Attitude Sensing Using a Global-positioning-system Antenna on a Turntable

A new attitude sensor is proposed that consists of a single global positioning system (GPS) antenna mounted on a turntable with its phase center offset from the turntable's spin axis. It is being considered as a means of sensing 3-axis attitude information. It is attractive because its GPS receiver could have a low number of channels and, therefore, be smaller and use less power. The system senses attitude by demodulation of periodic oscillations of the GPS carrier phase. These oscillations are caused by the turntable’s rotation, and their amplitude and phase depend on the direction vector to the tracked GPS satellite. This system is described in detail, its demodulation phase-locked loop is designed, and its performance is analyzed and evaluated via simulation. The computer simulation results show that, when using a turntable radius of 0.1 m, a rotation rate of 4,000 RPM, and an ovenized crystal oscillator for the receiver clock, the system can sense vector attitude with a 1-σ accuracy of 1.4 deg at a bandwidth of 0.64 Hz. Accuracy can be improved by increasing the turntable radius or by reducing multi-path reflections. Introduction Many different air, space, and marine vehicles need a 3-axis attitude determination system, and various types of sensor data can be used to determine the roll, pitch, and yaw orientations. The measured carrier phase of a Global Positioning System (GPS) signal is one such data type . Attitude sensing based on GPS signals is attractive for several reasons. One is that a GPS receiver often is already part of a system because of its ability to sense position and velocity. If it can be made to sense attitude, then there will be a weight and power savings for the overall system because no additional attitude sensors will be needed. Alternatively, a GPS-based system can provide attitude determination redundancy. Yet a third attractive feature of GPS-based attitude sensing is the continuous availability of its signal. In low-Earth-orbit spacecraft applications, attitude data from sun sensors and from horizon sensors may not be ∗ Associate Professor, Sibley School of Mech. & Aero. Engr. Associate Fellow, AIAA. Copyright  2000 by Mark L. Psiaki. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. continuously available , but a well designed GPSbased system will not have this problem. The standard GPS-based attitude sensing method uses multiple GPS antennas that are spatially distributed on the user vehicle. The receiver measures the carrier phase differences of the signal from a given GPS satellite. Each phase difference between an antenna pair gives the cosine of the angle between the vector to the GPS satellite and the vector from one antenna to the other. The former vector is known in inertial coordinates; the latter vector is known in vehicle body coordinates. Given enough of these cosine measurements, the full 3-axis attitude of the vehicle can be determined. The minimum number of receiver antennas is 3, and the minimum number of received GPS signals is 2 . There are several difficulties with the standard approach. One is the need to resolve phase ambiguities, which are integer cycle uncertainties in the carrier phase differences. Another problem is that the receiver may need to have many channels, one per antenna per satellite. A system that uses 4 antenna to track 6 GPS satellites might require 24 channels. This can require a high processor speed, which can increase the receiver's weight and power consumption. Yet a third problem with the traditional approach is that cosine-type attitude measurements are more difficult to use in a full 3-axis solution procedure than are vector-type measurements . Alternate schemes have been pursued for doing GPS-based attitude determination using fewer than 3 antennas . The idea of Ref. 4 is to compute a pseudo-attitude based on the usual relationships between acceleration, velocity, and attitude for an aircraft. References 5 and 6 use two GPS antennas that are mounted on a spinning satellite. Their approach makes use of the known dynamics of a spinning, nutating spacecraft and deduces 3-axis attitude and attitude rate from the carrier phase differences between the signals that are received at the two antennas. The present work presents a new way to use a single GPS antenna to sense 3 axis attitude information. A patch-type antenna is mounted on a spinning turntable. Its phase center is mounted off-axis, so that it translates around a circle as the turntable rotates. Its field of view is centered on the turntable's rotation axis so that its geometric gain pattern's inertial orientation does not vary significantly with table rotation. Figure