Method and apparatus for reliable inter-antenna baseline determination

• US 6,313,788 B1
• Filed: 08/14/1998
• Issued: 11/06/2001
• Est. Priority Date: 08/14/1998
• Status: Expired due to Term

• Current Assignee: Garmin International, Inc. (Garmin Limited)
• Original Assignee: Seagull Technology, Inc.
• Inventors: Wilson, John M.
• Primary Examiner(s): Tarcza, Thomas H.
• Assistant Examiner(s): Mull, Fred H
• Application Number:
  US09/134,228
• Time in Patent Office: 1,180 Days
• Field of Search: 342/357.11
• US Class Current: 342/357.38
• CPC Class Codes: G01S 19/55 Carrier phase ambiguity resolution; Floating ambiguity; LAMBDA [Least-squares AMBiguity Decorrelation Adjustment] method
• CPC Class Codes: G01S 5/0247 Determining attitude

Abstract

Disclosed is a method for inter-antenna baseline determination that uses an antenna configuration comprising a pair of relatively closely spaced antennas and other pairs of distant antennas. The closely spaced pair provides a short baseline having an integer ambiguity that may be searched exhaustively to identify the correct set of integers. This baseline is then used as a priori information to aid the determination of longer baselines that, once determined, may be used for accurate run time attitude determination.

Claims

1. A method for determining inter-antenna baselines from radio wave phase measurements, said method comprising:

identifying a first antenna and a second antenna located a distance D1 apart;

identifying a third antenna located a distance D2 from the second antenna;

determining a first baseline between the first antenna and the second antenna using a backward integer search procedure; and

determining a second baseline between the second antenna and the third antenna using a forward integer search procedure responsive to the first baseline.

2. The method of claim 1, wherein the distance D1 is less than or equal to 2.5 wavelengths of a carrier wave that emanates from a radio source.

3. The method of claim 1, wherein the distance D2 is greater than or equal to 1.0 wavelengths of a carrier wave that emanates from a radio source.

4. The method of claim 1, further comprising the steps:

establishing a fourth antenna; and

determining a third baseline between the fourth antenna and one of the first antenna, the second antenna, and the third antenna, the third baseline being determined using a forward integer search procedure responsive to at least one previously-determined baseline.

5. The method of claim 1, wherein the step of determining a first baseline between the first antenna and the second antenna using a backward integer search procedure comprises steps of:

performing a backward integer search procedure for each of a plurality of different radio source carrier frequencies;

identifying at least one baseline candidate at each of the different radio source carrier frequencies;

evaluating each of the baseline candidates with a baseline quality test; and

selecting as a first baseline the baseline candidate that satisfies the baseline quality test for each of the plurality of different radio source carrier frequencies.

6. The method of claim 1, wherein the step of determining a second baseline between the second antenna and the third antenna using a forward integer search procedure responsive to the first baseline comprises steps of:

performing a forward integer search procedure for each of a plurality of different radio source carrier frequencies;

identifying at least one baseline candidate at each of the different radio source carrier frequencies;

evaluating each of the baseline candidates with a baseline quality test; and

selecting as a first baseline the baseline candidate that satisfies the baseline quality test for each of the plurality of different radio source carrier frequencies.

7. The method of claim 1, wherein the step of determining a first baseline between the first antenna and the second antenna using a backward integer search procedure comprises steps of:

ranking a plurality of baseline candidates on the basis of a ranking criterion;

identifying a highest ranked baseline candidate and a next highest ranked baseline;

computing a difference measure related to the difference between the highest ranked baseline candidate and the next highest ranked baseline candidate; and

selecting the highest ranked baseline candidate as the first baseline if the difference measure is above a predetermined threshold.

8. The method of claim 1, wherein the step of determining a second baseline between the second antenna and the third antenna using a forward integer search procedure responsive to the first baseline comprises steps of:

determining a plurality of attitude component estimates from the first baseline;

selecting a limited geometric region responsive to the plurality of attitude component estimates, vectors in the limited geometric region corresponding to candidate integer sets;

determining a second baseline responsive to the candidate integer sets corresponding to vectors in the limited geometric region.

9. A method for selecting a baseline from a plurality of baseline candidates comprising:

ranking the plurality of baseline candidates on the basis of a ranking criterion;

identifying a highest ranked baseline candidate and a next highest ranked baseline;

computing a difference measure related to the difference between the highest ranked baseline candidate and the next highest ranked baseline candidate; and

selecting the highest ranked baseline candidate if the difference measure is above a predetermined threshold.

