Development of an artificial radio transmitter on the Moon for precise measurement of lunar physical librations, tides and gravity fields

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We have developed an artificial radio transmitter for differential VLBI in order to measure lunar physical librations and lunar tides precisely. We completed the total aystem including a 2.2GHz transmitter, a battery package, a timer and an antenna by the end of 1992, and confirmed that the frequency stability is better than 10 ^<-6> and that the system can survive under acceleration of 10,000g (1g=9.8ms ^<-2>), which is equivalent to that received when the transmitter dropped from a lunar orbiter on the altitude of 200Km collides to the lunar surface. We prepared sands which have similar electrical properties to the lunar regolith, and measured radiation patterns and intensities of CW radio signals emitted from the transmitter buried in the sands by changing the depth and the agale between the anttena surafce and the sand surface. As the results, we could obtain the condition which the transmitter should satisfy on the Moon.This research is characterized by 1)development of the radio transmitter tolerating a hard landing on the lunar surface and 2)development of a method of the differential VLBI which can measure an angular distance between two radio sources on the Moon precisely, eliminating the effect of atmospheric fluctuation. We confirmed that the differential VLBI can determine the amplitude of the lunar physical librations with an accuracy of 10 ^<-5>, which is 10 times better than the previous results, and that of the lunar tides with an accuracy of 10 ^<-1>, respectively.This radio transmitter will have great power and possibilities and wide applications in planetary science. The method of setting the radio transmitter on the lunar surface by hard landing has been established and a new approach into the planetary science has been prospected through this research.