A new method to calculate relative distance of closest terrain point using interferometric radar altimeter output in real flight environment

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. With IRA, more complicated calculation is required to determine the precise relative distance of the closest terrain point using the sensor output. 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. The flight test result shows that there was a difference between the Euler angles and the effective observation 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 was evaluated by comparing the altitude of the closest terrain point and terrain DB obtained by LiDAR(Light Detection and Ranging) during flight test. The new method has smaller mean and standard deviation of the altitude error, implying the superiority of the proposed algorithm.