputs and the rise in the plurality of third correlator outputs.

18. The method as in claim 15 wherein detecting the jamming signal further comprises:

selecting a plurality of PN codes not contained within the received GPS signal;

correlating the received GPS signal in a plurality of third correlators with the plurality of selected PN codes; and

detecting a rise in the correlated output of the plurality of third correlators;

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of at least two conditions selected from The group of the sudden drop in the signal to noise ratio, the observation of a difference between the expected and actual clock drift, the sudden increase in the signal strength value of the received GPS signal, a coincident reversal in the first and second correlator outputs and the rise in the plurality of third correlator outputs.

19. The method as in claim 1 wherein detecting the jamming signal further comprises:

detecting a sudden increase in a signal strength value of the received GPS signal at the GPS frequency; and

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of the observation of a difference between the expected and actual clock drift and the sudden increase in the signal strength value of the received GPS signal.

20. The method as in claim 19 wherein detecting the jamming signal further comprises:

correlating the received GPS signal with a first PN code in a first correlator, wherein the first correlator output indicates that the first PN code is present in the received GPS signal;

correlating the received GPS signal with a delayed version of the first PN code in a second correlator wherein the delay is sufficient so that the second correlator output indicates that the delayed PN code is not present in the received GPS signal; and

monitoring the output of the first correlator for a drop in output value which coincides with an increase in the output of the second correlator; and

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of at least two conditions selected from the group of the observation of a difference between the expected and actual clock drift, the sudden increase in the signal strength value of the received GPS signal and a coincident reversal in the first and second correlator outputs.

21. The method as in claim 20 wherein detecting the jamming signal further comprises:

selecting a PN code not contained within the received GPS signal;

correlating the received GPS signal with the selected PN code in a third correlator; and

detecting a rise in the correlated output of the third correlator;

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of at least two conditions selected from the group of the observation of a difference between the expected and actual clock drift, the sudden increase in the signal strength value of the received GPS signal, a coincident reversal in the first and second correlator outputs and the rise in the third correlator output.

22. The method as in claim 21 wherein detecting the jamming signal further comprises:

selecting a PN code not contained within the received GPS signal;

correlating the received GPS signal in a plurality of third correlators with successively delayed versions of the selected PN code; and

detecting a rise in the correlated output of the plurality of third correlators;

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of at least two conditions selected from the group of the observation of a difference between the expected and actual clock drift, the sudden increase in the signal strength value of the received GPS signal, a coincident reversal in the first and second correlator outputs and the rise in the plurality of third correlator outputs.

23. The method as in claim 21 wherein detecting the jamming signal further comprises:

selecting a plurality of PN codes not contained within the received GPS signal;

correlating the received GPS signal in a plurality of third correlators with the plurality of selected PN codes; and

detecting a rise in the correlated output of the plurality of third correlators;

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of at least two conditions selected from the group of the observation of a difference between the expected and actual clock drift, the sudden increase in the signal strength value of the received GPS signal, a coincident reversal in the first and second correlator outputs and the rise in the plurality of third correlator outputs.

24. The method as in claim 1 wherein detecting the jamming signal further comprises:

correlating the received GPS signal with a first PN code in a first correlator, wherein the first correlator output indicates that the first PN code is present in the received GPS signal;

correlating the received GPS signal with a delayed version of the first PN code in a second correlator wherein the delay is sufficient so that the second correlator output indicates that the delayed PN code is not present in the received GPS signal; and

monitoring the output of the first correlator for a drop in output value which coincides with an increase in the output of the second correlator; and

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of the observation of a difference between the expected and actual clock drift and a coincident reversal in the first and second correlator outputs.

25. The method as in claim 24 wherein detecting the jamming signal further comprises:

selecting a PN code not contained within the received GPS signal;

correlating the received GPS signal with the selected PN code in a third correlator; and

detecting a rise in the correlated output of the third correlator;

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of at least two conditions selected from the group of the observation of a difference between the expected and actual clock drift, a coincident reversal in the first and second correlator outputs and the rise in the third correlator output.

26. The method as in claim 25 wherein detecting the jamming signal further comprises:

selecting a PN code not contained within the received GPS signal;

correlating the received GPS signal in a plurality of third correlators with successively delayed versions of the selected PN code; and

detecting a rise in the correlated output of the plurality of third correlators;

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of at least two conditions selected from the group of the observation of a difference between the expected and actual clock drift, a coincident reversal in the first and second correlator outputs and the rise in the plurality of third correlator outputs.

