GNSS time fraction calibration of a real time clock

• US 7,791,534 B1
• Filed: 03/13/2009
• Issued: 09/07/2010
• Est. Priority Date: 05/19/2006
• Status: Active Grant

• Current Assignee: Trimble Navigation Limited (Trimble Inc.)
• Original Assignee: Trimble Navigation Limited (Trimble Inc.)
• Inventors: Fontes, Joseph W., Zhi, Xiaorong, Yu, Yiming
• Primary Examiner(s): Tarcza; Thomas H
• Assistant Examiner(s): Nguyen; Nga X
• Application Number:
  US12/381,562
• Time in Patent Office: 543 Days
• Field of Search: 34235701-35717
• US Class Current: 342/357.22
• CPC Class Codes: G01S 19/23 Testing, monitoring, correcting or calibrating of receiver elements

Abstract

A generic navigation satellite system (GNSS) signal receiver having a fast time to first fix by calibrating a low power always-on real time clock (RTC). The receiver includes an RTC calibrator having a fraction calculator. The RTC calibrator may also include a time expander. Before the receiver is powered off, the fraction calculator uses the fine resolution of GNSS time for determining a time fraction for RTC time. When the receiver is powered back on, the time expander uses an estimate of RTC time drift during the time that GNSS receiver had power off and the time fraction for calibrating and increasing the resolution of the RTC time for an RTC time tick. A signal navigation processor uses the calibrated RTC time for assisting a first fix with code phase search, integration time periods, resolution of epoch integer and/or location-in-space of GPS satellites.

Claims

1. A global navigation satellite system (GNSS) receiver, comprising:

an always-on real time clock to maintain an RTC time having RTC time ticks of a certain time period;

an RTC calibrator to determine a time fraction of said certain time period while the receiver has power on wherein said time fraction is a difference at a one of said RTC time ticks between GNSS-based time at a fine resolution and said GNSS-based time at a time resolution of said certain time period; and

to combine said time fraction with a turn on said RTC time corresponding to turn-on of the receiver at a one of said time ticks after a power down time of the receiver to calibrate said turn-on RTC time when the receiver is switched from power off to power on; and

a signal navigation processor to use said calibrated RTC time to assist in determination of a GNSS-based fix.

2. The receiver of claim 1, wherein:

the RTC calibrator includes an RTC time expander to calibrate said turn-on RTC time by adding said time fraction to said turn-on RTC time.

6. The receiver of claim 1, wherein:

the signal navigation processor is configured to receive a GNSS signal having spread spectrum code epochs, use said calibrated RTC time to define an integration time period covering several said code epochs, integrate said GNSS signal over said integration time period to acquire said GNSS signal, and use said acquired GNSS signal to determine said fix.

7. The receiver of claim 1, wherein:

the signal navigation processor is configured to receive a GNSS signal having a spread spectrum code having many code phases, use said calibrated RTC time to designate a subset of said code phases, search only said code phase subset to acquire said GNSS signal, and use said acquired GNSS signal to determine said fix.

8. The receiver of claim 1, wherein:

the signal navigation processor is configured to receive a GNSS signal having data bits having code epochs, use said calibrated RTC time to determine an integer number of said code epochs in a pseudorange between the receiver and a GNSS signal source transmitting said GNSS signal, and use said pseudorange to determine said fix.

9. The receiver of claim 1, wherein:

the signal navigation processor is configured to receive a GNSS signal from a GNSS signal source, use said calibrated RTC time to determine a location-in-space for said GNSS signal source, and use said location-in-space to determine said fix.

10. The receiver of claim 1, wherein:

the real time clock is configured to maintain said RTC time with said certain time period of about one second derived from a resonator having a resonant frequency in a range of 32 to 33 kHz while operating on less than one milliwatt of power while the receiver has power off.

3. A global navigation satellite system (GNSS) receiver, comprising:

an always-on real time clock to maintain an RTC time having increments of a certain time period;

an RTC calibrator to determine a time fraction of said certain time period while the receiver has power on; and

to combine said time fraction with said RTC time to calibrate said RTC time when the receiver is switched from power off to power on; and

a signal navigation processor to use said calibrated RTC time to assist in determination of a GNSS-based fix;

wherein;

the RTC calibrator includes a power down calculator to use said RTC time to determine an elapsed power down time; and

an RTC time expander to scale said power down time with an estimated RTC drift rate to calibrate said power down time and use said calibrated power down time to calibrate said RTC time when the receiver is switched from power off to power on.

4. A global navigation satellite system (GNSS) receiver, comprising:

an always-on real time clock to maintain an RTC time having increments of a certain time period;

an RTC calibrator to determine a time fraction of said certain time period while the receiver has power on; and

to combine said time fraction with said RTC time to calibrate said RTC time when the receiver is switched from power off to power on; and

a signal navigation processor to use said calibrated RTC time to assist in determination of a GNSS-based fix; and

wherein;

the RTC calibrator further includes a drift rate calculator to estimate an RTC drift rate of said real time clock by comparisons between said RTC time and a GNSS-based time while said GNSS receiver has power on; and

the signal navigation processor is configured to use said RTC drift rate estimate to calibrate said RTC time when the receiver is switched from power off to power on.

