The VEGA balloons made in situ measurements of pressure, temperature, vertical wind velocity, ambient light, frequency of lightning, and cloud particle backscatter. Both balloons encountered highly variable atmospheric conditions, with periods of intense vertical winds occurring sporadically throughout their flights. Downward winds as large as 3.5 meters per second occasionally forced the balloons to descend as much as 2.5 kilometers below their equilibrium float altitudes. Large variations, in pressure, temperature, ambient light level, and cloud particle backscatter (VEGA-1 only) correlated well during these excursions, indicating that these properties were strong functions of altitude in those parts of the middle cloud layer sampled by the balloons.

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[1] In spacecraft borne radar investigations of planets such as Venus and Mars the waves must penetrate an ionospheric layer which causes absorption and dispersive phase delay if the waves reach the surface at all. If the purpose of the radar system is to explore the planetary subsurface, the radar frequency should be as low as possible for maximum skin depth, yet high enough to reach the surface. In order to resolve subsurface discontinuities the radar must use short pulses and because of the low frequency the dispersive pulse distortion in the ionosphere becomes a problem. In this paper we discuss ways to avoid the pulse distortion and to recover the original pulse shape. As the ionosphere is unknown and changes with time and position, the necessary ionospheric data must be derived from the radar observations themselves. The scheme described is designed to do this. We also discuss the effect of additive noise on the accuracy of this observation scheme. The investigation was motivated by the MARSIS experiment on Mars Express and by a similar experiment on the Russian spacecraft Mars96. The implementation of the scheme in MARSIS will depend on the computational resources which can be allocated to the task. In future space missions with the proper planning it is thought that the scheme presented will be even more attractive.

A global array of 20 radio observatories was used to measure the three-dimensional position and velocity of the two meteorological balloons that were injected into the equatorial region of the Venus atmosphere near Venus midnight by the VEGA spacecraft on 11 and 15 June 1985. Initial analysis of only radial velocities indicates that each balloon was blown westward about 11,500 kilometers (8,000 kilometers on the night side) by zonal winds with a mean speed of about 70 meters per second. Excursions of the data from a model of constant zonal velocity were generally less than 3 meters per second; however, a much larger variation was evident near the end of the flight of the second balloon. Consistent systematic trends in the residuals for both balloons indicate the possibility of a solar-fixed atmospheric feature. Rapid variations in balloon velocity were often detected within a single transmission (330 seconds); however, they may represent not only atmospheric motions but also self-induced aerodynamic motions of the balloon.