Optimum utilization of pseudorange and range rate corrections by SATPS receiver

• US 5,748,651 A
• Filed: 05/05/1995
• Issued: 05/05/1998
• Est. Priority Date: 05/05/1995
• Status: Expired due to Fees

• Current Assignee: Trimble Navigation Limited (Trimble Inc.)
• Original Assignee: Trumble Navigation Limited
• Inventors: Sheynblat, Len
• Primary Examiner(s): Baker, Stephen M.
• Application Number:
  US08/435,552
• Time in Patent Office: 1,096 Days
• Field of Search: 371/49.1
• US Class Current: 714/800
• CPC Class Codes: G01S 19/41 Differential correction, e.g. DGPS [differential GPS]

Abstract

The partial decoding algorithm for decoding the partially damaged differential SATPS messages and the decoding device is disclosed. The algorithm is based on the modified parity test. The information contained in the decoded messages is used for the high level differential SATPS testing.

Claims

1. A method of checking a partially damaged message using a parity mechanism, said method being used by a differential satellite positioning system (DSATPS), said partially damaged message being transmitted from a base to a rover, said method allows to recover data contained in said damaged message;

each said message including a plurality of words;

each said message including a header and a plurality of data words following said header, wherein said header includes a first header word and a second header word, wherein said first header word includes a specific PREAMBLE field;

wherein said first header word indicates the start of a new message; and

wherein said second header word provides the message frame length information;

each said header word including n bits including m parity bits;

each said data word including n bits including m parity bits;

n and m being integers;

said method comprising the steps of;

initially synchronizing after a loss of lock said message, wherein said first header word of said message including specific PREAMBLE field is employed for identifying the received information as said message;

testing the parity of said first header word of said message frame using an initial parity test;

testing the parity of said second header word of said message frame using the standard parity test when said first header word passes the initial parity test;

testing the parity of said first data word of said message frame using a standard parity test when said second header word passes the standard parity test;

designating as a "hit" word said first data word if said first data word fails the standard parity test and designating as a "pass" word said first data word if said first data word passes the standard parity test;

testing the parity of each said data word following said "hit" data word using a modified parity test and testing the parity of each said data word following said "pass" data word using the standard parity test;

starting the resynchronization process I, said resynchronization process I comprising the steps of;

searching for the PREAMBLE at the beginning of said current message if said last data word of said preceding message is a "hit" data word;

checking for the initial parity test that does not require the knowledge of the last two bits d29* and d30* of said last "hit" data word of the preceding message said first header word of said current message, wherein said last two bits d29* and d30* of said last "hit" data word of the preceding message are both assumed to be zeroes;

checking for the standard parity test said second header word of said current message; and

reacquiring the lock of the current message if the PREAMBLE in said first header word of the current message is found, and said first header word of said current message passes the initial parity test, and said second header word of said current message passes the standard parity test;

starting the resynchronization process II, said resynchronization process II comprising the steps of;

search for the PREAMBLE at the beginning of said current message if said last data word of said preceding message is a "pas" data word;

checking for the standard parity test said first header word and said second word of said current message, wherein when said first header word of said current message is tested the standard parity test employs the known last two bits d29* and d30* of said last "pass" data word of the preceding message, and wherein the standard parity test employs the known last two bits d29** and d30* of said first header word of said current message when said second header word of said current message is tested; and

reacquiring the lock of the current message if the PREAMBLE in said first header word of the current message is found, and said first header word of said current message passes the standard parity test, and said second header word of said current message passes the standard parity test; and

assigning the "pass" or "fail" grade for each said header word and for each said data word, wherein the record of all "pass" and "fail" words is used for the higher level testing of the differential corrections.

2. The method of claim 1, wherein said message is an RTCM1 message generated by a Reference Station (RefSta) and received by the SATPS receiver, and wherein the step of synchronizing said RTCM1 message further includes the step of:

matching said PREAMBLE 1 field in the data stream with said PREAMBLE 1 data contained in the user receiver.

3. The method of claim 1, wherein said message is an RTCM2 message generated by a Reference Station (RefSta) and received by the SATPS receiver, and wherein the step of synchronizing said RTCM2 message further includes the step of:

matching said PREAMBLE 2 field in the data stream with said PREAMBLE2 data contained in the user receiver.

4. The method of claim 1, wherein said message is an RTCM message generated by a Reference Station (RefSta) and received by the GLONASS receiver, and wherein the step of synchronizing said RTCM message further includes the step of:

matching said PREAMBLEg field in the data stream with said PREAMBLEg data contained in the user receiver.

5. The method of claim 1, wherein each said message consists of 30-bit words numbered 1, 2, . . . M, wherein each said 30-bit word consists of d1, d2, . . . d24, d25, d26, . . . d29, d30 bits, d25 through d30 being the parity bits, wherein bits d29* and d30* indicate the last two bits of the said parity bits of the previous word, and wherein the standard parity test includes the steps of:

computing the checksum based on d29*, d30* and d1 through d24; and

checking said checksum for the match with the parity bits contained in the bits d25 through d30;

wherein if there is a match the standard parity test has been passed, and wherein the standard parity test has been failed if there is no match.

