LOCAL CLOCK FREQUENCY CALIBRATION USING LOW EARTH ORBIT (LEO) SATELLITES

• US 20100171652A1
• Filed: 01/06/2009
• Published: 07/08/2010
• Est. Priority Date: 01/06/2009
• Status: Active Grant

• Current Assignee: The Boeing Co.
• Original Assignee: The Boeing Co.
• Inventors: Ferrell, Barton G., Gutt, Gregory M., Whelan, David A., Brumley, Robert W., Martens, Christopher J., Haddad, Anne T.
• Granted Patent: US 7,859,455 B2
• Time in Patent Office: Days
• US Class Current: 342/352
• CPC Class Codes: G01S 19/235 Calibration of receiver components

Abstract

Various techniques are provided for calibrating a frequency of a local clock using a satellite signal. In one example, a method of transferring frequency stability from a satellite to a device includes receiving a signal from the satellite. The method also includes determining a code phase from the satellite signal. The method further includes receiving aiding information. In addition, the method includes calibrating a frequency of a local clock of the device using the code phase and the aiding information to substantially synchronize the local clock frequency with a satellite clock frequency.

Claims

1. A method of transferring frequency stability from a satellite to a device, the method comprising:

receiving a signal from the satellite;

determining a code phase from the satellite signal;

receiving aiding information; and

calibrating a frequency of a local clock of the device using the code phase and the aiding information to substantially synchronize the local clock frequency with a satellite clock frequency.

2. The method of claim 1, wherein:

the calibrating comprises correcting the local clock frequency to compensate for a Doppler shift associated with relative motion of the satellite and the device.

3. The method of claim 1, wherein:

the calibrating comprises measuring a frequency of the satellite signal using the code phase and the aiding information.

4. The method of claim 1, wherein the calibrating comprises:

measuring a frequency of the satellite signal using the code phase and the aiding information;

correcting the measured frequency of the satellite signal to compensate for a Doppler shift associated with relative motion of the satellite and the device;

determining the satellite clock frequency; and

generating a frequency correction for the local clock frequency using the satellite clock frequency and the corrected measured frequency of the satellite signal.

5. The method of claim 1, wherein:

the satellite signal comprises a plurality of message frames, wherein each message frame comprises a time transfer structure message.

6. The method of claim 5, wherein:

each message frame further comprises payload bits associated with communication data.

7. The method of claim 5, wherein:

the determining the code phase comprises identifying a code that is unique for each of the time transfer structure messages of at least one of the message frames.

8. The method of claim 7, wherein:

the code is a binary code having a code set, wherein cross-correlation between distinct members of the code set is relatively low and auto-correlation for each individual member of the code set is relatively high.

9. The method of claim 1, wherein:

the aiding information comprises satellite orbit information.

10. The method of claim 1, wherein:

the satellite is a low earth orbit (LEO) satellite.

11. The method of claim 10, further comprising:

using the calibrated local clock frequency to lock onto a Global Positioning System (GPS) signal; and

determining a position of the device using the GPS signal.

12. The method of claim 11, wherein:

the calibrating comprises calculating a range rate from an orbital model of the LEO satellite and an approximate location of the device.

13. A receiver unit comprising:

an antenna adapted to receive a signal from a satellite;

a local clock;

an analog-to-digital (A/D) converter adapted to sample data from a time transfer structure message in the satellite signal;

a memory adapted to store aiding information; and

a computer adapted to determine a code phase using the sampled data and determine a frequency correction for the local clock using the code phase and the aiding information to substantially synchronize the local clock frequency with a clock frequency of the satellite.

14. The receiver unit of claim 13, wherein:

the computer is adapted to measure a frequency of the satellite signal using the code phase and aiding information; and

the frequency correction includes a Doppler frequency correction to the measured frequency of the satellite signal to compensate for a Doppler shift associated with relative motion of the satellite and the receiver unit.

15. The receiver unit of claim 13, wherein:

the aiding information comprises ephemeredes for the satellite orbit; and

the aiding information comprises an approximate location of the receiver unit.

16. The receiver unit of claim 13, wherein:

the time transfer structure message comprises a binary code having a code set, wherein cross-correlation between distinct members of the code set is relatively low, auto-correlation for each individual member of the code set is relatively high, and each member of the code set uniquely identifies a message frame of the time transfer structure message.

17. The receiver unit of claim 13, wherein:

the identity of the message frame determines a code phase; and

the aiding information includes a time slot offset and a distance from the satellite to the receiver unit.

18. A method of receiving a Global Positioning System (GPS) signal in attenuated environments, the method comprising:

detecting a frame structure of a low earth orbit (LEO) satellite signal;

aligning a local clock to the frame structure;

generating a plurality of time estimates respectively separated according to the frame structure;

providing respective ones of the time estimates to respective ones of a plurality of parallel correlators;

time-aligning the parallel correlators using the time estimates; and

using a time estimate from the time-aligned correlators to lock on to the GPS signal.

19. The method of claim 18, wherein:

the generating comprises generating the time estimates until at least one frame of the LEO signal is substantially aligned with a GPS second.

20. The method of claim 18, further comprising:

determining a position using the GPS signal.