Instantaneous GPS Attitude Tests: Cycle Ambiguity Resolved

Geoffrey Hazel and Stacey Smith

Abstract:	As a result of 6 years of research supported by the U.S. Coast Guard and the U.S. Army, Adroit Systems, Inc. (ASI) has developed a GPS based Attitude Determining technique. The technique is tested and proven and has been applied in numerous systems with a range of applications. One of these systems is a general navigation instrument that is currently commercially available for marine appli- cations. Other applications for which the technique has been implemented include general navigation, hydro- graphic surveying, and positioning of buoys, as well as pointing, targeting and communication satellite location for antenna pointing. GPS Attitude Determination is accomplished by carrier phase interferometry. The phase measurements from at least two satellites are taken at several antennas and then compared to compute attitude. The key difficulty with GPS attitude determination is cycle ambiguity, the unknown number of whole carrier cycles which will fit between a pair of GPS antennas. Conventional approaches to resolv- ing this ambiguity all require observation of the measure- ments over several epochs. By comparing measurements taken over time while the satellites and/or antennas move, candidate solutions are gradually eliminated and a solution is found. The problem with this approach is that it requires a continuous period of measurement of time-correlated phases in order to achieve a solution. This implies that the system requires a significant initialization period after power up as well as a recovery period following signal blockages or cycle slips which are in addition to the initial- ization and recovery times required for the receivers to regain phase tracking lock. ASI’s novel approach to attitude determination utilizes redundant measurements at a single epoch to achieve instantaneous attitude solutions without the need for ini- tialization or recovery times. Essentially, for each epoch in which the receivers can provide phase measurements, an attitude solution is computed and the cycle ambiguity is resolved, without dependence on previous measurement epochs. As a result the system recovers immediately from signal blockages and cycle slips and it provides attitude solutions at power up as soon as the receivers provide the first phase measurements. In addition, the algorithm includes a real-time measurement of confidence in the solution. This is used to ensure correct ambiguity resolu- tion. The algorithm presently provides attitude updates at a rate of 1Hz. This, however, is limited only by the measure- ment rate of the receivers. The algorithm has been proven to provide solutions at over 20Hz with that limitation removed. This paper presents a set of test results that demonstrate instantaneous cycle ambiguity resolution and attitude determination. The tests are designed to evaluate the sys- tem’s ability to compute attitude immediately upon mea- surement availability and to recover from tracking loss immediately. Static and pseudo-static test results are com- pared for short 17cm baselines (which do not suffer from cycle ambiguity) and longer 85x-t baselines. Accuracy levels are also measured and compared.
Published in:	Proceedings of the 7th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1994) September 20 - 23, 1994 Salt Palace Convention Center Salt Lake City, UT
Pages:	683 - 688
Cite this article:	Hazel, Geoffrey, Smith, Stacey, "Instantaneous GPS Attitude Tests: Cycle Ambiguity Resolved," Proceedings of the 7th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1994), Salt Lake City, UT, September 1994, pp. 683-688.
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