Ion and Neutral Wind Vectors back
Vector drift mesasurements of both the ion and neutral gases utilize the fact that the orbital motion of the satellite is much larger than the thermal velocity and the ambient bulk flow of the gas.
In each case the instruments are mounted to view approximately parallel to the satellite velocity. Then a measurement of the kinetic energy of the gas along the sensor look direction can be used to derive the gas velocity along that direction. This measurement, called Vram, is accomplished using a retarding potential analyzer.
The gas velocity perpendicular to the sensor look direction is obtained by measuring the angle of arrival of the gas with respect to two mutually perpendicular directions perpendicular to the look direction.
These so-called cross-track velocity components, called Vvert and Vtran, are calulated using a simple geometric relationship shown below.