Precision Alignment Of A New Static Horizon Sensor With Four Telescopes

Abstract

A new static IR horizon earth sensor is used in Indian Remote Sensing (IRS) satellite for precision attitude determination. Due to a large subtended angle of the Earth at 904 km IRS satellite's polar orbit, the static earth sensor has a unique design consisting of four telescopes mounted on a pyramid shaped base. Each of the telescopes contains an IR optics and thermopile detectors of special geometry. These detectors are positioned precisely and aligned with respect to earth's image at the horizon. Sensitive axis of each telescope is defined at sub-assembly level using extended area black body source and a two-axis indexing table. This sensitive axis is transferred to an alignment mirror mounted on each telescope. The four telescopes carrying individual alignment mirrors are finally aligned to a central optical cube on an integrated sensor assembly. This alignment is carried out using a two-axis servo-table and an alignment facility. In addition, there are four sun interference sensors mounted on the main base and aligned to sensitive axis of the individual telescope. This alignment is carried out using a sun simulator and a two-axis servo table. The alignment of integrated static sensor is counter-checked using the large earth simulator both for altitude and null angles, and calibration is carried out for roll and pitch measurements. The central optical cube is finally used for aligning the sensor with respect to the satellite reference axes and in turn to the camera systems on the satellite. This paper discusses the methodology used for the alignment of individual telescopes, four telescopes on the main base and sun interference sensors.