PRECISE ABSOLUTE TIME TRANSFER FROM A SATELLITE SYSTEM

• US 20090315764A1
• Filed: 05/30/2008
• Published: 12/24/2009
• Est. Priority Date: 05/30/2008
• Status: Active Grant

• Current Assignee: The Boeing Co.
• Original Assignee: The Boeing Co.
• Inventors: Ferrell, Barton G., Cohen, Clark E., Gutt, Gregory M., Whelan, David A., Brumley, Robert W.
• Granted Patent: US 8,035,558 B2
• Time in Patent Office: Days
• US Class Current: 342/357.640
• CPC Class Codes: G01S 19/05 providing aiding data
• 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 19/23 Testing, monitoring, correcting or calibrating of receiver elements
• CPC Class Codes: G01S 19/256 relating to timing, e.g. time of week, code phase, timing offset

Abstract

Systems and methods according to one or more embodiments are provided for obtaining a precise absolute time using a satellite system. The precise absolute time may be used, for example, as an aid for positioning systems including navigation in attenuated or jammed environments. A method of obtaining precise absolute time transfer from a satellite according to an embodiment comprises: receiving a precision time signal from a satellite, wherein the precision time signal comprises a periodic repeating code; determining a timing phase of the code; receiving additional aiding information; and using the timing phase and the additional aiding information to determine a precise absolute time.

Claims

1. A method of obtaining precise absolute time transfer from a satellite, the method comprising:

receiving a precision time signal from a satellite, wherein the precision time signal comprises a periodic repeating code;

determining a timing phase of the code;

receiving additional aiding information; and

using the timing phase and the additional aiding information to determine a precise absolute time.

2. The method of claim 1, wherein the receiving additional aiding information comprises receiving the additional aiding information through a network.

3. The method of claim 2, wherein the network is a cellular network, a WiFi network, or an Internet network.

4. The method of claim 1, wherein the code alternates between a coarse timing code and a pseudorandom code.

5. The method of claim 1, wherein the satellite is a first satellite, the method further comprising:

using the precise absolute time to align positioning correlators of a receiver unit to determine a positioning signal from a second satellite in an attenuated or jammed environment.

6. The method of claim 5, wherein the second satellite is a Global Positioning System (GPS) satellite.

7. The method of claim 1, wherein the satellite is a Low Earth Orbit (LEO) satellite.

8. The method of claim 7, wherein the LEO satellite is an Iridium satellite or a Globalstar satellite.

9. The method of claim 1, wherein the additional aiding information is orbit information associated with the satellite, an approximate time within about 5 seconds, an approximate range between the satellite and a receiver unit within about 3000 meters, or clock offset information associated with a clock of the satellite.

10. The method of claim 1, further comprising determining the additional aiding information from a Doppler profile of the satellite.

11. A method of performing navigation in attenuated or jammed environments, the method comprising:

detecting a frame structure of a signal received from a first satellite;

aligning a clock of a receiver unit to the detected frame structure;

generating multiple time estimates respectively separated according to the frame structure, wherein at least one time estimate will be aligned to a signal of a second satellite;

providing the time estimates to system correlators of the receiver unit;

aligning the system correlators according to the time estimates; and

identifying the at least one time estimate that is aligned to the signal source from the second satellite, wherein the at least one time estimate provides successful aiding information to the receiver unit and significantly improves detection efficiency.

12. The method of claim 11, wherein the receiver unit is an iGPS timing receiver.

13. The method of claim 11, wherein the first satellite is a Low Earth Orbit (LEO) satellite.

14. The method of claim 13, wherein the LEO satellite is an Iridium satellite.

15. The method of claim 14, wherein the frame structure comprises a 90 millisecond L band frame.

16. A receiver unit adapted for use in attenuated or jammed environments comprising:

an antenna adapted to receive a precision time signal from a satellite and receive additional aiding information, wherein the precision time signal comprises a periodic repeating code;

a processor; and

a memory adapted to store a plurality of computer readable instructions which when executed by the processor are adapted to cause the receiver unit to;

determine a timing phase of the code, and

use the timing phase and the additional aiding information to determine a precise absolute time.

17. The receiver unit of claim 16, wherein the satellite is a first satellite, the receiver unit further comprising:

system correlators adapted to be aligned based on the precise absolute time to perform navigation based on a positioning signal received from a second satellite.

18. The receiver unit of claim 17, wherein the system correlators are implemented by the processor.

19. The receiver unit of claim 16, wherein the satellite is a Low Earth Orbit (LEO) satellite.

20. The receiver unit of claim 19, wherein the LEO satellite is an Iridium satellite or a Globalstar satellite.

21. The receiver unit of claim 16, wherein the network is a cellular network, a WiFi network, or an Internet network.

22. The receiver unit of claim 16, wherein the code alternates between a coarse timing code and a pseudorandom code.

23. The receiver unit of claim 16, wherein the receiver unit is a cellular telephone, a handheld navigation device, a vehicle-based navigation device, or an aircraft-based navigation device.

24. The receiver unit of claim 16, wherein the precision time signal further comprises data and wherein the determining the timing phase of the code further comprises:

receiving the data over a frequency band of the satellite;

downconverting and sampling the received data;

capturing and storing the sampled data in appropriate blocks;

performing a course acquisition search of the sampled data;

screening for peaks in the sampled data;

performing a fine acquisition search at a location of a screened peak determined from the course acquisition search so that a best peak is determined;

determining the timing phase of the code based on an identified message that generated the best peak and relative time.

25. The receiver unit of claim 24, wherein the performing the course acquisition search of the sampled data further comprises:

selecting a portion of the sampled data for detailed processing;

estimating Doppler of the portion of the sampled data using a known orbit model and estimated time;

digitally demodulating the portion of the sampled data with sine and cosine functions based on a known frequency sub band;

decimating the portion of the sampled data;

applying a Fast Fourier Transform to the decimated portion of the sampled data so that a highest peak and associated frequency are determined;

continuing to a next portion of the sampled data and repeating the detailed processing.

26. A receiver unit adapted for use in attenuated or jammed environments comprising:

an antenna adapted to receive a precision time signal from a first satellite wherein the precision time signal comprises a frequency band frame structure, and to receive a second signal from a second satellite;

a processor; and

a memory adapted to store a plurality of computer readable instructions which when executed by the processor are adapted to cause the receiver unit to;

detect the frame structure of the first satellite;

align an internal clock of the receiver unit to the frame structure of the first satellite;

generate multiple time estimates respectively separated according the frame structure of the first satellite, wherein at least one time estimate is aligned to the signal from the second satellite;

align system correlators of the receiver unit according to the time estimates; and

identify the at least one time estimate that is aligned to the signal from the second satellite such that successful aiding information is provided to the receiver unit.

27. The receiver unit of claim 26, wherein the frequency band frame structure comprises an L-band frame structure.