Attitude determination from a codeless GPS signal processing system

This work describes a codeless GPS receiver designed to determine the yaw of a low Earth orbit satellite. The GPS signals are received at two directional, moderately high gain antennas at opposite ends of a structurally fixed baseline on the user satellite. The relative phase difference between the two signals received from each GPS satellite is directly dependent on the user satellite's attitude. In order to effectively de-spread and process the GPS satellites 1575.42MHz signals without knowledge of the codes used for the spread spectrum generation, the GPS satellite's signals are squared to produce the second harmonic of the suppressed carrier (2L1). The 2L1 signals from the GPS satellites in view are converted into a low frequency band for signal processing and identification. Each GPS source is distinctly identified by the Doppler shift and Doppler rate caused by the motion of the GPS satellite's relative to the user satellite. Ground based demonstration results are presented from a one meter antenna baseline.