Navigation data prediction for GPS and GLONASS weak signal tracking

• US 6,731,701 B2
• Filed: 08/27/2001
• Issued: 05/04/2004
• Est. Priority Date: 08/27/2001
• Status: Active Grant

• Current Assignee: Topcon GPS, LLC
• Original Assignee: Topcon GPS, LLC
• Inventors: Vorobiev, Michael V., Ashjaee, Javad
• Primary Examiner(s): Fan, Chieh M.
• Application Number:
  US09/939,976
• Publication Number:
  US 20030152134A1
• Time in Patent Office: 981 Days
• Field of Search: 375/130, 375/140, 375/147, 375/340, 375/342, 375/343, 375/354, 375/373, 375/376, 375/365, 375/366, 375/367, 375/368, 342/357-1-3, 701/200, 701/207, 701/213, 701/214, 701/215, 701/216, 455/130
• US Class Current: 375/342
• 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 19/27 creating, predicting or correcting ephemeris or almanac data within the receiver
• CPC Class Codes: H04B 2201/70715 with application-specific features

Abstract

Prediction methods for navigation message symbols of satellite systems, such as GPS (Global Positioning System) and GLONASS (Global Orbiting Navigation System), that use information repetition in navigation data messages are disclosed. The methods enable one to eliminate the impact of symbol modulation for a received signal on the threshold performance of any navigation receiver, thereby permitting its operation in severe environments when the carrier-to-noise ratio C/N₀ of “weak” satellite signals falls to levels below C/N₀=20 dB*Hz.

Claims

1. A method of predicting navigation symbols for use in a global positioning tracking process, the navigation symbols being transmitted from a global positioning satellite in the form of a carrier signal modulated by a data-stream signal comprising the symbols, the navigation data symbols being conveyed in a sequence of one or more repeating frames, said data-stream signal having a first group of symbols which is repeated within the stream once every M number of frames at predetermined locations in the frames, M being equal to or greater than 1, said method comprising:

(a) detecting the symbols of the first group within the frames of the data-stream signal;

(b) storing in a memory a sequence of the detected symbol values of the first group over at least M frames;

(c) providing the sequence stored in the memory to the global positioning tracking process;

(d) subsequently monitoring the data-stream signal to detect a change in the symbol values of the first group, said subsequently monitoring comprising the steps of detecting the symbols of the first group within subsequently transmitted frames, comparing their values with the sequence of previously detected symbol values stored in memory to find differences, and counting the number of differences encountered since the start of a group of frames; and

(e) repeating steps (a) and (b) when the number of differences exceeds a predetermined number.

2. The method of claim 1 wherein said predetermined number is in a range between 10 and 30.

3. A method according to claim 1 wherein each frame comprises a plurality of sub-frames,

wherein each sub-frame comprises a telemetry word, a hand-over word, and a plurality of data words, the telemetry word comprising a plurality of symbols, the hand-over word comprising a plurality of symbols, and each data word comprising a plurality of symbols, and

wherein the first group of symbols comprises the symbols of the data words of the sub-frames of all of the frames.

4. A method according to claim 3 wherein the data words of the sub-frames comprise ephemeris and almanac data.

5. A method according to claim 3 wherein the first group of symbols further comprises the symbols of the telemetry words of the sub-frames of all of the frames.

6. A method according to claim 3 wherein each sub-frame is assigned a predetermined numerical identifier depending upon its position within its frame, and

wherein the hand-over word of each sub-frame comprises a first set of symbols representing the current time of the week as of the transmission time of the hand-over word, a second set of symbols representing data-flags which have constant values over the time duration of several frames, a third set of symbols representing the sub-frame'"'"'s numerical identifier, a fourth set of symbols holding a set of parity bits which are computed on the basis of the values of the symbols in the hand-over word, and a fifth set of adjustment symbols having values which cause the last two parity bits to have zero values.

7. A method according to claim 6 wherein said method further comprises the steps of:

obtaining the values of the data-flags of each hand-over word from previously known information;

generating a time signal which is at least approximately representative of the current time of the week;

predicting the numerical identity of an upcoming sub-frame that will be received from the satellite,

predicting the value of the time of the week which said upcoming sub-frame will contain;

generating a replica of the hand-over word for the upcoming sub-frame comprising the steps of generating a first set of symbols representing the predicted value of the time of the week, generating a second set of symbols representative of the data-flags, generating a third set of symbols representative of the predicted numerical identity of the upcoming sub-frame, generating a fourth set of parity-bit symbols and related fifth set of adjustment symbols on the basis of the values of the three previously generated sets of symbols, and

providing the replica of the hand-over word to the global positioning tracking process.

8. A method according to claim 1 wherein the carrier signal, as modulated by the data-stream signal, is transmitted by one of the GPS navigation system satellites.

9. A method according to claim 1 wherein the carrier signal, as modulated by the data-stream signal, is transmitted by one of the GLONASS navigation system satellites.

10. A method according to claim 1 wherein the global positioning tracking process is performed by a navigation station, and wherein steps (a)-(e) are performed by the same navigation station.

