GPS-Based Attitude Determination for a Spinning Rocket

An algorithm is developed for determining the attitude of a spinning sounding rocket. This algorithm is able to track signals with low availability duty cycles, but with enough accuracy to yield phase observables for the precise, 3-axis attitude determination of a nutating rocket. Raw GPS RF front-end data are processed by several filters to accomplish this task. First, a Levenberg-Marquardt algorithm (LMA) batch filter performs a leastsquares fit on a bank of correlators to generate GPS observables for multiple satellites. These observables are then used as measurements in a Rauch-TungStriebel (RTS) smoother that optimizes estimates of carrier phase, Doppler shift, and code phase. Finally, attitude determination is carried out by another batch filter that uses the single-differenced optimizal carrier phase estimates between two antennas. The batch filter itself is a combination of a substantially modified form of the LMA and the Least-Squares Ambiguity Decorrelation (LAMBDA) method. It is configured to deal with integer ambiguities that are constantly changing due to the long data gaps between times of carrier phase availability. The algorithm presented in this paper is applied to recorded RF data from a spinning sounding rocket mission in order to produce attitude quarternion and spin-rate estimates. These results are confirmed by another set of quaternions and spin-rate vectors independently estimated from magnetometer and horizon crossing indicator data.