Geo-Location Using Direction Finding Angles

254 INTRODUCTION The detection, classification, and geo-location of ground-based signal sources is one of the primary missions of modern military surveillance aircraft. Example signals of interest are the search radar of an enemy missile site, the radio communications between enemy combatants, and the distress radio beacon being transmitted by a downed friendly pilot. The ability to locate such signal sources is of obvious importance, especially during times of conflict. The geo-location of ground targets is performed by surveillance aircraft using the output of sensors attached to the aircraft. In order to determine the target’s location on Earth, the sensor measurements are processed by some form of estimation algorithm. This geo-location algorithm provides both a location estimate and an estimation error covariance matrix. The covariance matrix gives a statistical representation of the uncertainty in the location estimate and is used to construct a target location confidence region, such as an “error ellipse,” which can be displayed to the signals intelligence system operator on the aircraft or in a remote ground station. Geo-location performance is driven by the number of measurements, measurement error statistics, and engagement geometry. assive geo-location of ground targets is commonly performed by surveillance aircraft using direction finding angles. These angles define the line of sight from the aircraft to the target and are computed using the response of an antenna array on the aircraft to the target’s RF emissions. Direction finding angles are the inputs required by a geo-location algorithm, which is typically an extended Kalman filter or a batch processor. This modality allows a single aircraft to detect, classify, and localize ground-based signal sources. In this article, the direction finding angles used for geo-location are defined and a mathematical model is developed that relates measurements of these angles to the target’s position on Earth. Special emphasis is given to the angle measurement provided by a linear antenna array. An algorithm is presented that uses iterated least squares to estimate a target’s position from multiple angle measurements. Simulation results are shown for a single aircraft locating a stationary ground target. Geo-Location Using Direction Finding Angles