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Algorithms of Spacebased SAR Antenna Adjustment Using Results of Non-Simultaneous Doppler and Range Measurements when Non-Regular Spaceships Orientation Errors Take Place


S.G. Likhansky

The task of adjustment of spacebased SAR antenna using results of non-simultaneous measurements of central Doppler frequency and slant range along pointing direction and spaceship current orientation data knowledge (in Greenwich Coordinate System – GCS) is considered in the paper. Suppose that described in the paper antenna allows ray switching (scanning) in azimuth plane and continuous mechanical or electronic rotation of azimuth plane by tilt angle relatively fixed axis. The antenna adjustment is understood in the paper as estimation of regular (i.e. constant or very-slow changing in time) errors of the whole set of parameters describing both the antenna orientation in Internal Coordinate System (ICS) of spaceship and the internal geometrical configuration of antenna system. The whole adjustment algorithm can be divided into four steps (in brief). The first step is the most important and difficult one. It consists of azimuth and tilt angles estimation using current spaceship orientation data knowledge in GCS for different rays (two or more) and different emitter positions (three or more) relatively rotation axis. It is proved that it is necessary to measure Doppler frequency and slant range on different very short time intervals: the Doppler frequency must be measured on low repetition frequency, but the slant range must be measured on high repetition frequency. The second step consists of antenna orientation orthogonal basic estimation (called in the paper as «Mechanical Coordinate System – MCS») relatively spaceship ICS basis. (The first axis of the MCS is emitter rotation axis.) The third step consists of emitter (or emitter-reflector system) internal orthogonal basis estimation for middle («zero») position of this system (called in the paper as «Optical Coordinate System – OpCS») relatively spaceship ICS basis. The fourth step consists of calculation of quaternion pair – MCS relatively ICS orientation quaternion and OpCS relatively ICS orientation quaternion. This quaternion pair (the eight real numbers) is the output data of the whole adjustment algorithm. Then it is shown in the paper how to use the results of adjustment algorithm (the quaternion pair) for correct antenna ray pointing during SAR data processing.
June 24, 2020
May 29, 2020

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