Overview and preliminary results of end of mission experiments of the Tropical Rainfall Measurement Mission Satellite (TRMM)

Tropical rainfall measurement mission satellite (TRMM) faced out of fuel for the orbit maintenance in July 2014. After that the satellite was gradually descending its altitude (nominal altitude is 402.5km). During the descending, the precipitation radar (PR) onboard TRMM faces the difficulty to implement the nominal observation because of PR uses fixed pulse repetition frequency (PRF) of 2776 Hz that means the fixed sampling window from the satellite. In this situation, the rain echo (and the ground clutter) sometimes disappears from the windows and sometimes partly appears at the top of the window. Data during the descending are not suitable for the higher algorithm processing (e.g. level 2 or higher) because of lack of information for the algorithm (e.g. full profile of the rain, normalized backscattering cross-section of surface (sigma-zero)). Therefore, special experiments are proposed and implemented during the descending. The purposes of the special experiment are to investigate the observation capability of the future spaceborne radar mission and to increase the meaningful data of rain during the descending. Both purposes are possible by changing the scan geometry. Several experiments are implemented during the descending: 1) 90 degree yaw maneuver, 2) wider swath observation and 3) dense sampling observation. In the 90-degree yaw maneuver experiment is that the satellite flies with the 90-degree yaw turn. In this situation the cross-track scanning radar (PR) became the along-track scanning radar. Therefore, the quite dense data from various incident angles are obtained in this experiment. The wider swath experiment tries to observe the wider incident angle data than the nominal TRMM/PR observation (+/- 17 degrees) to confirm the possibility of the wide swath observation of spaceborne radar. Since the observation window of the satellite is fixed for PR, wider swath observation gains the possibility of the rain observation. The dense sampling experiment is to scan the overlapping the footprint three or four times denser than the nominal observation. The purpose of this experiment is survey the capability to obtain the fine structure of the rainfall. The experiments are implemented from mid October 2014 to mid February 2015. The preliminary results indicate that each experiment was performed as expected. From mid February to the end of March 2015, PR returned nominal observation mode because the satellite altitude reached to around 350 km that is the original nominal altitude of TRMM. After the satellite altitude reached to 340km PR was shut down for the reentry to the atmosphere.