A Novel Sensor for Attitude Determination Using Global Positioning System Signals

An entirely new sensor approach for attitude determination using Global Positioning System (GPS) signals is proposed. The concept involves the use of multiple GPS antenna elements arrayed on a single sensor head to provide maximum GPS space vehicle availability. A number of sensor element configurations are discussed. In addition to the navigation function, the array is used to find which GPS space vehicles are within the field-of-view of each antenna element. Attitude determination is performed by considering the sightline vectors of the found GPS space vehicles together with the fixed boresight vectors of the individual antenna elements. This approach has clear advantages over the standard differential carrier-phase approach. First, errors induced by multipath effects can be significantly reduced or eliminated altogether. Also, integer ambiguity resolution is not required, nor do line biases need to be determined through costly and cumbersome self-surveys. Furthermore, the new sensor does not require individual antennas to be physically separated to form interferometric baselines to determine attitude. Finally, development potential of the new sensor is limited only by antenna and receiver technology development unlike the physical limitations of the current interferometric attitude determination scheme. Initial simulation results indicate that accuracies of about 1 degree (3σ) are possible. .Copyright c 1998 by John L. Crassidis. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. Introduction The Global Positioning System (GPS) constellation was originally developed to permit a wide variety of user vehicles an accurate means of determining position for autonomous navigation. The constellation includes 24 space vehicles (SVs) in semi-synchronous (12 hour) orbits, providing a minimum of six SVs in view for ground-based navigation. The underlying principle involves geometric triangulation with the GPS SVs as known reference points to determine the user’s position to a high degree of accuracy. The GPS was originally intended for ground-based and aviation applications, gaining much attention in the commercial community (e.g., automobile navigation, aircraft landing, etc.). However, in recent years there has been a growing interest in space-based applications. Since the GPS SVs are in approximately 20,000 km circular orbits, the position of any potential user below the constellation may be easily determined. A minimum of four SVs are required so that in addition to the three-dimensional position of the user, the time of the solution can be determined and in turn employed to correct the user’s clock. Since its original inception, there have been many innovative improvements to the accuracy of the GPS determined position. These include using local area as well as wide area differential GPS, carrier-phase differential GPS, and so-called “pseudolites” (groundbased GPS transmitters). In particular, carrier-phase differential GPS measures the phase of the GPS carrier relative to the phase at a reference site, which dramatically improves the position accuracy. Also, for spacecraft applications dynamically aided GPS using AIAA-98-4228