Randhir Singh, B. Simon and P. C. Joshi
Atmospheric Sciences Division, Meteorology and Oceanography Group
Space Applications Centre, ISRO, Ahmedabad – 380 015
India has launched several remote sensing satellite under the series of IRS satellites. IRS-P4 satellite is the fourth satellite launched by the launch vehicle PSLV from Shriharikota India. IRS – P4 was flown in May, 1999 and is a sun-synchronous, near polar and near circular orbit at an altitude of 720 km with local time of equator crossing in descending node at 1200 Hrs. The Multichannel Scanning Microwave Radiometer (MSMR) on board IRS P4 with capability of global coverage with two-day repetivity has been providing data since its launch. MSMR is a eight band (four frequency in dual polarizations) passive microwave radiometer with a conical scan mechanism. Microwave sensor MSMR (Multi channel Scanning Microwave Radiometer) on board IRS-P4 is used to develop a retrieval method for near surface specific humidity by means of multivariate regression technique. The MSMR measures the microwave radiances at frequencies of 6.6, 10.7, 18 and 21 GHz for both vertical and horizontal polarizations. It is found that the humidity parameter is sensitive for 18 and 21 GHz vertical as well as horizontal polarization. The coefficients were derived using NCEP data of three months (June, September and November) 1999. Daily 2o by 2o average bins in the latitude, longitude were used to derive the coefficients. The derived coefficients were validated with in-situ observations over Indian Ocean and with NCEP analysed data for rest of the places. A linear relationship is established to determine the near surface specific humidity from MSMR brightness temperature (Tb) with an accuracy of 1.4 g/kg.