TOPEX POSEIDON MICROWAVE RADIOMETER (TMR) .1. INSTRUMENT DESCRIPTION AND ANTENNA TEMPERATURE CALIBRATION/

The TOPEX/Poseidon Microwave Radiometer (TMR) is a three-frequency rad iometer flown on the TOPEX/Poseidon (T/P) satellite in low Earth orbit , It operates at 18, 21, and 37 GHz in a nadir-only viewing direction which Is co-aligned with the TIP radar altimeters, TMR monitors and co rrects for the propagation path delay of the altimeter radar signal du e to water vapor and nonprecipitating liquid water in the atmosphere, This paper describes the TMR instrument and the radiometric instrument calibration required to derive antenna temperature (T-A) from the raw digital data, T-A precision of 0.4 degrees K is predicted on orbit in ah expected thermal environments, T-A accuracy of 0.5-0.6 degrees K i s expected following a post-launch field calibration campaign, These p erformance figures represent a significant improvement over those of t he Seasat and Nimbus-G Scanning Multichannel Microwave Radiometer on w hich TMR is based, The improvements are the result of specific hardwar e design and calibration changes, Hardware changes include a redesigne d feed horn, to reduce impedance mismatches, and the addition of radom es over the feed and sky horns, to reduce thermal variations. Calibrat ion changes involve more extensive temperature cycling and data analys is during thermal/vacuum testing.