10. The method of claim 9, wherein the difference measure is the difference of the magnitude of the least squares residuals of the baselines associated with the highest ranked candidate integer set and the next highest ranked candidate integer set.

11. A method of determining an inter-antenna baseline from estimates of two or more orientation angles of the baseline, the method comprising:

determining a limited geometric region from estimates of the two or more orientation angles of the baseline;

identifying a plurality of baseline candidate vectors in the limited geometric region from candidate integer sets that resolve an integer ambiguity associated with the inter-antenna baseline;

selecting the inter-antenna baseline from the plurality of baseline candidate vectors.

12. The method of claim 11, wherein the limited geometric region is determined by deviating the estimates of the two or more orientation angles by a predetermined amount.

13. The method of claim 11, wherein the step of selecting the inter-antenna baseline from the plurality of baseline candidate vectors comprises steps of:

ranking the plurality of baseline candidates on the basis of a ranking criterion;

determining a highest ranked candidate and a next highest ranked baseline candidate;

computing a difference measure related to the difference between the highest ranked baseline candidate and the next highest ranked baseline candidate; and

rejecting the plurality of baseline candidates if the difference measure is within a predetermined range; and

selecting the inter-antenna baseline from the plurality of baseline candidates if difference measure is outside the predetermined range.

14. The method of claim 13, wherein the difference measure is the difference of the magnitude of the least squares residuals of the baseline candidates.

15. The method of claim 11, wherein the step of selecting the inter-antenna baseline from the plurality of baseline candidate vectors comprises steps of:

evaluating the plurality of baseline candidate vectors with a baseline quality test; and

selecting the inter-antenna baseline from baseline candidate vectors passing the baseline quality test.

16. The method of claim 15, wherein the baseline quality test is based on the baseline'"'"'s magnitude.

17. The method of claim 15, wherein the baseline quality test is based on the baseline'"'"'s least squares residual.

18. The method of claim 15, wherein the baseline quality test is based on a weighted sum of the errors in the baseline'"'"'s computed magnitude, the magnitude of the baseline'"'"'s least squares residual, and an error in the computed angle formed with the first baseline, wherein some weights may be zero.

19. An apparatus configured to determine inter-antenna baselines from radio wave phase measurements;

the apparatus having a processor, and a memory, the apparatus comprising;

means for determining a first baseline between a first antenna and a second antenna using a backward integer search procedure, wherein the first antenna and the second antenna are located a distance D1 apart; and

means for determining a second baseline between the second antenna and a third antenna using a forward integer search procedure responsive to the first baseline, wherein the second antenna and the third antenna are located a distance D2 apart.

20. The apparatus of claim 19, wherein the distance D1 is less than about 2.5 wavelengths of a carrier wave that emanates from a Global Positioning System satellite.

21. The apparatus of claim 19, wherein the distance D2 is greater than about 1.0 wavelengths of a carrier wave that emanates from a pseudolite.

22. The apparatus of claim 19 further comprising:

means for determining a third baseline between a fourth antenna and one of the first antenna, the second antenna, and the third antenna, the third baseline being determined using a forward integer search procedure responsive to at least one previously-determined baseline.

23. The apparatus of claim 19 wherein the means for determining a first baseline between the first antenna and the second antenna using a backward integer search procedure comprises:

means for ranking a plurality of baseline candidates on the basis of a ranking criterion;

means for identifying a highest ranked baseline candidate and a next highest ranked baseline;

means for computing a difference measure related to the difference between the highest ranked baseline candidate and the next highest ranked baseline candidate; and

;

means for selecting the highest ranked baseline candidate as the first baseline if the difference measure is above a predetermined threshold.

24. The apparatus of claim 19, wherein the means for determining a second baseline between the second antenna and the third antenna using a forward integer search procedure responsive to the first baseline comprises:

means for determining a plurality of attitude component estimates from the first baseline;

means for selecting a limited geometric region responsive to the plurality of attitude component estimates, vectors in the limited geometric region corresponding to candidate integer sets;

means for determining a second baseline responsive to the candidate integer sets corresponding to vectors in the limited geometric region.

25. The apparatus of claim 23 wherein the difference measure is the difference of the least squares residuals of the baselines associated with the highest ranked candidate integer sets and the next highest ranked candidate integer sets.

