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From earth‐based Doppler and interferometric radio observations we determined the paths, in three dimensions and as functions of time, taken by the Pioneer probes as they fell to the surface of Venus. From the motion of each probe below about 65‐km altitude we were able to infer the ambient wind velocity with an estimated uncertainty of about 1 m s−1 in each vector component. The magnitude of the velocity was about 1 m s−1 or less near the surface of the planet and about 100 m s−1 near 65‐km altitude at all four probe locations. Distinct strata of high wind shear were centered at altitudes of 15, 45, and 60 km, where the atmosphere is most stable against verticle motion. Except within a few kilometers of the surface, the wind velocity was always directed within a few degrees of due west. At the day and the night probe sites, which were separated by 100° in longitude and 3° in latitude, the altitude profiles of the westward velocity were virtually identical. Thus the dominant motion of the lower atmosphere seems to be a retrograde zonal rotation. Eddies appear to account for most of the instantaneous meridional velocity. However, in the radiatively heated middle cloud layer, between 50‐ and 55‐km altitude, equatorward flow of 1–7 m s−1 was observed for all four probes. These observations, coupled with other observations of ∼10 m s−1 poleward average meridional velocity for cloud top features at about 65‐km altitude, suggest that within the clouds a thermally driven mean meridional circulation is superimposed on the much more rapid zonal rotation.

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The 1.24‐km base line vector between the two antennas of the Haystack Observatory was determined from X band radio interferometric observations of extragalactic sources via a new method that utilizes the precision inherent in fringe phase measurements. This method was employed in 11 separate experiments distributed between October 1974 and January 1976, each being between about 5 and 20 hours in duration. The rms scatters about the means for the vertical and the two horizontal components of the base line obtained from the 11 independent determinations were 7, 5, and 3 mm, respectively. The corresponding scatter for the base line length was 3 mm; the mean differed from the result obtained in a conventional survey by 8 mm, well within the 20‐mm uncertainty of the survey. (The determination of the direction from the survey was too crude to be useful.) Another external check on our data was possible, since the azimuth and elevation axes of one of the antennas do not intersect but are separated by 318 mm. We estimated this horizontal offset from the radio interferometry data and found a difference of 10 ± 9 mm from the directly measured value, the relatively large rms scatter being due to the ∼0.96 correlation between the estimate of this offset and that of the vertical component of the base line. Use of a newly completed calibration system in future experiments should allow the scatter to be reduced to the millimeter level in all coordinates for short base lines. For long base lines, such repeatability should be degraded only to about the centimeter level if calibrated observations with sufficient sensitivity are made simultaneously at two frequency bands. An assessment of the accuracy of either our present or future base line results awaits the availability of an accepted, more accurate, standard for comparison. Nonetheless, base line changes can be determined reliably at any established level of repeatability.

We consider the theoretical and the practical aspects of using radiointerferometric observations of the Global Positioning System (GPS) satellites to establish a three‐dimensional, multistation, geodetic control network. We discuss various ways of processing GPS data and try them with observations from a 35‐station network in Germany. We examine the reproducibility of interferometric determinations of individual baseline vectors and the three‐dimensional vector closure of subnetworks and the whole network. We compare these measures of precision with corresponding predictions based on statistics of interferometric phase residuals. We conclude that the relative positions of stations in the network were determined by GPS interferometry within about 1 part per million (ppm) in both horizontal coordinates and about 1.6 ppm in the vertical. As an external test of horizontal accuracy, we compare baseline lengths with electro‐optical distance measurements. The vertical accuracy is tested against determinations, by means of spirit leveling and gravimetry, of the height differences between stations. The differences between the interferometric and the independent determinations are consistent with the uncertainties of the latter. Interferometry with GPS is by far the most efficient method of establishing geodetic control on local and regional scales. This is already true, even though the constellation of satellites is incomplete. The accuracy of regional control by GPS should improve to about 0.1 ppm when interferometry is also used to determine the satellite orbits.