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Atmospheric Drag (OAD)

NSSDCA ID: 1978-051A-19

Mission Name: Pioneer Venus Orbiter
Principal Investigator:Dr. Gerald M. Keating


The Orbiter Atmospheric Drag (OAD) experiment made use of the spacecraft S-band and X-band radio signals for data measurements. The objectives were (1) to establish the diurnal variation of thermospheric density and density scale height (2) to determine the relationship of solar wind variations to variations in atmospheric density, (3) to determine the relationship of long and short term variation in solar extreme UV radiation to density variations, (4) to search for phenomena such as a semi-annual variation and super rotation of the thermosphere, and (5) to formulate a thermospheric model for the Venusian atmosphere.

The OAD experiment worked by measuring the Doppler shift in X and S band transmissions to the Deep Space Network. The shift would give the spacecraft acceleration along a line-of-sight from the spacecraft to the receiver. The accelerations at periapsis, when the orbiter was in the region of highest atmospheric density and the drag effect was greatest, were used, along with the spacecraft mass, and spacecraft orientation (giving the cross-section), to give a drag coefficient. This, combined with atmospheric models, gave an estimate of the atmospheric density at that altitude. Since the periapsis altitude changed with time, different depths in the atmosphere would be sampled. The effects of the drag on the evolution of the orbital period and inclination were also used with, and fed into, an atmospheric model. The results could be used to study diurnal and semi-annual variations, variations with solar activity, and possibly the super-rotation of the upper atmosphere.

Pioneer Venus Orbiter Radio System

1.09 m diameter high-gain parabolic dish antenna mounted on a 2.9 meter mast on the top deck of the spacecraft. It is despun from the rotating spacecraft so that it always faces Earth. It is also capable of being pointed up to 15 degrees from the Earth during Venus atmospheric occultations, to account for the refraction of the beam as it passed through the atmosphere. It operated in both S and X bands. At S-band, the antenna beam is 7.6 degrees. The orbiter carries a 750 mW X band (8.415 GHz) transmitter and a 2.295 GHz S band transmitter. The frequencies are set so that the X-band frequency is 11/3 of the S band.

Alternate Names

  • OAD
  • PioneerVenusOrbiter/OAD


  • Planetary Science: Atmospheres

Additional Information

Questions and comments about this experiment can be directed to: Dr. Dieter K. Bilitza



NameRoleOriginal AffiliationE-mail
Dr. Gerald M. KeatingPrincipal InvestigatorNASA Langley Research Center

Selected References

  • Keating, G. M., et al., Venus thermosphere and exosphere: First satellite drag measurements of an extraterrestrial atmosphere, Science, 203, No. 4382, 772-774, doi:10.1126/science.203.4382.772, Feb. 1979.
  • Keating, G. M., et al., Short-term cyclic variations and diurnal variations of the Venus upper atmosphere, Science, 205, No. 4401, 62-64, doi:10.1126/science.205.4401.62, July 1979.
  • Colin, L., Ed., and D. M., Ed. Hunten, Pioneer Venus experiment descriptions, Space Sci. Rev., 20, No. 4, 451-525, doi:10.1007/BF02186463, June 1977.
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