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Satellite-to-satellite Doppler tracking between the ATS 6 and the GEOS 3 spacecraft is used to measure the high-degree and -order gravity field over an 80 ø area in the central Pacific Ocean. Doppler tracking provides velocity measurements of GEOS 3 as a function of time, which are easily converted to line-ofsight accelerations. Since the low-degree and -order gravity field is known well, each GEOs 3 revolution is reduced relative to a twelfth-degree and -order field model. The method is then capable of measuring wavelengths in the range from about 3500 to 1000 kin. Accelerations computed along 40 revolutions, roughly half of them descending and the others ascending, can be contoured into a map of the gravity field at the GEOS 3 altitude of 840 kin. The dominant wavelength of the map is about 2000 km. A comparison of this field with an altimeter-derived geoid and the GEM 10B gravity field shows good agreement. Since each map is essentially independent of the others, this comparison is critical in establishing the validity of any of the maps. Each map shows virtually the same areas of positive and negative anomalies. These areas are essentially the same in the central Pacific as those found in our earlier study. Compared with surface amplitudes these anomalies are only one-tenth as large at the GEOS 3 altitude. This observation and the correlation of gravity with residual depth anomalies imply that the cause of some of these anomalies may be due to bumps in the upper surface of the lithosphere that are compensated at depths of about 30-100 km. Some of the anomalies apparently do not correlate with residual depth anomalies.

The satellite to satellite tracking (SST) technique has been used to measure the earth's gravity field. Data from the Apollo/Soyuz and GEOS 3 missions, obtained at greatly differing altitudes, have been used in combination to determine 5° and 10° equal area mean gravity anomalies in the South Atlantic and Indian Oceans and over the African continent. The Apollo/ATS 6 SST data consist of 47 passes of data taken from the 230‐km Apollo altitude, while the GEOS 3/ATS 6 data set consists of 45 passes taken from 840‐km altitude. Through a combination of the two data sets with the GEOS 3 SST data providing stability for the solution at the longer wavelengths, excellent (5° at ±3 mgal, 10° ±1 mgal) results were obtained for this largely unsurveyed region. These results have been compared to an altimetric surface at comparable wavelengths, providing good agreement (±1 mgal for 10° blocks and ±5 mgal for 5° blocks) between these independent measuring techniques. The terrestrial data over the African continent is shown to differ significantly (rms difference of 8 mgals with a maximum difference of 23 mgals) from a comparison of 10° GEOS 3 SST derived blocks. A simulation has also been performed to assess the data requirements, aliasing, and noise characteristics of a low spacecraft being tracked in a high‐low SST configuration. The results of the simulation were employed in the design of the SST data reduction procedures.

Summary A geodynamics experiment was performed in 1975 July during the joint US/USSR Apollo/Soyuz Mission. The experiment's main purpose was to demonstrate that the‘high–low’satellite‐to‐satellite tracking (SST) technique could be used to resolve the high‐frequency components of the Earth's gravitational field. During this experiment, expected high‐frequency‐range‐rate changes were observed between the‘high’Applications Technology Satellite‐6 (ATS‐6) in a geosynchronous orbit and the‘low’flying Apollo spacecraft in a nearcircular, near‐Earth orbit. A total of 348 5°× 5° mean free air gravity anomalies have been recovered using data from 30 processed Apollo orbits (52° N to 52° s) and the area covered by the ATS‐6 spacecraft (64° W to 116° E). A comparison with anomalies independently determined from only that GEOS‐3 altimeter data which were available in the same area was found to be in agreement within 7 mgals. This estimated accuracy includes the effects of unadjusted anomalies on the solution. The magnitude of this‘aliasing’effect is still under study.