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S-Band Transponder

NSSDCA ID: 1972-031D-03

Mission Name: Apollo 16 Subsatellite
Principal Investigator:Mr. William L. Sjogren


The purpose of this experiment (S-164) was to measure the lunar gravitational field based on dynamical motion of the spacecraft in free-fall orbit by precise earth-based radio tracking measurements. A stable frequency of 2115 MHz obtained from a cesium reference was transmitted to the subsatellite which transponded the received signal, after multiplying it by the constant 240/221 (to avoid self-lockup), back to Earth. At the Earth receiver, the initial transmitted frequency was multiplied by 240/221 and subtracted, and the resulting cycle-count differences were accumulated for data reduction. 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. Because the fractional part of a cycle count was measured, the resolution was 0.01 Hz, or 0.6 mm s. For a 5-day period after May 9, 1972, subsatellite periapsis altitudes near 12 km provided new detailed gravity measurements of many near-side features such as Copernicus, Sinus Medii, and Mare Fecunditatis.

Alternate Names

  • Apollo16Subsatellite/S-BandTransponder
  • S164

Funding Agency

  • NASA-Office of Manned Space Flight (United States)


  • Planetary Science: Geology and Geophysics

Additional Information

Questions and comments about this experiment can be directed to: Dr. David R. Williams



NameRoleOriginal AffiliationE-mail
Mr. William L. SjogrenPrincipal InvestigatorNASA Jet Propulsion

Selected References

  • Sjogren, W. L., et al., S-band transponder experiment, in Apollo 16, Prelim. Sci. Rept., NASA SP-315, 24-1, Wash., DC, 1972.
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