27. The method as in claim 25 wherein detecting the jamming signal further comprises:

selecting a plurality of PN codes not contained within the received GPS signal;

correlating the received GPS signal in a plurality of third correlators with the plurality of selected PN codes; and

detecting a rise in the correlated output of the plurality of third correlators;

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of at least two conditions selected from the group of the observation of a difference between the expected and actual clock drift, a coincident reversal in the first and second correlator outputs and the rise in the plurality of third correlator outputs.

28. The method as in claim 1 wherein detecting the jamming signal further comprises:

selecting a PN code not contained within the received GPS signal;

correlating the received GPS signal with the selected PN code in a third correlator; and

detecting a rise in the correlated output of the third correlator;

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of the observation of a difference between the expected and actual clock drift and the rise in the third correlator output.

29. The method as in claim 28 wherein detecting the jamming signal further comprises:

selecting a PN code not contained within the received GPS signal;

correlating the received GPS signal in a plurality of third correlators with successively delayed versions of the selected PN code; and

detecting a rise in the correlated output of the plurality of third correlators;

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of the observation of a difference between the expected and actual clock drift and the rise in the plurality of third correlator outputs.

30. The method as in claim 28 wherein detecting the jamming signal further comprises:

selecting a plurality of PN codes not contained within the received GPS signal;

correlating the received GPS signal in a plurality of third correlators with the plurality of selected PN codes; and

detecting a rise in the correlated output of the plurality of third correlators;

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of the observation of a difference between the expected and actual clock drift and the rise in the plurality of third correlator outputs.

31. The method as in claim 1 wherein detecting the jamming signal further comprises:

detecting a sudden increase in a signal strength value of the received GPS signal at the GPS frequency; and

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of the sudden drop in the signal to noise ratio and the sudden increase in the signal strength value of the received GPS signal.

32. The method as in claim 31 wherein detecting the jamming signal further comprises:

correlating the received GPS signal with a first PN code in a first correlator, wherein the first correlator output indicates that the first PN code is present in the received GPS signal;

correlating the received GPS signal with a delayed version of the first PN code in a second correlator wherein the delay is sufficient so that the second correlator output indicates that the delayed PN code is not present in the received GPS signal; and

monitoring the output of the first correlator for a drop in output value which coincides with an increase in the output of the second correlator; and

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of at least two conditions selected from the group of the sudden drop in the signal to noise ratio, the sudden increase in the signal strength value of the received GPS signal and a coincident reversal in the first and second correlator outputs.

33. The method as in claim 32 wherein detecting the jamming signal further comprises:

selecting a PN code not contained within the received GPS signal;

correlating the received GPS signal with the selected PN code in a third correlator; and

detecting a rise in the correlated output of the third correlator;

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of at least two conditions selected from the group of the sudden drop in the signal to noise ratio, the sudden increase in the signal strength value of the received GPS signal, a coincident reversal in the first and second correlator outputs and The rise in the third correlator output.

34. The method as in claim 32 wherein detecting the jamming signal further comprises:

selecting a PN code not contained within the received GPS signal;

correlating the received GPS signal in a plurality of third correlators with successively delayed versions of the selected PN code; and

detecting a rise in the correlated output of the plurality of third correlators;

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of at least two conditions selected from the group of the sudden drop in the signal to noise ratio, the sudden increase in the signal strength value of the received GPS signal, a coincident reversal in the first and second correlator outputs and the rise in the plurality of third correlator outputs.

35. The method as in claim 32 wherein detecting the jamming signal further comprises:

selecting a plurality of PN codes not contained within the received GPS signal;

correlating the received GPS signal in a plurality of third correlators with the plurality of selected PN codes; and

detecting a rise in the correlated output of the plurality of third correlators;

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of at least two conditions selected from the group of the sudden drop in the signal to noise ratio, the sudden increase in the signal strength value of the received GPS signal, a coincident reversal in the first and second correlator outputs and the rise in the plurality of third correlator outputs.

36. The method as in claim 1 wherein detecting the jamming signal further comprises:

correlating the received GPS signal with a first PN code in a first correlator, wherein the first correlator output indicates that the first PN code is present in the received GPS signal;

correlating the received GPS signal with a delayed version of the first PN code in a second correlator wherein the delay is sufficient so that the second correlator output indicates that the delayed PN code is not present in the received GPS signal; and

monitoring the output of the first correlator for a drop in output value which coincides with an increase in the output of the second correlator; and

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of the sudden drop in the signal to noise ratio and a coincident reversal in the first and second correlator outputs.

37. The method as in claim 36 wherein detecting the jamming signal further comprises:

selecting a PN code not contained within the received GPS signal;

correlating the received GPS signal with the selected PN code in a third correlator; and

detecting a rise in the correlated output of the third correlator;

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of at least two conditions selected from the group of the sudden drop in the signal to noise ratio, a coincident reversal in the first and second correlator outputs and the rise in the third correlator output.

