Method and apparatus for direct re-acquisition of precision-code after a short power interruption

• US 5,615,236 A
• Filed: 03/29/1995
• Issued: 03/25/1997
• Est. Priority Date: 03/29/1995
• Status: Expired due to Term

• Current Assignee: Trimble Navigation Limited (Trimble Inc.)
• Original Assignee: Trimble Navigation Limited (Trimble Inc.)
• Inventors: Turney, Paul F.
• Primary Examiner(s): Bocure, Tesfaldet
• Assistant Examiner(s): WEBSTER, BRYAN E
• Application Number:
  US08/412,554
• Time in Patent Office: 727 Days
• Field of Search: 375/200, 375/208, 375/357, 375/365-367, 455/13.2, 455/51.1, 455/51.2, 342/104, 342/107, 342/108, 342/118, 342/342, 342/357, 342/358
• US Class Current: 375/365
• CPC Class Codes: G01S 19/235 Calibration of receiver components
• CPC Class Codes: G01S 19/30 code related G01S19/246 takes precedence
• CPC Class Codes: G01S 19/32 Multimode operation in a single same satellite system, e.g. GPS L1/L2

Abstract

An improved single or dual-frequency navigation satellite receiver that can directly re-acquire P-code code-phase after a brief loss of P-code tracking. A digital signal processor (DSP) writes its current position fixes to a non-volatile memory. The DSP also updates a clock that keeps a running estimate of precise global positioning system time. When P-code code-phase is lost, it is re-acquired by reading the clock back in, adjusting for its drift, reading in the last position fix from the non-volatile memory, and then beginning an advanced P-code code-phase search based on such head start information.

Claims

1. A method for directly re-acquiring precision-code (P-code) code phase in a global position system (GPS) receiver after a brief loss of primary-power supply affecting code tracking, the method comprising the steps of:

periodically storing a position fix in a non-volatile memory while tracking P-code transmissions from GPS satellites;

periodically updating a clock to synchronize it with global positioning system time while tracking said P-code transmissions from said GPS satellites; and

re-acquiring a P-code code-phase lock to said GPS satellites by reading said non-volatile memory to obtain information about said receiver'"'"'s position and by reading an estimate of said global positioning system time from said clock and by using said position and time information to index into said P-code for a starting point in a P-code code-phase search then code-phase locking onto said P-code when a correlation is detected.

2. A computer-implemented process for execution by a digital signal processor (DSP) in a dual-frequency navigation satellite receiver for directly re-acquiring precision-code (P-code) code-phase lock after losing lock, the process comprising the steps of:

reading-in a position fix from a non-volatile memory that was periodically updated with position fixes of said receiver before losing a P-code code-phase lock;

reading-in an estimate of precise satellite system time from a clock that was periodically updated before losing said P-code code-phase lock;

determining a drift value of said clock;

adjusting a GPS satellite system time kept by said clock by a drift correction factor computed in the previous step of determining, wherein an enhanced estimate of said satellite system time is obtained;

indexing into a P-code sequence at a point that is predicted to be correct for said enhanced satellite system time and at a position indicated by a last position fix read from said memory; and

slipping said P-code sequence through a correlator initially based on the step of indexing in order to obtain a P-code code-phase lock.

3. A global position system (GPS) receiver for directly re-acquiring precision-code (P-code) code phase after a brief loss of P-code tracking, comprising:

Last position fix storage means for periodically storing a position fix in a non-volatile memory while tracking a P-code transmissions from GPS satellites;

synchronizing means for periodically updating a clock to synchronize it with global positioning system time while tracking said P-code transmissions from said GPS satellites; and

digital signal processor means for re-acquiring a P-code code-phase lock to said GPS satellites by reading said non-volatile memory to obtain information about said receiver'"'"'s position and by reading an estimate of said global positioning system time from said clock and by using said position and time information to index into said P-code for a starting point in a P-code code-phase search then code-phase locking onto said P-code when a correlation is detected.

4. A digital signal processor (DSP) for use in a dual-frequency navigation satellite receiver in directly re-acquiring precision-code (P-code) code-phase lock after losing lock, comprising:

position fix means for reading-in a position fix from a non-volatile memory that was periodically updated with position fixes of said receiver before losing a P-code code-phase lock;

satellite system time means for reading-in an estimate of precise satellite system time from a clock that was periodically updated before losing said P-code code-phase lock;

computing means for determining a drift value of said clock;

correction means for adjusting a satellite system time kept by said clock by a drift correction factor computed in the previous step of determining, wherein an enhanced estimate of said satellite system time is obtained;

P-code storage means for indexing into a P-code sequence at a point that is predicted to be correct for said enhanced satellite system time and at a position indicated by a last position fix read from said memory; and

P-code correlation means for slipping said P-code sequence through a correlator initially based on the step of indexing in order to obtain a P-code code-phase lock.

5. A dual-frequency navigation satellite receiver, comprising:

a first receiver channel for receiving coarse/acquisition-code (C/A-code) transmissions on a first microwave frequency from orbiting navigation satellites and for down-converting and despreading said C/A-code transmissions;

a second receiver channel for receiving precision-code (P-code) transmissions on a second microwave frequency from said orbiting navigation satellites and for down-converting and despreading said P-code transmissions;

carrier generator means for supplying a carrier and a carrier phase for said down-converting in the first and second receiver channels;

code generator means for supplying a code and a code phase for said despreading in the first and second receiver channels;

digital signal processing (DSP) means connected to the first and second receiver channels and the carrier and code generators for controlling said carrier and code phase according to information received in said transmissions from said satellites;

non-volatile position memory means connected to the DSP means for storing position fixes determined by the DSP means from said information received in said P-code transmissions from said satellites;

system clock means connected to the DSP means for keeping satellite system time and for receiving periodic updates of satellite system time determined by the DSP means from said information received in said P-code transmissions from said satellites; and

direct P-code re-acquisition means included in the DSP means for reading a last position fix from the non-volatile position memory means and a satellite system time estimate from the system clock means and for determining from a combination of said position fix and satellite system time an initial point in a P-code sequence to begin a P-code code-phase search and code-phase locking means.