urn:nasa:pds:context:instrument:pvo.oad 1.0 1.20.0.0 Product_Context 2024-03-04 1.0 Initial creation of the context products urn:nasa:pds:context:instrument_host:spacecraft.pvo instrument_to_instrument_host 10.1126/science.203.4382.772 Keating, G. M., et al., Venus thermosphere and exosphere: First satellite drag measurements of an extraterrestrial atmosphere, Science, 203, No. 4382, 772-774, Feb. 1979. reference.KEATINGETAL_1979a 10.1126/science.205.4401.62 Keating, G. M., et al., Short-term cyclic variations and diurnal variations of the Venus upper atmosphere, Science, 205, No. 4401, 62-64, July 1979. reference.KEATINGETAL_1979b 10.1007/BF02186463 Colin, L., Ed., and D. M., Ed. Hunten, Pioneer Venus experiment descriptions, Space Sci. Rev., 20, No. 4, 451-525, June 1977. reference.COLINETAL_1977 ORBITER ATMOSPHERIC DRAG (OAD) FOR PIONEER VENUS Radio Science not applicable not applicable INSTRUMENT OVERVIEW =================== 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.