6. The method of claim 1, wherein each said message consists of 30-bit words numbered 1, 2, . . . M, wherein each said 30-bit word consists of d1, d2, . . . d24, d25, d26, . . . d29, d30 bits, d25 through d30 being the parity bits, wherein bits d29* and d30* indicate the last two bits of the said parity bits of the previous word, and wherein the initial parity test further includes the steps of:

assuming that said d29* and d30* bits are both zeroes;

making the direct comparison of said d1 through d8 bits to said PREAMBLE stored in the receiver;

setting d30* to a one, inverting data bits d1 through d8, inverting said stored PREAMBLE, and making the inverted comparison of said inverted data bits d1 through d8 to said inverted PREAMBLE if the direct comparison fails;

computing the checksum based on d29* d30* and d1 through d24 if either direct or inverted comparison does not fail;

checking the checksum for the match with the parity bits contained in the bits d25 through d30; and

recomputing the checksum based on d29*, d30 and d1 through d24 by setting d29* to a one if the match is not found and checking the checksum for the match again.

7. The method of claim 1, wherein the modified parity test further includes the step of:

modifying a "hit" word by changing the last two bits of said "hit" word;

wherein said current word immediately follows said "hit" word;

wherein said last two bits of said "hit" word have four different sates; and

wherein said first state is a "0" state for said last but one bit d29*; and

a "0" state for said last bit d30*; and

wherein said second state is a "0" state for said last but one bit d29*, and a "1" state for said last bit d30*; and

wherein said third state is a "1" state for said last but one bit d29*, and a "0" state for said last bit d30*; and

wherein said fourth state is a "1" state for said last but one bit d29*, and a "1" state for said last bit d30*;

computing for said first embodiment of said "hit" word a first checksum based on d29* and d30* and d1 through d24 and matching said first checksum with the parity bits d25 through d30;

computing for said second embodiment of said "hit" word a second checksum based on d29* and d30* and d1 through d24 and matching said second checksum with the parity bits d25 through d30 if there is no match for said first checksum;

computing for said third embodiment of said "hit" word a third checksum based on d29* and d30* and d1 through d24 and matching said third checksum with the parity bits d25 through d30 if there is no match for said second checksum;

computing for said fourth embodiment of said "hit" word a fourth checksum based on d29* and d30* and d1 through d24 and matching said fourth checksum with the parity bits d25 through d30 if there is no match for said third checksum wherein if for any of said four embodiments there is a match the modified parity test has been passed, and wherein if for all four embodiments there is no match the modified parity test has been failed.

8. A method of checking a partially damaged message using a parity mechanism, said method being used by a differential satellite positioning system (DSATPS), said partially damaged message being transmitted from a base to a rover, said method allows to recover data containing in said damaged message;

each said message including a plurality of words;

each said message including a header and a plurality of data words following said header, wherein said header includes a first header word and a second header word, wherein said first header word includes a specific PREAMBLE field;

wherein said first header word indicates the start of a new message; and

wherein said second header word provides the message frame length information;

each said header word including n bits including m parity bits;

each said data word including n bits including m parity bits;

n and m being integers;

said method comprising the steps of;

matching said PREAMBLE field in the data stream with said PREAMBLE data contained in the user receiver;

checking said first header word for an initial parity test;

checking said first header word for the station ID if said first header word does not pass the initial parity test;

checking said second header word of the initial parity test if said first header word does not pass the initial parity test and if said station ID contained in said message matches with the station ID contained in the user receiver;

acquiring the lock if said second header word passes the initial parity test and if said station ID contained in said message matches with the station ID contained in the user receiver;

testing the parity of each said current data word of said message using a standard parity test;

designating as a "hit" word said first data word if said first data word fails the standard parity test and designating as a "pass" word said first data word if said first data word passes the standard parity test;

testing the parity of each said data word following said "hit" data word using a modified parity test and testing the parity of each said data word following said "pass" data word using the standard parity test;

starting the resynchronization process;

said process comprising the steps of;

searching for the PREAMBLE at the beginning of said current message;

checking said first header word and said second word of said current message for the initial parity test that does not require the knowledge of the last two bits d29* and d30* of said last data word of said preceding message;

checking said first header word for the station ID; and

reacquiring the lock if the PREAMBLE in said first header word of the current message is found, and if station ID contained in said message matches with the station ID contained in the user receiver, and if said first header word of said current message does not pass the initial parity test, and if said second header word of said current message passes the initial parity test that does not require the knowledge of the last two bits d29* and d30* of said first header word of said current message; and

assigning the "pass" or "fail" grade for each said header word and for each said data word, wherein the record of all "pass" and "fail" words is used for the higher level testing of the differential corrections.

9. The method of claim 8, wherein the step of checking said first header word for the station ID further includes the step of entering the station ID by the user.

10. The method of claim 8, wherein the step of checking said first header word for the station ID further includes the step of storing the previously received station ID.

11. The method of claim 8, wherein said message is an RTCM1 message generated by a RefSta and received by the SATPS receiver, and wherein the step of matching said PREAMBLE field in the data stream with said PREAMBLE data contained in the user receiver further includes the step of:

matching said PREAMBLE 1 field in the data stream with said PREAMBLE 1 data contained in the user receiver.

12. The method of claim 8, wherein said message is an RTCM2 message generated by a RefSta and received by the SATPS receiver, and wherein the step of matching said PREAMBLE field in the data stream with said PREAMBLE data contained in the user receiver further includes the step of:

matching said PREAMBLE 2 field in the data stream with said PREAMBLE2 data contained in the user receiver.

13. The method of claim 8, wherein said message is a GLONASS RTCM message generated by a RefSta and received by the GLONASS receiver, and wherein the step of matching said PREAMBLE field in the data stream with said PREAMBLEg data contained in the user receiver further includes the step of:

matching said PREAMBLEg field in the data stream with said PREAMBLEg data contained in the user receiver.