11. A method according to claim 1 wherein the global positioning tracking process is performed by a first navigation station, wherein steps (a)-(e) are performed by a second navigation station, and wherein step (c) comprises the step of transmitting the stored sequence from the second station to the first station.

12. A method according to claim 1 wherein the global positioning tracking process provides the stored sequence to a discriminator of a phase-lock loop.

13. A method according to claim 1 wherein the global positioning tracking process provides the stored sequence to a discriminator of a delay-lock loop.

14. A method according to claim 1 wherein the global positioning tracking process comprises a discrete signal record process which processes a stored record of satellite signal information with one or more records of intermediate data, and wherein said discrete signal record process multiplies at least one of the one or more records of intermediate data by the sequence stored in memory.

15. A method according to claim 1 wherein the global positioning tracking process multiplies a reference code signal by the stored sequence of symbols to generate a modified reference code signal which is correlated against a transmitted signal of the satellite or a frequency down-converted version thereof.

16. A method according to claim 1 wherein the global positioning tracking process multiplies a reference carrier signal by the stored sequence of symbols to generate a modified reference carrier signal which is correlated against a transmitted signal of the satellite or a frequency down-converted version thereof.

17. A method according to claim 1 wherein the global positioning tracking process multiplies a frequency down-converted version of the carrier signal modulated by the data-stream signal by the stored sequence of symbols before the down-converted version is correlated against a reference signal.

18. A computer program product for directing a data processor to predict navigation symbols for use in a global positioning tracking process, the navigation symbols being transmitted from a global positioning satellite in the form of a carrier signal modulated by a data-stream signal comprising the symbols, the navigation data symbols being conveyed in a sequence of one or more repeating frames, the data-stream signal having a first group of symbols which is repeated within the stream once every M number of frames at predetermined locations in the frames, M being equal to or greater than one, the data processor having a data input port and a data output port, the computer program product comprising:

a first computer-readable medium;

a first set of instructions embodied on the first computer-readable medium which directs the data processor to read from the data input port the symbols of the first group at their predetermined locations within the frames of the data-stream signal thereby providing detected symbol values;

a second set of instructions embodied on the first computer-readable medium which directs the data processor to store in a second computer-readable medium a sequence of the detected symbol values of the first group over a set of M frames and a representation of the location of each detected symbol within the set of M frames; and

a third set of instructions embodied on the first computer-readable medium which directs the data processor to generate at the data output port two or more repeating sets of M frames of predicted navigation symbols, said third set of instructions directing the data processor to read the symbols of the first group stored in the second computer-readable medium and to output values representative of the stored symbols of the first group to the data output port at the locations within the frames at which the symbols were initially detected.

19. The computer program product of claim 18 further comprising a fourth set of instructions embodied on the first computer-readable medium which directs the data processor to collect a second sequence of detected symbol values of the first group over a subsequent set of M frames and to compare the detected symbol values of the subsequent set to the detected symbol values of the set stored in the second computer-readable medium to find differences, said fourth set of instructions comprising a subset of instructions which invokes an execution of the second set of instructions when the number of differences exceeds a pre-set amount.

20. The computer program product of claim 18 wherein each frame comprises a plurality of sub-frames, each sub-frame comprises a telemetry word, a hand-over word, and a plurality of data words, the telemetry word comprising a plurality of symbols, the hand-over word comprising a plurality of symbols, and each data word comprising a plurality of symbols, wherein the first group of symbols comprises the symbols of the data words of the sub-frames of all of the frames, wherein each sub-frame is assigned a predetermined numerical identifier depending upon its position within its frame, and wherein the hand-over word of each sub-frame comprises a first set of symbols representing the current time of the week as of the transmission time of the hand-over word, a second set of symbols representing data-flags which have constant values over the time duration of several frames, a third set of symbols representing the sub-frame'"'"'s numerical identifier, a fourth set of symbols holding a set of parity bits which are computed on the basis of the values of the symbols in the hand-over word, and a fifth set of adjustment symbols having values which cause the last two parity bits to have zero values, wherein the computer program product further comprises:

a fifth set of instructions embodied on the first computer-readable medium which directs the data processor to generate and output on the data output port predicted symbol values of the hand-over words, said fifth set of instructions comprising;

a first sub-set of instructions which directs the data processor to obtain the values of the data-flags of each hand-over word from previously known information;

a second sub-set of instructions which directs the data processor to generate a time signal which is at least approximately representative of the current time of the week;

a third sub-set of instructions which directs the data processor to predict from the time signal the numerical identity of an upcoming sub-frame that will be transmitted by the satellite and to predict the value of the time of the week which said upcoming sub-frame will contain; and

a fourth sub-set of instructions which directs the data processor to generate a replica of the hand-over word for the upcoming sub-frame by generating a first set of symbols representing the predicted value of the time of the week, generating a second set of symbols representative of the data-flags of the upcoming hand-over word, generating a third set of symbols representative of the predicted numerical identity of the upcoming sub-frame, and generating a fourth set of parity-bit symbols and related fifth set of adjustment symbols on the basis of the three previously generated sets of symbols.