A Single-Baseline Attitude Measurement System Using Low-Cost C/A-Code Receivers

This paper describes a single-axis, dual-antenna, GPS attitude measurement system developed by SRI International to determine the azimuth and elevation angles of military weapons, such as the 120-mm gun on the Abrams tank, during training exercises. The pointing angle is used to pair the weapon with the intended target. The system uses low-cost, six-channel, commercially available, single-frequency, C/A-code GPS receivers. No attempt is made to synchronize the receivers’ clocks. Antenna separation is approximately six wavelengths. Ambiguity resolution is achieved using a modified Hatch integer search algorithm. Resolution is facilitated by a clinometer that provides approximate elevation angle information when the vehicle and weapon are stationary; angular motion limits the search sector when the weapon is moving. With six satellites in track, ambiguities are almost always resolved in less than 100 seconds and are typically resolved in less than 10 seconds. Once angular “track” has been achieved, the validity of the integer set is revalidated each epoch by monitoring the residuals, the computed baseline length, and the elevation or azimuth sector limits that can be inferred from the clinometer and from relative antenna motion. At the end of the paper, the results of this constrained design are compared with results achieved using longer baselines, better receivers, and better multipath conditions.