Satellite-based positioning system receiver for weak signal operation

• US 20020005802A1
• Filed: 05/08/2001
• Published: 01/17/2002
• Est. Priority Date: 05/08/2000
• Status: Active Grant

• Current Assignee: U-Blox AG (u-blox Holding AG)
• Original Assignee: SigNav Pty Ltd. (u-blox Holding AG)
• Inventors: Glennon, Eamonn P., Bryant, Roderick C., Dougan, Stanley L., Dempster, Andrew G.
• Granted Patent: US 6,642,884 B2
• Time in Patent Office: Days
• US Class Current: 342/357.01
• CPC Class Codes: G01S 19/24 Acquisition or tracking or demodulation of signals transmitted by the system synchronisation aspects of direct sequence spread spectrum modulation H04B1/7073
• CPC Class Codes: G01S 19/25 involving aiding data received from a cooperating element, e.g. assisted GPS
• CPC Class Codes: G01S 19/252 Employing an initial estimate of location in generating assistance data
• CPC Class Codes: G01S 19/28 Satellite selection
• CPC Class Codes: G01S 19/42 Determining position

Abstract

A method, device and system for determining a receiver location using weak signal satellite transmissions. The invention involves a sequence of exchanges between an aiding source and a receiver that serve to provide aiding information to the receiver so that the receiver'"'"'s location may be determined in the presence of weak satellite transmissions. With the aiding information, the novel receiver detects, acquires and tracks weak satellite signals and computes position solutions from calculated pseudo ranges despite the inability to extract time synchronization date from the weak satellite signals.

Claims

1. An SPS system for identifying the location of a receiver in the presence of satellite signal attenuation comprising:

a plurality of orbital satellites sending synchronized encoded signals on a carrier frequency wherein said encoded signals have repeated epochs containing synchronization data;

a receiver for detecting, acquiring, tracking a set of the encoded signals and simultaneously determining the code phases of said set with respect to said epochs; and

an aiding source to transmit an approximate location of the receiver and the position and velocity of a plurality of satellites;

wherein the receiver determines a precise location using the approximate location, the satellite positions, the code phases and carrier frequency differences.

2. The system of claim 1 wherein the precise location is determined using the offsets between the code phase of a reference signal and each of the code phases from additional signals.

3. The system of claim 1 wherein the receiver transmits a request to the aiding source and in response the aiding source transmits an approximate location for the receiver.

4. The system of claim 1 wherein the receiver detects and acquires the set of encoded signals using a search method in which the receiver first detects and acquires stronger encoded signals, if and when available, and then detects and acquires weaker encoded signals.

5. The system of claim 1 wherein the receiver conducts concurrent searches for multiple satellites signals.

6. The system of claim 1 wherein the receiver restricts its search by estimating code phase differences for additional satellites after acquiring at least one satellite.

7. The system of claim 6 wherein the receiver broadens its restricted search if any additional satellites are not detected and acquired.

8. The system of claim 1 wherein the receiver, after acquisition of a plurality of satellites, transmits a request that includes the identities of a plurality of satellites and in response, the aiding source transmits each satellite'"'"'s location and velocity.

9. The system of claim 1 wherein the aiding source determines and transmits the position and velocity of a plurality of satellites in response to a request from the receiver.

10. The system of claim 9 wherein the aiding source determines location and velocity of the plurality of satellites using ephemerides data received from the satellites, the time of receipt of the receiver'"'"'s request and the transmission latency of the request.

11. The system of claim 1 wherein the receiver transmits to the aiding source receiver information selected from a group of receiver information including:

location;

speed;

heading;

height;

solution mode;

satellite identities used in a recent location solution; and

code phase differences.

12. The system of claim 1 further comprising a call taker for using or processing receiver location information.

13. The system of claim 12 wherein the aiding source further transmits the approximate location of the receiver to the call taker.

14. The system of claim 12 wherein the aiding source further transmits the precise location of the receiver to the call taker.

15. The system of claim 1 wherein the aiding source determines a precise location for the receiver based upon information that was transmitted from the receiver.

16. The system of claim 1 wherein the receiver performs a smoothing operation on the code phases of the attenuated signals.

17. A method for determining a precise location of a receiver in the presence of attenuated satellite signals carrying synchronization data comprising:

receiving and storing almanac data for regional satellites by a receiver;

sending an approximate location of the receiver from an aiding source in response to a request from the receiver;

detecting, acquiring and tracking a set of signals from a plurality of satellites;

determining simultaneously the code phases from the set of signals and then determining an offset for each code phase with respect to a reference code phase;

sending a request including the identities of a set of satellites from the receiver to the aiding source and returning the positions and velocities of each requested satellite;

determining a position solution using the approximate location, almanac data, code phase differences and satellite positions; and

18. The method of claim 17 further comprising sending receiver data to the aiding source.

19. The method of claim 18 wherein said receiver data is data from a group of data including:

receiver location;

receiver speed;

receiver heading;

receiver height;

receiver solution mode;

satellite identities used by the receiver in determination of a location solution; and

receiver determined code phase differences.

