IRS-1C User's Handbook

PAN Camera

Optics and electronics

The Panchromatic camera uses reflective optics alongwith 4096 element CCD linear array (7 micron x 7 micron) for imaging. A special arrangement comprising of an isoceles prism reflector is used for covering the full swath of 70 km. Each detector has separate interference filters and 4 Light Emitting Diodes (LEDs) along with a cylindrical lens. Two LEDs are for optical biasing and two are for inflight calibration of the sensor. Four selectable gains are provided for PAN camera.

PAN camera

Inflight calibration

The detector characteristics that are evaluated on the ground are Light Transfer Characteristics (LTC), spectral responsivity, dark current, dynamic range and shading characteristics. Besides the main detectors, optical components like lenses and filters are thoroughly performance tested on ground and extensive calibration data is generated on ground for radiometric correction. Regular inflight calibration helps to study the response degradation of the CCD output. The inflight calibration of the camera will be carried out using LEDs. LEDs have the advantage of low power consumption, low thermal dissipation and fast response time. The scheme envisages the calibration of CCDs excluding optics. LEDs are operated in pulse mode at higher currents resulting in higher intensities.

A calibration cycle comprises of 2048 lines. The time taken for one calibration cycle for PAN camera is approximately 1.8 sec. The LEDs are operated at pulsed mode and the duration for which the LEDs are 'ON' is varied in specific steps. The CCD detector integrates the light falling on it during one readout period. Six non-zero exposure levels spanning the full dynamic range are provided for each detector.