In the area of platform pointing and stability, a linear array CCD-based star sensor was flown in IRS-1A/1B for measuring attitude of the satellite as a supplement to the earth sensor. Subsequently CCD-based star sensor was developed and flown in IRS-1C/1D as a full-fledged three-axis attitude sensor. Then onwards, star sensor is the main attitude sensor onboard all IRS satellites. The spacecraft pointing accuracy, which is measure of the geometrical accuracy of the imagery, has been improved from 0.4 deg in IRS-1A to 0.05 deg in Cartosat. These star sensors in conjunction with highly stable mechanical gyroscopes resulted in platform stability of about 5 × 10e–5 deg/s. Table 4 provides the different types of attitude sensors and their evolution. Development of fibre optic gyro, which will give better performance in terms of drift stability, is under way.
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Sensor Type Accuracy (degree) No of Sensors per spacecraft
4PI Sun Sensor 1.0 4
Fine Sun Sensor 0.5 1
Precision Yaw Sensor 0.2 2
Digital Sun Sensor 0.125 2
Conical Scanning Earth Sensor 0.1 2
Area Array CCD Star Sensor 0.01 2