The IRS-P4 (Oceansat-1), the eighth satellite built in India under the indigenous Indian Remote Sensing Satellite programme was successfully launched on May,26,1999 at 11.52 A.M from Sriharikota, India using the indigenously developed Polar Satellite Launch Vehicle (PSLV).

IRS-P4 carries two sensors onboard, Ocean Color Monitor (OCM) and Multi-frequency Scanning Microwave Radiometer (MSMR). Several new technologies like Dual Cone Earth Sensor, improved Digital Sun Sensor and Satellite Positioning System (SPS) were introduced in the satellite. OCM data products are available to the User community acquired from July,01,1999 onwards.

Ocean Color Monitor (OCM)

OCM is a eight channel sensor, operating in the visible and NIR regions of the electromagnetic spectrum.

The OCM camera can be tilted by +/- 20 degrees in the along track direction, to avoid the sun glint. As the OCM has narrow bands and the total upwelling radiation from ocean surface is weak, the sensor is so designed to give a high radiometric performance, spanning the entire dynamic range. Also, the field of view of the optics is + 43degrees, providing a swath of 1420km, from 720km altitude. This requires a complex lens design known as Telecentric. Hence the optics of OCM is a wide-angle Telecentric and Refractive system. In order to take care of the thermal control of the payload and electrical noise, a scheme of using unused CCD pixels, as the reference for calibration of temperature variation, is being utilized. With these features, OCM provides more insight into oceanic and atmospheric applications.

Applications of OCM

Development of algorithms for retrieval of ocean and atmospheric parameters.

Ocean color applications

• identification of potential fishing zones in coastal waters.
• exploration of deep sea fishery resources .
• primary production model and fish stock assessment.
• selection and monitoring of algal blooms.

Coastal processes

• sediment dynamics
• dynamics of estuarine/tidal inlets.
• circulation and dispersal pattern.
• Upwelling; coastal/oceanic fronts and surface currents.
• marine pollution and oil slicks.
• coral reef studies.

MSMR

MSMR works on the principle of collecting radiation from earth in the microwave region, which gives the brightness temperature of the surface. Physical temperature when multiplied with the emissivity of the object (here ocean water), gives the brightness temperature of the object. The radiation emitted by the ocean surface passes through earth atmosphere, gets modified and sensed by MSMR. Various constituents of the atmosphere have different and specific effect in a particular microwave frequency. In order to assess and eliminate the contribution by atmospheric constituents, four frequencies using vertical and horizontal polarizations have been chosen. These frequencies also help in observing water vapor and liquid water content in the atmospheric column through suitable modeling, using radiation transfer models. These frequencies have evolved after considering the emissivity variations with frequency and polarization, due to various geophysical parameters.

The brightness temperature of ocean surface is also a function of the surface roughness which in turn is a function of wind speed. Thus the data collected by MSMR provided information on wind speed

Since the weak signal due to microwave radiation is used to measure brightness temperature, any corruption of the signal by the receiver, which is mainly because of temperature, instability of the components of the sensor needs to be overcome. In order to cover 1440 km swath every orbit and for achieving two day repetivity, the antenna footprint is scanned across the orbital trace. As the surface emissivity has strong dependence on the angle of observation, data over each pixel should correspond to the same look angle. MSMR has been, therefore, configured with Conical Scan System to ensure incidence of 50deg at each scan position.

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