Study On Baseline Incline Angle Inversion Method Of Interference Imaging Altimeter

After the Tiangong-2 Interferometric Imaging Radar Altimeter(In IRA)was launched,the sea surface height(SSH)can be retrieved according to the geometric relationship between the two antennas and the observed position.Since the orbit of Tiangong-2 is about 400 km,even though the baseline attitude has a slight variation,a great influence on the measurement accuracy can be resulted.The baseline is mainly affected by the change in the roll angle of the platform.Therefore,in order to achieve centimeter SSH measurement,the measurement accuracy of the platform roll angle needs to be less than 0.1 arc seconds,which cannot be achieved by the state-of-art star tracker.In order to solve this problem,we proposed a method to use the interferometric phase to invert the baseline incline angle.Based on the principle of interferometry,we analyzed the impact of incline angle error on the measurement accuracy,and then introduced the principle and the main factors affecting its inversion accuracy.The inversion experiment was first carried on the airborne data obtained by a nadir-looking interferometric radar altimeter.After aligned with the rolling angle of the platform in geometry and amplitude,the measured interferometric phase is well-matched with it.Compared with the filtered results,it can be seen that the measurement has more details than the roll angle measured by the platform,which shows that the accuracy of the phase measurement is relatively higher.In addition,the advantage of multi-frequency measurement is also proved.Finally,the interferometric phases of the echoes acquired by the Tiangong-2 In IRA were processed through the following steps: range compression,range migration correction,and multi-looking.Then,the inclination of the baseline was inverted according to the geometric relationship.The results indicate there is a systematic error between the inversion angles and the measured roll angle by platform.This is because the earth is an ellipsoid,and the attitude control of the platform is conducted with respect to the earth center,while the baseline inclination from the interferometric echoes inversion is relative to the surface,there is a systematic deviation between them along the orbit.After this systematic errors corrected,the expected results were obtained: both the trend and standard deviation are consistent with each other.