System for, and method of, determining the speed of an airborne vehicle

This system measures the speed of an airborne vehicle relative to the surrounding atmosphere. The measurement is based on the scattering of pulses of coherent laser radiation, generated in the vehicle, preferably in the infrared region of the electromagnetic spectrum, by particles naturally present in the atmosphere at all times. The pulses are focused into the atmosphere at a sufficient distance from the vehicle, preferably 10-30 meters, to be beyond that region perturbed by the passage of the vehicle. The frequency of the radiation scattered by the particles differs from the frequency of the transmitted pulses by virtue of the relative motion of the vehicle and the atmosphere. Equipment in the vehicle digitally processes the received energy to determine this frequency difference for each pulse, and hence the component of the vehicle's velocity in the direction of the pulse transmission. Successive pulses are transmitted into the atmosphere in differing directions lying on the surface of a cone whose axis is fixed with respect to the vehicle, making possible the vectorial determination of the vehicle's relative motion. This conical scan is repeated without interruption over successive cycles of pulses. In determining the vehicle's velocity vector from the measured velocity components, account is taken, through weighting factors, of the statistically variable quality of the individual measurements from successive pulses. These weighting factors are derived from the properties of the measurements themselves and are applied to the data to enhance both accuracy and continuity of information.