38. The method as in claim 36 wherein detecting the jamming signal further comprises:

selecting a PN code not contained within the received GPS signal;

correlating the received GPS signal in a plurality of third correlators with successively delayed versions of the selected PN code; and

detecting a rise in the correlated output of the plurality of third correlators;

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of at least two conditions selected from the group of the sudden drop in the signal to noise ratio, a coincident reversal in the first and second correlator outputs and the rise in the plurality of third correlator outputs.

39. The method as in claim 36 wherein detecting the jamming signal further comprises:

selecting a plurality of PN codes not contained within the received GPS signal;

correlating the received GPS signal in a plurality of third correlators with the plurality of selected PN codes; and

detecting a rise in the correlated output of the plurality of third correlators;

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of at least two conditions selected from the group of the sudden drop in the signal to noise ratio, a coincident reversal in the first and second correlator outputs and the rise in the plurality of third correlator outputs.

40. The method as in claim 1 wherein detecting the jamming signal further comprises:

correlating the received GPS signal with a first PN code in a first correlator, wherein the first correlator output indicates that the first PN code is present in the received GPS signal;

correlating the received GPS signal with a delayed version of the first PN code in a second correlator wherein the delay is sufficient so that the second correlator output indicates that the delayed PN code is not present in the received GPS signal; and

monitoring the output of the first correlator for a drop in output value which coincides with an increase in the output of the second correlator; and

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of the sudden drop in the signal to noise ratio and a coincident reversal in the first and second correlator outputs.

41. The method as in claim 40 wherein detecting the jamming signal further comprises:

selecting a PN code not contained within the received GPS signal;

correlating the received GPS signal with the selected PN code in a third correlator; and

detecting a rise in the correlated output of the third correlator;

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of at least two conditions selected from the group of the sudden drop in the signal to noise ratio, a coincident reversal in the first and second correlator outputs and the rise in the third correlator output.

42. The method as in claim 41 wherein detecting the jamming signal further comprises:

selecting a PN code not contained within the received GPS signal;

correlating the received GPS signal in a plurality of third correlators with successively delayed versions of the selected PN code; and

detecting a rise in the correlated output of the plurality of third correlators;

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of at least two conditions selected from the group of the sudden drop in the signal to noise ratio, a coincident reversal in the first and second correlator outputs and the rise in the plurality of third correlator outputs.

43. The method as in claim 41 wherein detecting the jamming signal further comprises:

selecting a plurality of PN codes not contained within the received GPS signal;

correlating the received GPS signal in a plurality of third correlators with the plurality of selected PN codes; and

detecting a rise in the correlated output of the plurality of third correlators;

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of at least two conditions selected from the group of the sudden drop in the signal to noise ratio, a coincident reversal in the first and second correlator outputs and the rise in the plurality of third correlator outputs.

44. The method as in 1 wherein detecting the jamming signal further comprises:

selecting a PN code not contained within the received GPS signal;

correlating the received GPS signal with the selected PN code in a third correlator; and

detecting a rise in the correlated output of the third correlator;

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of the observation of the sudden drop in the signal to noise ratio and the rise in the third correlator output.

45. The method as in claim 44 wherein detecting the jamming signal further comprises:

selecting a PN code not contained within the received GPS signal;

correlating the received GPS signal in a plurality of third correlators with successively delayed versions of the selected PN code; and

detecting a rise in the correlated output of the plurality of third correlators;

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of the observation of the sudden drop in the signal to noise ratio and the rise in the plurality of third correlator outputs.

46. The method as in claim 44 wherein detecting the jamming signal further comprises:

selecting a plurality of PN codes not contained within the received GPS signal;

correlating the received GPS signal in a plurality of third correlators with the plurality of selected PN codes; and

detecting a rise in the correlated output of the plurality of third correlators;

deciding that the presence of the jamming signal is indicated based only on substantial coincidence of the observation of the sudden drop in the signal to noise ratio and the rise in the plurality of third correlator outputs.

47. A computer data signal embodied in a carrier wave comprising a method of minimizing the error within a GPS receiver in the presence of a jamming signal the method comprising:

receiving a GPS signal at a GPS frequency, the GPS signal having a jamming signal potentially associated therewith;

detecting the jamming signal using at least one detection scheme selected from the group of detection schemes consisting of;

(a) detecting a temperature change within the GPS receiver, determining an expected clock drift from the temperature change detected within the GPS receiver, determining an actual clock drift of the GPS receiver within the GPS receiver, and observing a difference between the expected clock drift and the actual clock drift of the GPS receiver whereby the presence of the jamming signal is detected;

(b) monitoring a signal to noise ratio for the received GPS signal, and detecting a sudden drop in the signal to noise ratio whereby the presence of the jamming signal is detected;

