GNSS signal processing methods and apparatus with tracking interruption

• US 8,704,709 B2
• Filed: 08/05/2009
• Issued: 04/22/2014
• Est. Priority Date: 08/19/2008
• Status: Active Grant

• Current Assignee: Trimble Navigation Limited (Trimble Inc.)
• Original Assignee: Trimble Navigation Limited (Trimble Inc.)
• Inventors: Vollath, Ulrich, Talbot, Nicholas Charles
• Primary Examiner(s): Keith, Jack W
• Assistant Examiner(s): Mull, Fred H
• Application Number:
  US13/059,428
• Publication Number:
  US 20110156949A1
• Time in Patent Office: 1,721 Days
• Field of Search: 342/357.27, 342/357.3, 701/477, 701479-480
• US Class Current: 342/357.27
• CPC Class Codes: G01S 19/44 Carrier phase ambiguity resolution; Floating ambiguity; LAMBDA [Least-squares AMBiguity Decorrelation Adjustment] method

Abstract

Methods and apparatus are provided for estimating parameters, i.e. ambiguities, derived from GNSS signals. Observations of each of received frequencies of a GNSS signal from a plurality of GNSS satellites are obtained for a plurality of instances in time (3120). The time sequence of observations is fed to a filter to estimate a state vector comprising float ambiguities, wherein each float ambiguity constitutes a non integer estimate of an integer number of wavelengths for a received frequency of a GNSS signal between a receiver of the GNSS signal and the GNSS satellite from which it is received and wherein the float ambiguities of the state vector are updated over time on the basis of the observations (3140). The occurrence of an interruption in tracking of at least one signal of a satellite is determined (3121). The float ambiguity of the state vector for the at least one signal for which an interruption in tracking occurred is maintained at the value before the interruption in tracking occurred (3122). Integer values are assigned to at least a subgroup of the estimated float values to define a plurality of integer ambiguity candidate sets (3160). A quality measure is determined for each of the candidate sets. A weighted average of the candidate sets is formed (3200). Ambiguities of the weighted average can be used in subsequent operations to aid in determining a position of the receiver or can be used to prepare data, e.g., in a network processor that can be used to augment position information of a rover.

Claims

1. Method of operating a processor having associated data storage and program code enabling the processor to estimate parameters derived from global navigational satellite system (GNSS) signals useful to determine a position, comprising

obtaining observations of each of received frequencies of a GNSS signal from a plurality of GNSS satellites to obtain observations at a plurality of instances in time;

feeding the time sequence of observations to a filter operated by the processor to estimate a state vector at least comprising float ambiguities, wherein each float ambiguity constitutes a real number estimate of an integer number of wavelengths for a received frequency of a GNSS signal between a receiver of the GNSS signal and the GNSS satellite from which it is received and wherein the float ambiguities of the state vector are updated over time on the basis of the observations;

determining that an interruption in tracking of at least one signal of a satellite occurred;

maintaining the float ambiguity of the state vector for the at least one signal for which an interruption in tracking occurred at the value before the interruption in tracking occurred;

assigning integer values to at least a subgroup of the estimated float values to define a plurality of integer ambiguity candidate sets;

determining a quality measure for each of the candidate sets; and

forming a weighted average of the candidate sets.

2. Method of claim 1, wherein it is determined that an interruption in tracking of at least one signal of a satellite occurred if an observation for the at least one signal is not available for at least one of the instances in time.

3. Method of claim 1, wherein it is determined that an interruption in tracking of at least one signal of a satellite occurred, if a cycle slip occurred.

4. Method of claim 1 comprising, if, after an interruption in tracking of a signal, tracking of the signal resumes, maintaining the float ambiguity of the state vector for the signal for which an interruption in tracking occurred at the value before the interruption in tracking occurred as a first float ambiguity and introducing into the state vector a second float ambiguity for the signal after resuming tracking.

5. Method of claim 1, wherein forming the weighted average comprises

selecting a first number of candidate sets having a quality measure better than a first threshold, wherein the first threshold is determined based on a reference quality measure of a reference candidate set;

and

forming the weighted average of the selected candidate sets by weighing each candidate set in the weighted average based on its quality measure.

6. Method of claim 5, comprising using the weighted average to estimate a position of the receiver of the GNSS signals.

7. Method of claim 5, wherein the reference candidate set is the candidate set having the best quality measure.

8. Method of claim 5, wherein the quality measure of the candidate sets is constituted by a residual error norm value, the residual error norm value of a candidate set being a measure for a statistical distance of the candidate set to the state vector having the float ambiguities.

9. Method of claim 5, wherein the first threshold is determined as at least one of a fraction of, a multiple of, and a distance to the reference quality measure.

10. Method of claim 5, comprising, if the first number of selected candidate sets is smaller than a second threshold, selecting, for forming the weighted average, a second number of further candidate sets on the basis of the quality measures of the candidate sets in decreasing order starting with the non-selected candidate set having the best quality measure, the second number being constituted by the difference between the first number of selected candidate sets and a second threshold defining a minimum number of candidate sets to be included in the weighted average.

