Apollo 15 Mission

Science Experiments - S-Band Transponder

The S-Band Transponder Experiment measured regional variations in the Moon's gravitational acceleration.

The S-band Transponder Experiment was performed on Apollo 14, 15, 16, and 17. In this experiment, the frequency of radio waves transmitted by the spacecraft was accurately measured at Earth and compared with the frequency of the waves as transmitted by the spacecraft. Changes in the frequency of the radio waves occur because the spacecraft is moving, a phenomenon known as the Doppler effect. By measuring the frequency of the radio waves at the spacecraft and at Earth, the spacecraft's velocity can be determined with very high accuracy. Changes in the spacecraft's velocity are due to the force of gravity acting on the spacecraft. The primary gravitational influence on the spacecraft is the Moon's gravity. Other objects, particularly the Earth and Sun, also affect the spacecraft, but the contributions of these objects can be readily calculated. As a result, this experiment provided maps of how the Moon's gravity varies with location across its surface. In turn, this information was used by geophysicists to study how material is distributed inside the Moon. Among the features studied using the Apollo observations are impact basins such as Crisium, Imbrium, Nectaris, and Serenitatis, impact craters such as Copernicus, Ptolemaeus, and Theophilus, the Apennine Mountains, and the Marius Hills.