On-orbit measurement of TOPEX/Poseidon altimeter antenna pattern

The NASA and CNES altimeters on the TOPEX/Poseidon satellite share a 1.5-m antenna. Early data from the NASA altimeter suggested that the beam was broader than measured preflight. An altimeter transponder was modified to output a relative measurement of received power. The instrument is briefly described. The instrument was deployed on the TOPEX ground track at the coast near Los Angeles, California. Measurements from three of these deployments are presented to show the on-orbit antenna pattern. The measured pattern is effectively broader than preflight in the central region, particularly the part of the pattern which corresponds to the tail of the altimeter waveform where the attitude is determined. This result is consistent with the general shape of both the TOPEX and Poseidon waveforms. As the TOPEX corrections which depend on values from the tail of the waveform have been compensated for deviations from the preflight measurements, no appreciable effect on the final data is expected.