11. Method of claim 5, comprising, if the first number of selected candidate sets is larger than a third threshold, excluding a third number of selected candidate sets from forming the weighted average in decreasing order starting with the selected candidate set having the worst quality measure, wherein the third number is constituted by the difference between the first number of selected candidate sets and a third threshold defining a maximum number of candidate sets to be included in the weighted average.

12. Method of claim 1, wherein forming the weighted average comprises:

determining the best quality measure of the candidate sets;

determining an expectation value of the candidate set having the best quality measure;

determining an error measure as a ratio of the best quality measure to the expectation value;

adapting the quality measures of the candidate sets as a function of the error measure;

performing a selection of the first number of candidate sets based on the adapted quality measures; and

forming the weighted average on the basis of the quality measures.

13. Method of claim 12, comprising adjusting a variance-covariance matrix of the filter using the error measure, if the error measure is in a predetermined range.

14. Method of claim 12, comprising adjusting a variance-covariance matrix of the filter using the error measure, if the error measure is larger than one.

15. Method of claim 1, wherein forming the weighted average comprises:

determining the best quality measure of the candidate sets;

determining an expectation value of the candidate set having the best quality measure;

determining an error measure as a ratio of the best quality measure to the expectation value;

adapting the quality measures of the candidate sets as a function of the error measure; and

forming the weighted average based on the adapted quality measures.

16. Method of claim 1, comprising adapting the quality measures of the candidate sets as a function of the error measure by adjusting a variance-covariance matrix of the float solution so as to change the distribution of the quality measures of the candidate sets relative to the float solution.

17. Method of claim 1, wherein defining the integer candidate sets comprises:

selecting a subset of float ambiguities of the state vector to form the subgroup of the float ambiguities of the state vector used to define the plurality of integer ambiguity candidate sets; and

assigning integer values to the estimated float values of the float ambiguities of the subset to define the plurality of integer ambiguity candidate sets.

18. Method of claim 17, wherein the selecting comprises selecting, as float ambiguities of the subset, the float ambiguities of frequencies tracked continuously for the longest period of time.

19. Method of claim 1, comprising

estimating by the filter a float value for each float ambiguity of the state vector and covariance values associated with the state vector;

determining a formal precision value based on covariance values of the filter, the formal precision value being a measure for an achievable precision;

determining an achieved precision value of the weighted average;

comparing the achieved precision value with the formal precision value to obtain a convergence value; and

indicating a convergence of the determination of the state vector based on the convergence value.

20. Method of claim 19, wherein the convergence value is obtained as a ratio of the achieved precision value to the formal precision value.

21. Method of claim 19, comprising

determining an instance in time when the convergence value of the position is better than a convergence threshold;

and

indicating a convergence of the determination of the state vector at and after the determined instance in time.

22. Method of claim 19, comprising

estimating an achieved precision of a position of the receiver determined based on the weighted ambiguities;

and

indicating a convergence of the determination of the state vector, if the achieved precision of the position is better than an inclusion threshold.

23. Method of claim 19, comprising

estimating an achieved precision of a position of the receiver determined based on the weighted ambiguities;

and

indicating a non-convergence of the determination of the state vector, if the achieved precision of the position is worse than an exclusion threshold.

24. Computer-readable physical storage medium embodying instructions configured, when executed on a computer processing unit, to carry out the method of claim 1.

25. Apparatus to estimate parameters derived from global navigational satellite system (GNSS) signals useful to determine a position, comprising:

a processor having associated data storage and program code enabling the processor to

obtain observations of each of received frequencies of a GNSS signal from a plurality of GNSS satellites to obtain observations at a plurality of instances in time;

operate a filter to estimate a state vector at least comprising float ambiguities based on the time sequence of observations, wherein each float ambiguity constitutes a real number estimate of an integer number of wavelengths for a received frequency of a GNSS signal between a receiver of the GNSS signal and the GNSS satellite from which it is received and wherein the float ambiguities of the state vector are updated over time on the basis of the observations;

determine that an interruption in tracking of at least one signal of a satellite occurred;

maintain the float ambiguity of the state vector for the at least one signal for which an interruption in tracking occurred at the value before the interruption in tracking occurred;

assign integer values to at least a subgroup of the estimated float values to define a plurality of integer ambiguity candidate sets;

determine a quality measure for each of the candidate sets; and

form a weighted average of the candidate sets.

26. Apparatus of claim 25, wherein the program code enables the processor to determine that an interruption in tracking of at least one signal of a satellite occurred if an observation for the at least one signal is not available for at least one of the instances in time.

27. Apparatus of claim 25, wherein the program code enables the processor to determine that an interruption in tracking of at least one signal of a satellite occurred, if a cycle slip occurred.

28. Apparatus of claim 25 wherein the program code enables the processor to, if, after an interruption in tracking of a signal, tracking of the signal resumes, maintain the float ambiguity of the state vector for the signal for which an interruption in tracking occurred at the value before the interruption in tracking occurred as a first float ambiguity and introducing into the state vector a second float ambiguity for the signal after resuming tracking.

