Real Time Measurement and Resolver Calibration of Ocean Color Monitor Oceansat-2 Spacecraft using Close Range Photogrammetry

摘要

Ocean Color monitor is flown to orbit with Oceansat-2 spacecraft. During in-orbit operations provision is made to tilt the Electro-optics about the Pitch axis by ± 20° with reference to nadir to avoid the sun glint from sea surface. An on-board angular resolver is used to read the angular rotation of Ocean Color Monitor (OCM) for the given pulses through telemetry command. This study is carried out to calibrate the resolver during ground level measurements, which involves real time measurement of the system to derive 3D rotations and translations of the system in response to pulse through telemetry command. Close Range Photogrammetry (CRP) is used to measure the rotation of OCM in response to a given telemetry pulse. In the integrated level there is no physical feature accessible to define either OCM or spacecraft axis. CRP is used to measure the 3D-coordinates of retro reflective targets fixed on OCM and Spacecraft. A total of14 sets of independent measurements were carried out using CRP. Mathematical transformation and best-fit analysis is carried out on 3D measured data to derive the axis of rotation for OCM. Further computation is made to find the stability of OCM about Roll and Yaw axis during the rotation. Auto-collimation measurement data is used as reference to compare the accuracy of CRP and resolver. Comparison between Auto-collimation and CRP shows accuracy lie within 0.002° and the difference between CRP and resolver angle (commanded angle through telemetry) lie within 0.01°. This paper describes the measurements and mathematical computation carried out using CRP, resolver and auto-collimation. The paper further claims advantages of CRP over other methods of measurement like time and effort saving, real time measurement capability, measurement of critical geometries in inaccessible orientations and development of methodology to derive axis of a rotating members without the use of physical features as references.