Time-to-first-fix for position determination
View Patent ↗An improved time-to-first-fix (TTFF) for GPS systems is provided through a comparison of the time-of-week (TOW) to the sub-frame identification (ID). In one embodiment, this comparison comprises dividing the TOW to form a ratio and performing a modulus operation on the ratio to form a remainder, which is then incremented. If the incremented remainder equals the sub-frame ID, the TOW is assumed to be valid, thereby providing a time of transmission. The time of transmission may then be used to calculate pseudoranges and determine a receiver's location.
1. A method, comprising:
receiving GPS-encoded signals from a satellite, wherein the GPS-encoded signals are organized into sub-frames;
for a given received sub-frame, identifying a potential preamble and a potential time of week (TOW) and a potential sub-frame identification (ID);
comparing the potential sub-frame ID to the potential TOW to confirm that the potential preamble is an actual preamble to thereby confirm that the potential TOW is an actual TOW; and
using the actual TOW to determine a time of transmission for the given received sub-frame.
2. The method of claim 1 , further comprising:
using the time of transmission to determine pseudoranges.
3. The method of claim 1 , wherein the comparing act comprises performing a modulus operation on the TOW.
4. The method of claim 3 , wherein the modulus operation is a modulo-5 operation.
5. The method of claim 1 , wherein the comparing act comprises:
dividing the TOW by an integer to form a ratio; and
performing a modulo operation on the ratio to provide a remainder;
incrementing the remainder to form an incremented remainder; and
comparing the incremented remainder to the sub-frame ID.
6. The method of claim 5 , wherein incrementing the remainder comprises incrementing the remainder by one.
7. The method of claim 5 , wherein comparing the incremented remainder to the sub-frame ID comprises determining whether the incremented remainder and the sub-frame ID are equal.
8. The method of claim 6 , further comprising:
if the sub-frame ID equals the incremented remainder, using the TOW to establish a time of transmission for the given received sub-frame.
9. The method of claim 8 , further comprising:
using the time of transmission in determining a pseudorange.
10. A receiver, comprising:
a GPS signal processor adapted to decode GPS-encoded signals to provide decoded signals; and
a processor adapted to identify sub-frames in the decoded signals, the processor being further adapted to identify, for a given identified, sub-frame, a potential preamble, a potential time-of-week (TOW) message, and a potential sub-frame identification (ID), the processor being further adapted to compare the potential sub-frame ID to the potential TOW to confirm that the potential preamble is an actual preamble to thereby determine that the potential TOW is an actual TOW, the processor being further adapted to use the actual TOW so as to determine a time of transmission for the given identified sub-frame.
11. The receiver of claim 10 , wherein the processor is configured to compare the sub-frame ID to the TOW by performing a modulus operation on the TOW.
12. The receiver of claim 11 , wherein the modulus operation is a modulo-5 operation.
13. The receiver of claim 10 , wherein the processor is configured to perform the comparison by:
dividing the TOW by an integer to form a ratio; and
performing a modulo operation on the ratio to provide a remainder;
incrementing the remainder to form an incremented remainder; and
comparing the incremented remainder to the sub-frame ID.
14. The receiver of claim 13 , wherein the processor is configured to compare the incremented remainder to the sub-frame ID by determining whether the incremented remainder and the sub-frame ID are equal.
15. The receiver of claim 14 , wherein the processor is further configured to establish the time of transmission for the given received sub-frame if the sub-frame ID equals the incremented remainder by using the TOW.
16. The receiver of claim 15 , wherein the processor is further configured to use the time of transmission to determine a pseudorange.
17. An apparatus, comprising:
means for decoding GPS-encoded signals so as to retrieve a potential preamble, a potential time-of-week (TOW) message, and a potential sub-frame ID for a given transmitted sub-frame from a GPS satellite; and
means for comparing the potential sub-frame ID to the potential TOW to confirm that the potential preamble is an actual preamble to thereby determine that the potential TOW is an actual TOW and for using the actual TOW so as to determine a time of transmission for the given transmitted sub-frame.
18. The apparatus of claim 17 , wherein the means for comparing the sub-frame ID to the TOW is configured to perform the comparison by:
dividing the TOW by an integer to form a ratio; and
performing a modulo operation on the ratio to provide a remainder;
incrementing the remainder to form an incremented remainder; and
comparing the incremented remainder to the sub-frame ID.
19. The apparatus of claim 18 , wherein the means for comparing the sub-frame ID to the TOW is configured to compare the incremented remainder to the sub-frame ID by determining whether the incremented remainder and the sub-frame ID are equal.