Software GNSS receiver for high-altitude spacecraft applications

• US 8,259,012 B2
• Filed: 04/14/2010
• Issued: 09/04/2012
• Est. Priority Date: 04/14/2010
• Status: Active Grant

• Current Assignee: The Boeing Co.
• Original Assignee: The Boeing Co.
• Inventors: Li, Rongsheng, Ghassemi, Kamran, Kelley, Clifford W.
• Primary Examiner(s): Liu, Harry
• Application Number:
  US12/760,294
• Publication Number:
  US 20110254734A1
• Time in Patent Office: 874 Days
• Field of Search: 342/357.77
• US Class Current: 342/357.77
• CPC Class Codes: G01S 19/14 specially adapted for specific applications
• CPC Class Codes: G01S 19/23 Testing, monitoring, correcting or calibrating of receiver elements
• CPC Class Codes: G01S 19/246 involving long acquisition integration times, extended snapshots of signals or methods specifically directed towards weak signal acquisition
• CPC Class Codes: G01S 19/46 the supplementary measurement being of a radio-wave signal type

Abstract

A system that provides GPS-based navigation/orbit determination capabilities for high-altitude spacecraft. The system uses an existing spacecraft processor and an easy-to-space-qualify minimum-hardware front end to minimize the need for new space-qualified hardware. The system also uses coherent integration to acquire and track the very weak GPS signals at high altitudes. The system also uses diurnal thermal modeling of a spacecraft clock and precision orbit propagation to enable longer coherent integration, a special Kalman filter to allow weak signal tracking by integrated operation of orbit determination and GPS signal tracking, and a segment-by-segment, post-processing, delayed-time approach to allow a low-speed spacecraft processor to provide the software GPS capability.

Claims

1. A Global Navigation Satellite System (GNSS) receiver onboard a satellite, comprising:

an antenna for generating analog Global Positioning System (GPS) signal data in response to detection of analog GPS signals;

front-end circuitry connected to receive said analog GPS signal data from said antenna, said front-end circuitry comprising an analog-to digital converter which converts analog GPS signal data to digital GPS signal data; and

a computer processor programmed to perform the following steps;

(a) buffering digital GPS signal data outputted by said front-end circuitry over a time interval that is greater than the duration of a bit of a navigation message;

(b) calculating a respective set of parameters based in part on the digital GPS signal data remaining after navigation data wipe-out for each frequency/time data point in a search array, and based in part on modeled clock errors calculated by using a periodic function corresponding to a periodic thermal profile of said satellite to model clock errors;

(c) calculating time-domain averaged I and Q signal components as a function of said respective set of parameters for each frequency/time data point in said search array;

(d) calculating respective correlation magnitudes based on said time- domain averaged I and Q signal components for each frequency/time data point in said search array; and

(e) determining an orbit state correction based in part on said correlation magnitudes.

2. The receiver as recited in claim 1, wherein said set of parameters comprises predicted carrier intermediate frequency.

3. The receiver as recited in claim 1, wherein said set of parameters comprises predicted code frequency.

4. The receiver as recited in claim 1, wherein said set of parameters comprises predicted code phase.

5. The receiver as recited in claim 1, wherein said time interval is on the order of one second.

6. The receiver as recited in claim 1, wherein said computer processor is further programmed to perform the steps of propagating orbit state data of said satellite during said time interval and then buffering said orbit state data propagated during said time interval, and said respective set of parameters are calculated in step (b) based in part on said buffered orbit state data.

7. The receiver as recited in claim 1, wherein said computer processor is further programmed to perform the step of estimating corrections for the clock error model and a thruster error model.

8. A method of acquiring weak Global Positioning System (GPS) signals onboard a satellite orbiting at high altitudes, comprising the following steps:

(a) generating analog GPS signal data in response to detection of analog GPS signals;

(b) converting said analog GPS signal data to digital GPS signal data;

(c) buffering said digital GPS signal data over a time interval that is greater than the duration of a bit of a navigation message;

(d) calculating a respective set of parameters based in part on the digital GPS signal data remaining after navigation data wipe-out for each frequency/time data point in a search array, and based in part on modeled clock errors calculated by using a periodic function corresponding to a periodic thermal profile of said satellite to model clock errors;

(e) calculating time-domain averaged I and Q signal components as a function of said respective set of parameters for each frequency/time data point in said search array;

(f) calculating respective correlation magnitudes based on said time-domain averaged I and Q signal components for each frequency/time data point in said search array; and

(g) determining an orbit state correction based in part on said correlation magnitudes.