29. Apparatus of claim 25, wherein the program code enables the processor to, in order to form the weighted average

select a first number of candidate sets having a quality measure better than a first threshold, wherein the first threshold is determined based on a reference quality measure of a reference candidate set;

and

form the weighted average of the selected candidate sets by weighing each candidate set in the weighted average based on its quality measure.

30. Apparatus of claim 29, wherein the program code enables the processor to use the weighted average to estimate a position of the receiver of the GNSS signals.

31. Apparatus of claim 29, wherein the reference candidate set is the candidate set having the best quality measure.

32. Apparatus of claim 29, wherein the quality measure of the candidate sets is constituted by a residual error norm value, the residual error norm value of a candidate set being a measure for a statistical distance of the candidate set to the state vector having the float ambiguities.

33. Apparatus of claim 29, wherein the program code enables the processor to determine the first threshold as at least one of a fraction of, a multiple of, and a distance to the reference quality measure.

34. Apparatus of claim 29, wherein the program code enables the processor to, if the first number of selected candidate sets is smaller than a second threshold, select, for forming the weighted average, a second number of further candidate sets on the basis of the quality measures of the candidate sets in decreasing order starting with the non-selected candidate set having the best quality measure, the second number being constituted by the difference between the first number of selected candidate sets and a second threshold defining a minimum number of candidate sets to be included in the weighted average.

35. Apparatus of claim 29, wherein the program code enables the processor to, if the first number of selected candidate sets is larger than a third threshold, exclude a third number of selected candidate sets from forming the weighted average in decreasing order starting with the selected candidate set having the worst quality measure, wherein the third number is constituted by the difference between the first number of selected candidate sets and a third threshold defining a maximum number of candidate sets to be included in the weighted average.

36. Apparatus of claim 25, wherein the program code enables the processor to, in order to form the weighted average:

determine the best quality measure of the candidate sets;

determine an expectation value of the candidate set having the best quality measure;

determine an error measure as a ratio of the best quality measure to the expectation value;

adapt the quality measures of the candidate sets as a function of the error measure;

perform a selection of the first number of candidate sets based on the adapted quality measures; and

form the weighted average on the basis of the quality measures.

37. Apparatus of claim 36, wherein the program code enables the processor to adjust a variance-covariance matrix of the filter using the error measure, if the error measure is in a predetermined range.

38. Apparatus of claim 36, wherein the program code enables the processor to adjust a variance-covariance matrix of the filter using the error measure, if the error measure is larger than one.

39. Apparatus of claim 25, wherein the program code enables the processor to, in order to form the weighted average:

determine the best quality measure of the candidate sets;

determine an expectation value of the candidate set having the best quality measure;

determine an error measure as a ratio of the best quality measure to the expectation value;

adapt the quality measures of the candidate sets as a function of the error measure; and

form the weighted average based on the adapted quality measures.

40. Apparatus of claim 25, wherein the program code enables the processor to adapt the quality measures of the candidate sets as a function of the error measure by adjusting a variance-covariance matrix of the float solution so as to change the distribution of the quality measures of the candidate sets relative to the float solution.

41. Apparatus of claim 25, wherein the program code enables the processor to, in order to define the integer candidate sets:

select a subset of float ambiguities of the state vector to form the subgroup of the float ambiguities of the state vector used to define the plurality of integer ambiguity candidate sets; and

assign integer values to the estimated float values of the float ambiguities of the subset to define the plurality of integer ambiguity candidate sets.

42. Apparatus of claim 41, wherein the program code enables the processor to select, as float ambiguities of the subset, the float ambiguities of frequencies tracked continuously for the longest period of time.

43. Apparatus of claim 25, wherein the filter is operated by the processor to

estimate a float value for each float ambiguity of the state vector and covariance values associated with the state vector;

and

wherein the program code enables the processor to

determine a formal precision value based on covariance values of the filter, the formal precision value being a measure for an achievable precision;

determine an achieved precision value of the weighted average;

compare the achieved precision value with the formal precision value to obtain a convergence value; and

indicate a convergence of the determination of the state vector based on the convergence value.

44. Apparatus of claim 43, wherein the program code enables the processor to obtain the convergence value as a ratio of the achieved precision value to the formal precision value.

45. Apparatus of claim 43, wherein the program code enables the processor to

determine an instance in time when the convergence value of the position is better than a convergence threshold;

and

indicate a convergence of the determination of the state vector at and after the determined instance in time.

46. Apparatus of claim 43, wherein the program code enables the processor to

estimate an achieved precision of a position of the receiver determined based on the weighted ambiguities;

and

indicate a convergence of the determination of the state vector, if the achieved precision of the position is better than an inclusion threshold.

47. Apparatus of claim 43, wherein the program code enables the processor to

estimate an achieved precision of a position of the receiver determined based on the weighted ambiguities;

and

indicate a non-convergence of the determination of the state vector, if the achieved precision of the position is worse than an exclusion threshold.

48. Rover comprising an apparatus according to claim 25.

49. Network station comprising an apparatus according to claim 25.