(c) monitoring a signal strength value of the received GPS signal at the GPS frequency, and detecting a sudden increase in the signal strength value of the received GPS signal at the GPS frequency whereby the presence of the jamming signal is detected;

(d) correlating the received GPS signal with a first PN code in a first correlator, wherein the first correlator output indicates that the first PN code is present in the received GPS signal, correlating the received GPS signal with a delayed version of the first PN code in a second correlator wherein the delay is sufficient so that the second correlator output indicates that the delayed PN code is not present in the received GPS signal, and monitoring the output of the first correlator for a drop in output value which coincides with an increase in the output of the second correlator whereby the presence of the jamming signal is detected;

(e) selecting a PN code not contained within the received GPS signal, correlating the received GPS signal with the selected PN code, and detecting a rise in the correlated output whereby the presence of the jamming signal is detected; and

(f) selecting a plurality of PN codes not contained within the received GPS signal, correlating the received GPS signal in a plurality of correlators with the plurality of selected PN codes, and detecting a rise in the correlated output of the plurality of correlators, whereby the presence of the jamming signal is detected; and

applying a countermeasure whereby the error induced into the GPS receiver by the jamming signal is reduced.

48. An apparatus for minimizing the error within a GPS receiver in the presence of a jamming signal wherein the apparatus comprises:

means for receiving a GPS signal at a GPS frequency, the GPS signal having a jamming signal potentially associated therewith;

means for detecting the jamming signal, wherein the jamming signal detecting means is selected from the group consisting of;

(a) a temperature sensor operably associated with the GPS receiver, a computational circuit operably connected to the temperature sensor for determining an expected clock drift due to the change in the temperature of the GPS receiver, a circuit for determining actual clock drift, and a comparator for comparing the actual clock drift to the expected clock drift to determine a difference, whereby the presence of the jamming signal can be detected based on the difference;

(b) a circuit for determining the amount of signal present, a circuit for determining the amount of noise within the signal, a computational circuit for receiving the amount of signal present and for receiving the amount f noise present within the signal and determining signal to noise ratio, and a monitoring circuit coupled to the computational circuit for the purpose of receiving the signal to noise ratio and for detecting sudden changes in the signal to noise ratio for the purpose of detecting the presence of the jamming signal;

(c) a signal strength detection circuit for monitoring the strength of the signal in the GPS transmission band and a comparator circuit coupled to the signal strength detection circuit to receive the strength of the signal in the GPS transmission band and to compare the strength of the signal in the GPS transmission band to an expected maximum value, for the purpose of detecting the presence of the jamming signal;

(d) a first correlator, a PN code coupled to the first correlator whereby the presence of the PN code within the GPS signal is detected by a high output from the correlator, a second correlator, a delay circuit disposed between the PN code and the second correlator, for receiving the PN code and delaying the PN code prior to coupling it into the second correlator, whereby the delay is sufficient so that the delayed PN code is indicated, by a low output from the second correlator, as not being present, and a circuit for receiving the output of the first correlator and the second correlator for determining a drop in the value of the first correlator coincident with an increase in the output of the second correlator, whereby the presence of the presence of the jamming signal is detected;

(e) a chain of one or more correlators coupled to receive the GPS signal and a respective one of successively delayed PN code, and whereby the output of the correlators of the chain exhibit a low value and a monitoring circuit coupled to the outputs of the correlators for the purpose of detecting a rise in the output values of the correlators, whereby the presence of the jamming signal is detected; and

(f) a plurality of correlators coupled to receive the GPS signal and different PN codes, whereby the output of the correlators exhibit a low value and a monitoring circuit coupled to the outputs of the correlators for the purpose of detecting a rise in the output values of the correlators, whereby the presence of the jamming signal is detected; and

means for applying jamming signal countermeasures toward reducing the error induced in the GPS receiver upon detecting the jamming signal.

49. The apparatus as in claim 48 wherein the countermeasure means comprises:

a circuit for receiving in indication that the jamming signal is present whereby the circuit disables the output of GPS receiver.

50. The apparatus as in claim 48 wherein the countermeasure means comprises:

a circuit for receiving an indication that the jamming signal is present whereby the circuit disables the output of GPS receiver upon receiving an indication that a jamming signal is present; and

a circuit for applying dead reckoning as a means of updating the position of the GPS receiver upon receiving an indication that the jamming signal is present.

51. The apparatus as in claim 48 wherein the countermeasure means comprises.

a circuit for receiving an indication that the jamming signal is present whereby the circuit disables the GPS receiver upon receiving an indication that the jamming signal is present;

and

a circuit for defaulting to a backup position determination system for the purpose of updating the position of the GPS receiver upon receiving an indication that the jamming signal is present.