A New Method to Calculate Relative Distance of Closest Terrain Point Using Interferometric Radar Altimeter Output in Real Flight Environment
Juhyun Oh, Chang-Ky Sung, Jung-Shin Lee, Myeong-Jong Yu, Agency for Defense Development, Republic of Korea
The Terrain Referenced Navigation (TRN) in aviation is a position determination method by comparing the stored terrain DB and features of the terrain measured by radar altimeter. Conventional TRN algorithm considered Radar Altimeter (RA) measured altitude, the distance to the closest terrain point within the antenna beam width, as the direct relative altitude below the vehicle. The altitude error occurred in this process resulted in TRN errors. To overcome this drawback of the Rader Altimeter, the Interferometric Radar Altimeter (IRA) was developed jointly by Agency for Defense Development (ADD) and Hanwha Systems. The IRA can measure slant range and look angle of the closest terrain point on the zero Doppler line. When RA is used as a TRN sensor, the error occurs while calculating the position of the closest terrain point, simply subtracting the measured RA altitude from the position of the vehicle. With IRA, on the other hand, more complicated calculation is required to determine the precise relative distance of closest terrain point using the sensor output. There is a simple method to calculate the relative distance between the vehicle and the closest terrain point from zero Doppler line using IRA output in existing papers. The conventional method assumed that the direction of the vehicle nose and the velocity vector coincides, and the zero Doppler line is parallel to the aircraft lateral axis. This method could be used in the simulation environment, which does not consider wind condition. However, during actual flight, the zero Doppler line and the aircraft lateral axis are skew lines due to the influence of wind. Therefore, in order to calculate the relative distance to the closest terrain point using the output of the IRA in a real flight environment where the influence of the wind exists, a new approach different from the conventional one is necessary. In this paper, the new relative distance calculation method is proposed for that point, and proved mathematically. To evaluate the accuracy of the proposed approach, flight test was executed and analyzed. During the test, IRA, Inertial Navigation System, Global Positioning System and Light Detection and Ranging (LiDAR) are mounted to acquire IRA outputs, position information of the vehicle and Digital Terrain Map of the flight area. The flight test result shows that there was a difference between the Euler angles and the angles obtained by vehicle's velocity due to wind effects. With the wind effect, the zero Doppler line is located at the skewed position from the aircraft lateral axis, and the difference was confirmed between the proposed closest terrain point calculation method and that of the conventional. The accuracy of each relative distance calculation method were evaluated by comparing the altitude of the closest terrain point and terrain DB obtained by LiDAR during flight test. The new method has smaller mean and standard deviation of the altitude error, implying the superiority of the proposed algorithm.