S-Band Transponder

Description

The purpose of this experiment, carried on the Apollo 14 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 gravitational field was measured by observing the dynamical motion of a spacecraft in free fall orbits. The observational data were the precise earth-based 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 240/211 and retransmitted to Earth at the new frequency. The initial transmitted frequency, multiplied by 240/211, was subtracted from the signal from the spacecraft. 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 cycle count-differences were recorded, along with the time at which they were measured. Because the fractional part of a cycle count was measured, the resolution was 0.01 Hz or 0.6 mm/sec.