26. An apparatus for determining inter-antenna baselines among at least a first antenna, a second antenna, and a third antenna from a plurality of carrier waves, the apparatus comprising:

a microprocessor;

at least one receiver configured to report the carrier wave phases sensed by the antennas;

a storage medium coupled to the microprocessor and storing code which configure the microprocessor to perform steps of;

detecting a sensed phase difference between the phases reported by the first receiver and the second receiver for each of the plural carrier waves;

identifying candidate integer sets constituting an integer ambiguity associated with a first baseline, the first baseline representing the relative position of the first antenna and the second antenna;

determining the first baseline from the sensed phases differences and the candidate integer sets wherein the integer ambiguity is resolved with a backward integer search procedure; and

determining a second baseline representing the relative position of the second antenna and the third antenna;

using a forward integer search procedure responsive to the first baseline.

27. An apparatus for selecting a baseline from a plurality of baseline candidates, the apparatus comprising:

a microprocessor;

a storage medium coupled to the microprocessor and storing program instructions that configure the microprocessor to perform steps of;

ranking the plurality of baseline candidates on the basis of a ranking criterion;

identifying a highest ranked baseline candidate and a next highest ranked baseline;

computing a difference measure related to the difference between the highest ranked baseline candidate and the next highest ranked baseline candidate; and

selecting the highest ranked baseline candidate if the difference measure is above a predetermined threshold.

28. An apparatus for determining an inter-antenna baseline from estimates of two or more orientation angles of the baseline, the apparatus comprising:

a microprocessor;

a storage medium coupled to the microprocessor and storing program instructions that configure the microprocessor to perform steps of;

determining a limited geometric region from estimates of the two or more orientation angles of the baseline;

identifying a plurality of baseline candidate vectors in the limited geometric region from candidate integer sets that resolve an integer ambiguity associated with the inter-antenna baseline;

selecting the inter-antenna baseline from the plurality of baseline candidate vectors.

29. The apparatus of claim 28, wherein the step of selecting the inter-antenna baseline from the plurality of baseline candidate vectors comprises steps of:

ranking the plurality of baseline candidates on the basis of a ranking criterion;

identifying a highest ranked baseline candidate and a next highest ranked baseline;

computing a difference measure related to the difference between the highest ranked baseline candidate and the next highest ranked baseline candidate; and

selecting the highest ranked baseline candidate if the difference measure is above a predetermined threshold.

30. The apparatus of claim 29, wherein the step of selecting the inter-antenna baseline from the plurality of baseline candidate vectors comprises steps of:

evaluating the plurality of baseline candidate vectors with a baseline quality test; and

selecting the inter-antenna baseline from baseline candidate vectors passing the baseline quality test.

31. The apparatus of claim 30, wherein the baseline quality test is based on the baseline'"'"'s magnitude.

32. The apparatus of claim 30, wherein the baseline quality test is based on the baseline'"'"'s least squares residual.

33. The apparatus of claim 30, wherein the baseline quality test is based on a weighted sum of the errors in the baseline'"'"'s computed magnitude, the magnitude of the baseline'"'"'s least squares residual, and an error in the computed angle formed with the first baseline, wherein some weights may be zero.

34. An antenna configuration comprising

a first antenna

a second antenna located a distance D1 from the first antenna, wherein the distance D1 is less than or equal to 1.75 wavelengths of a predetermined carrier wave;

a third antenna located a distance D2 from the second antenna, wherein the distance D2 is greater than or equal to 2.75 wavelengths of the predetermined carrier wave; and

a fourth antenna located a distance D3 from said first antenna and a distance D4 from said third antenna, wherein said distance D3 is greater than or equal to 3.0 wavelengths of the predetermined carrier wave and wherein said distance D4 is greater than or equal to 1.0 wavelengths of the predetermined carrier wave.

35. The antenna configuration of claim 34 wherein the predetermined carrier wave is a carrier wave from a Global Positioning System satellite.

37. The antenna configuration of claim 34 wherein said fourth antenna is noncolinear with said first antenna and said third antenna.

36. A GPS attitude system comprising

a plurality of antenna-receiver units configured to report phases from GPS carrier waves;

and

a processing system coupled to the plurality of antenna-receiver units configured to process the phases to compute attitude, the processing system configured to initialize with steps comprising;

identifying a first antenna, a second antenna, and a third antenna associated with the plurality of antenna-receiver units;

determining a first baseline between the first antenna and the second antenna using a backward integer search procedure; and

determining a second baseline between the second antenna and the third antenna or between the first antenna and the third antenna using a forward integer search procedure responsive to the first baseline.