Radar altimeter as a navigation aid using hierarchical elevation map clustering

Radar altimeter is widely used as an independent sensor in navigation applications. It is commonly used for the purposes of enhanced flight safety and terrain aided navigation. In this study we aim to use radar altimeter as a navigation aid for a tactical grade inertial navigation system (INS) to reduce the vertical channel errors. Because terrain aided navigation methods have a quite limited area of use due to the changing requirements of the terrain slopes, the proposed method yields better performance in flat regions. Variations in the elevation maps and the radar altimeter data are fed into the Radar Altimeter/ INS Kalman Filter. Besides constant elevation areas such as seas, deserts and lakes, land areas which have varying elevations are also used for measurement. In order to estimate variations in elevation maps, we developed a hierarchical elevation map clustering (HEMC) of the entire world using various elevation data (especially DTED). HEMC includes mean and variances of the elevation data. Use of HEMC enables faster query of constant elevation regions and smaller sizes of map databases for embedded systems. Performance of the proposed method is tested using real flight data. Even if lateral and longitudinal positional errors increase with time, vertical channel accuracy is improved depending on the map characteristics of the flight region.