5. The receiver of claim 4, wherein:

the RTC calculator further includes a calibration error estimator to estimate a difference between said estimated RTC drift rate and an actual RTC drift rate to provide a calibration error estimate; and

the signal navigation processor is configured to use said calibration error estimate to determine a manner in which said calibrated RTC time is used to assist said determination of said fix.

11. A global navigation satellite system (GNSS) method in a GNSS receiver, comprising:

maintaining RTC time having RTC time ticks of a certain time period;

determining a time fraction of said certain time period while the receiver has power on wherein said time fraction is a difference at a one of said RTC time ticks between GNSS-based time at a fine resolution and said GNSS-based time at a time resolution of said certain time period; and

combining said time fraction with a turn on said RTC time corresponding to turn-on of the receiver at a one of said time ticks after a power down time of the receiver for calibrating said turn-on RTC time when the receiver is switched from power off to power on; and

using said calibrated RTC time for assisting in determination of a GNSS-based fix.

12. The method of claim 11,

wherein:

combining said time fraction is adding said time fraction to said turn-on RTC time for calibrating said turn-on RTC time.

16. The method of claim 11, further comprising:

receiving a GNSS signal having spread spectrum code epochs;

using said calibrated RTC time for defining an integration time period covering several said code epochs;

integrating said GNSS signal over said integration time period for acquiring said GNSS signal; and

using said acquired GNSS signal for determining said fix.

17. The method of claim 11, further comprising:

receiving a GNSS signal having a spread spectrum code having many code phases;

using said calibrated RTC time for designating a subset of said code phases;

searching only said code phase subset for acquiring said GNSS signal; and

using said acquired GNSS signal for determining said fix.

18. The method of claim 11, further comprising:

receiving a GNSS signal having data bits having code epochs;

using said calibrated RTC time for determining an integer number of said code epochs in a pseudorange between the receiver and a GNSS signal source transmitting said GNSS signal; and

using said pseudorange for determining said fix.

19. The method of claim 11, further comprising:

receiving a GNSS signal from a GNSS signal source;

using said calibrated RTC time for determining a location-in-space for said GNSS signal source; and

using said location-in-space for determining said fix.

20. The method of claim 11, wherein:

maintaining said RTC time includes maintaining said RTC time with said certain time period of about one second derived from a resonator having a resonant frequency in a range of 32 to 33 kHz while operating on less than one milliwatt of power while the receiver has power off.

13. A global navigation satellite system (GNSS) method in a GNSS receiver, comprising:

maintaining an RTC time having increments of a certain time period;

determining a time fraction of said certain time period while the receiver has power on; and

combining said time fraction with said RTC time for calibrating said RTC time when the receiver is switched from power off to power on;

using said calibrated RTC time for assisting in determination of a GNSS-based fix;

using said RTC time for determining an elapsed power down time;

estimating an RTC drift rate;

scaling said power down time with said estimated RTC drift rate for calibrating said power down time; and

using said calibrated power down time for calibrating said RTC time when the receiver is switched from power off to power on.

14. A global navigation satellite system (GNSS) method in a GNSS receiver, comprising:

maintaining an RTC time having increments of a certain time period;

determining a time fraction of said certain time period while the receiver has power on; and

combining said time fraction with said RTC time for calibrating said RTC time when the receiver is switched from power off to power on;

using said calibrated RTC time for assisting in determination of a GNSS-based fix;

estimating an RTC drift rate of said RTC time by comparisons between said RTC time and a GNSS-based time while said GNSS receiver has power on; and

using said RTC drift rate estimate for calibrating said RTC time when the receiver is switched from power off to power on.

15. The method of claim 14, further comprising:

estimating a difference between said estimated RTC drift rate and an actual RTC drift rate to provide a calibration error estimate; and

using said calibration error estimate for determining a manner in which said calibrated RTC time is used for assisting said determination of said fix.

21. An article of manufacture that may be read by a computing device for carrying out the following steps:

maintaining RTC time having RTC time ticks of a certain time period;

determining a time fraction of said certain time period while the computing device has power on wherein said time fraction is a difference at a one of said RTC time ticks between GNSS-based time at a fine resolution and said GNSS-based time at a time resolution of said certain time period; and

combining said time fraction with a turn on said RTC time corresponding to turn-on of the computing device corresponding to a one of said time ticks after a power down time of the computing device for calibrating said turn-on RTC time when the computing device is switched from power off to power on; and

using said calibrated RTC time for assisting in determination of a GNSS-based fix.