Effect of Phase Pattern of Antennas Onboard Flying Spin Satellites on Doppler Measurements

The effect of the phase patterns of antennas onboard flying spin satellites on the Doppler measurements is reported. Phase patterns mean that there are deviations in wave fronts from perfect sphericity, expressed as a function of the angular position around the antenna. We analyzed what effect the phase patterns of dipole and patch antennas onboard two flying spin satellites, Rstar and Vstar, used in the Japanese lunar mission, SELENE (KAGUYA), had on 2-way and 4-way Doppler measurements, and detected higher harmonics in the spin frequency up to an order of 26 in the Doppler frequency. We developed a low-pass filter (LPF) using a Kaiser window, with the optimal parameters empirically determined, to remove the influence of phase patterns and to precisely conserve information on the lunar gravity field. We processed the 2-way and 4-way Doppler data of SELENE by using LPF. After using LPF, a high degree of accuracy of about 0.001 Hz was achieved for the 2-way Doppler measurements, and signals that reflected the gravity field on the far side of the Moon were first detected from the 4-way Doppler data. We also suggested a method for estimating the phase response of satellite antennas using the Doppler frequency variations. In order to estimate the Doppler frequency variation, a filtering technique was adopted to extract the harmonics of interest in the residual signal, from which the antenna phase pattern was derived.