S-Band Transponder

Description

The purpose of this experiment, carried on the Apollo 15 mission, was to measure the lunar gravitational field, which in turn provided information on the lunar mass distribution and its correlation with surface features. The lunar gravitationaly field was measured by observing the dynamic motion of the spacecraft in orbit. The observational data were the precise radio tracking measurements used initially for real-time navigation. However, these line-of-sight velocity measurements could only be obtained while the spacecraft was in view of the Earth (thus, no farside data were available). The data were derived as follows -- a 2101.8-MHz radio signal was transmitted from the Earth to the spacecraft, where it was multiplied by a factor of 240/221 and retransmitted at the new frequency to Earth. At the earth, the initial transmitted frequency, multiplied by 240/221, was subtracted from the spacecraft signal. The motion of the spacecraft towards or away from the Earth caused a Doppler shift in the frequency, which could be used to estimate the velocity along the line of sight between the spacecraft and Earth. The resulting differences in cycle count, along with the time at which they were measured, were recorded. Because the fractional part of the cycle count was measured, the resolution was 0.01 Hz or 0.6 mm/sec. Because of an intermittent signal and poor coverage, only an insignificant amount of data resulted from this experiment.