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Achieving 12 Bit Resolution for Ocean Colour Monitor in Complex Electromagnetic Environment

Ocean Colour Monitor (OCM) and Multi-frequency Scanning Microwave Radiometer (MSMR), two payloads catering oceanographic studies share a common X-band downlink carrier. Both payloads offering 12 bit resolution have base band data rates of 10.4 M bits/sec. While MSMR operates at four distinct frequencies from 6.6 GHz to 21.0 GHz with both horizontal and vertical polarization's, the ocean colour monitor operates in eight bands with wavelength between 0.402 micron to 0.885 micron. The user need of 12 bit radiometric resolution boils down to containing the noise level at the ADC input to less than 1/2 the least significant bit weightage of 1.22 mV. Achieving this in the complex electromagnetic environment of the spacecraft posed challenges aplenty. The planning and precautions that went into it from conception to the realization phase, the strategy adopted, and the implementation aspects form the subject matter of this paper. Flown onboard IRS-P4 (christened OCEANSAT-1 in orbit), riding on PSLV-C2 on May 26, 1999 the payloads are fulfilling the user needs, to a Tee.