Journal of Global Positioning Systems (2002) Vol. 1, No. 2: 85-95 GPS Attitude Determination Reliability Performance Improvement Using Low Cost Receivers Chaochao Wang and Gérard Lachapelle Department of Geomatics Engineering, University of Calgary Received: 13 October 2002 / Accepted: 4 December 2002 Abstract. This paper describes different methods to improve reliability of attitude estimation using low cost GPS receivers. Previous work has shown that low cost receiver attitude determination systems are more susceptible to measurement errors, such as multipath, phase center offsets, and cycle slips. In some cases, these error sources lead to severely erroneous attitude estimates and/or to a lower availability. The reliability control in the attitude determination becomes imperative to users, as most attitude applications require a high level of reliability. The three methods tested herein to improve reliability are the use of a high data rate, fixed angular constraints, and a quality control algorithm implemented with a Kalman filter. The use of high rate measurements improves error detection as well as ambiguity fixing time. Fixed angular constraints in a multi-antenna attitude system is effective to reject incorrect solutions during the ambiguity resolution phase of the process. Utilizing a Kalman filter with a high data rate, e.g. 10 Hz, not only increases reliability through an increase of information, but also can improve accuracy and availability. The simultaneous utilization of the above methods significantly improves reliability, as demonstrated through a series of hardware simulations and field tests. The low cost receiver type selected is the CMC Allstar receiver equipped with a commercially available low cost antenna. Finally, the use of statistically reliability measures, namely internal and external reliability measures, shows the inherent limitations of a low cost system and the need to either use better antennas and/or external aiding in the form of low cost sensors. Key words: GPS, Attitude Determination, Low Cost Receiver 1 Introduction Multi-antenna GPS systems provide a high accuracy attitude solution without error drift over time [e.g., Lu 1994]. The performance of GPS attitude determination is a function of receiver firmware, satellite geometry, antenna carrier phase stability, multipath rejection ability and inter-antenna distances. With advances in GPS receiver technology, low cost receivers equipped with phase lock loops that output precisely time-synchronized carrier phase measurements are now available on the market. The use of this grade of GPS receiver for attitude determination has proven feasible [e.g., Hoyle et al. 2002]. However, it has been found that multipath, antenna phase center offsets and cycle slips are major error sources that mitigate the performance of low cost receiver attitude solutions. In worst-case scenarios, these errors severely affect the integrated carrier phase measurements and lead to incorrect attitude estimates in attitude determination. Therefore, the reliability of attitude estimation becomes a major issue. The objective of this paper is to investigate three methodologies to improve the reliability performance of attitude determination using low cost receivers. Three different schemes, namely the use of high rate carrier phase measurements, fixed angular constraints and a Kalman filter with a statistical quality control system, are used interactively to improve reliability. These schemes are implemented in a high performance, open architecture attitude determination software, namely HEADRT+ TM , for testing [e.g. Hoyle et al 2002]. The performances of different methods are examined both in hardware simulation mode and under field static and kinematic conditions.