Techniques for measuring bulk gas-motions from satellites

There is a growing appreciation for the dynamic nature of the upper atmosphere. This paper is concerned principally with discussing the methods for making in-situ measurements of the ambient gas velocities from satellites. Both neutral and ion instrumentation is discussed, but most attention is paid to the ion sensors on Atmosphere-Explorer-C, which are now providing the first detailed three-dimensional gas-velocity measurements from a satellite. While some data are presented, emphasis is placed on experimental difficulties and possible ways of alleviating these problems. It seems reasonable to expect that both neutral and ion gas velocities will be measured to an accuracy of the order of 20 m/s in future satellites, provided that a star tracker is available for the determination of vehicle attitude. The possible exception to this is the ram component of the neutral-gas velocity vector, for which no proven instrumentation yet exists.