9. The method as recited in claim 8, wherein said set of parameters comprises predicted carrier intermediate frequency.

10. The method as recited in claim 8, wherein said set of parameters comprises predicted code frequency.

11. The method as recited in claim 8, wherein said set of parameters comprises predicted code phase.

12. The method as recited in claim 8, wherein said time interval is on the order of one second.

13. The method as recited in claim 8, further comprising the steps of propagating orbit state data of said satellite during said time interval and then buffering said orbit state data propagated during said time interval, said respective set of parameters being calculated in step (d) based in part on said buffered orbit state data.

14. The method as recited in claim 8, further comprising the step of estimating corrections for the clock error model and a thruster error model.

15. A Global Navigation Satellite System (GNSS) receiver, comprising:

front-end circuitry operable to generate digital GPS signal data from received analog GPS signal data; and

a computer processor programmed to perform the following steps;

(a) buffering digital GPS signal data outputted by said front-end circuitry over a time interval that is greater than the duration of a bit of a navigation message;

(b) propagating orbit state data of a satellite during said time interval;

(c) buffering said orbit state data propagated during said time interval;

(d) calculating a respective set of parameters for each frequency/time data point in a search array, the values of said parameters being based in part on position and velocity data included in said buffered orbit state data and based in part on GNSS satellite position and velocity data derived from a navigation message received from a source other than said detected analog GPS signals;

(e) calculating time-domain averaged I and Q signal components as a function of said respective set of parameters for each frequency/time data point in said search array;

(f) calculating respective correlation magnitudes based on said time-domain averaged I and Q signal components for each frequency/time data point in said search array; and

(g) determining an orbit state correction based in part on said correlation magnitudes;

wherein steps (d)-(g) are performed in real time during real-time mode and performed in a long delayed manner, segment-by-segment, during a delayed processing mode.

16. The receiver as recited in claim 15, wherein said set of parameters comprises predicted carrier intermediate frequency, predicted code frequency and predicted code phase.

17. The receiver as recited in claim 15, wherein said computer processor is further programmed to perform the step of using a periodic function corresponding to a periodic thermal profile of said satellite to model clock errors, and said respective set of parameters are calculated in step (d) based in part on said modeled clock errors.

18. The receiver as recited in claim 15 wherein said computer processor is further programmed to perform the step of estimating corrections for the clock error model and a thruster error model.

19. The receiver as recited in claim 15, wherein:

the controller is programmed to enter the delayed-processing mode before a GPS signal is acquired, and to enter real-time mode after the GPS signal is acquired.

20. The receiver as recited in claim 15, wherein:

the orbit state data buffered in step (c) is buffered with time stamps to allow time-matched processing together with the GPS signal data buffered in step (a) during the delayed-processing mode.

21. The receiver as recited in claim 15, wherein:

the orbit state correction determined in step (g) is propagated from a time for which the correction is valid to a time when the correction is applied;

at the time when the correction is applied, the correction is sent to a Kalman filter, and the corrections received from the Kalman filter are applied.

22. A receiver, comprising:

front-end circuitry operable to generate digital Global Positioning System (GPS) signal data from a received analog GPS signal; and

a computer processor programmed to;

(a) calculate a respective set of parameters based in part on the digital GPS signal data for each frequency/time data point in a search array, and based in part on modeled clock errors calculated by using a periodic function corresponding to a periodic thermal profile of a satellite to model clock errors;

(b) calculate time-domain averaged I and Q signal components as a function of said respective set of parameters for each frequency/time data point in said search array;

(c) calculate respective correlation magnitudes based on said time- domain averaged I and Q signal components for each frequency/time data point in said search array; and

(d) determine an orbit state correction based in part on said correlation magnitudes.