20. The method of claim 17 further comprising sending receiver data from the aiding source to a call taker.

21. The method of claim 20 wherein said receiver data is data from a group of data including:

receiver location;

receiver speed;

receiver heading;

receiver height;

receiver solution mode;

satellite identities used by receiver in determination of a location solution; and

receiver determined code phase differences.

22. The method of claim 17 wherein said detecting, acquiring and tracking step includes a frequency and amplitude signal search in which a search amplitude threshold value starts at a some level and is successively reduced to allow stronger signals to be acquired before more attenuated signals.

23. The method of claim 22 wherein a frequency range sweep is performed prior to each successive reduction of the search amplitude threshold value.

24. The method of claim 17 wherein the step of determining a position solution includes a determination of pseudoranges to each satellite using said code phase differences.

25. The method of claim 24 wherein said determination of pseudoranges is made using calculated approximate ranges to each tracked satellite.

26. The method of claim 25 wherein said approximate ranges are calculated using the approximate location and the satellite positions.

27. A device for use in an SPS system for determining a precise location in the presence of attenuated satellite signals comprising:

a transmission means for sending requests for its approximate location and satellite identities to an external aiding source;

a reception means for receiving the approximate location and satellite positions and velocities from the external aiding source;

an antenna to receive transmitted satellite signals;

a circuit to filter and sample the signals received at the antenna;

a plurality of signal correllators each for detecting, acquiring and tracking a signal; and

a control circuit including a processor and memory;

wherein said control circuit determines the code phase offsets for a plurality of detected, acquired and tracked satellite signals in the plurality of signal correllators and using said code phase offsets in a determination of a location solution for the device.

28. The device of claim 27 wherein the control circuit adjusts a signal correllator based upon the received approximate location and stored almanac data.

29. The device of claim 27 wherein control circuit effects signal detection in which stronger encoded signals are acquired before more attenuated signals by successively increasing a correlator output signal integration period and successively reducing an amplitude threshold value.

30. The device of claim 29 wherein the control circuit effects a frequency range sweep prior to each successive reduction of the search amplitude threshold value.

31. The device of claim 30 in which the control circuit computes estimated Doppler offsets of the carrier and code frequencies of the satellite signals using stored almanac data, the approximate time and the approximate location to restrict the frequency range.

32. The device of claim 27 wherein the control circuit computes pseudoranges to each acquired satellite using said code phase offsets and calculated approximate ranges derived from the received approximate location and satellite positions.

33. In a receiver that receives signals from satellites in order for its own location to be computed based on its distance from the known or computed locations of those satellites and which incorporates a hardware correlator, a method for processing the signals at the correlator output in order to estimate the amplitude and frequency of the correlated signal and to detect the presence of a signal, comprising:

processing time domain signal samples over a fixed integration period;

transforming all or some of the samples within an integration period to the frequency domain in frequency bins;

identifying a peak frequency bin containing the largest complex magnitude;

applying a filter process involving the application of a window to the frequency bins with the window centered on the peak frequency bin;

estimating the amplitude and frequency of the signal from the windowed bins;

detecting the presence of a signal with the estimated frequency and amplitude;

34. The method of claim 33 wherein said transforming step involves the use of a transform method from a group of transform methods including a Discrete Fourier Transform, Fast Fourier Transform and Fourier Transform.

35. The method of claim 33 wherein the step of applying a filter process includes the autoconvolution of the vector of windowed bins to eliminate the effects of data transitions.

36. The method of claim 33 wherein said step of detecting the presence of a signal includes a comparison of a number of frequency estimates to see if they are consistent.

37. The method of claim 33 wherein said step of detecting the presence of a signal includes a comparison of the estimated amplitude with a threshold value.

38. The method of claim 37 wherein said step of detecting the presence of a signal further includes a comparison of a number of frequency estimates to see if they are consistent.

39. The method as in claim 38 wherein the steps are repeated with the fixed integration period set to a short time frame and the threshold value set to a high value to detect stronger signals and progressively using a longer fixed integration period and a lower threshold value to detect weaker signals.

40. The method of claim 39 wherein the threshold value is set using a function of the measured carrier amplitude for a previously detected signal.

41. The method of claim 40 wherein the function sets the threshold value sufficiently high to reduce the affect of cross correlation with the previously detected signal.

42. The method of claim 39 wherein the fixed integration period is set using a function of a fixed integration period used in the detection of a previous signal.