GPM Microwave Imager Key Technologies, Performance and Calibration Results

The Global Precipitation Measurement (GPM) Microwave Imager (GMI) Instrument was built and tested by Ball Aerospace and Technologies Corporation (Ball) under a contract with the GPM program at the National Aeronautics and Space Administration (NASA) Goddard Space Flight Center. The GMI instrument was delivered to Goddard in February 2012 and launched onboard the GPM spacecraft in late February 2014. This paper presents an overview of the GMI instrument, examines pre-flight radiometric accuracy and evaluates early on-orbit data versus pre-flight performance. The GPM Mission is an international effort managed by NASA to improve climate, weather, and hydro-meteorological predictions through more accurate and more frequent precipitation measurements [1]. The GPM Microwave Imager (GMI) infers precipitation by making calibrated passive radiometric measurements at multiple microwave frequencies. Also onboard the GPM spacecraft, the Dual-frequency Precipitation Radar (DPR) provides high resolution precipitation profiles by measuring the radar backscatter from the rain column. The data products from GPM afford frequent, near-global precipitation information for meteorologists and scientists making weather forecasts and performing research on the global energy and water cycle, precipitation, hydrology, and related disciplines. The GMI and DPR will be used together to develop a transfer standard for the purpose of calibrating precipitation retrieval algorithms and will establish a reference against which other satellites in the GPM constellation will be compared. The GMI instrument consists of 13 radiometric channels from 10.65 GHz to 183.31 GHz [2], providing accurate measurement of precipitation and multiple other environmental parameters. The GMI has a deployable antenna making it relatively compact for the 1.2 meter aperture size. For the GPM orbit, the GMI antenna provides 25 km native resolution at the lowest frequency and up to 5 km resolution at the higher frequencies. Multiple enhancements have been incorporated into the GMI to improve calibration accuracy over heritage systems. The calibration enhancements include tight shrouding around the hot load to keep out the sun, noise diodes on the 7 low frequency channels to provide a dual calibration system, and a proven robust reflective coating for the antenna [3]. The state-of-the-art receiver subsystem built by ITT Exelis provides low noise figures and very good stability for excellent radiometric performance. The GMI was extensively tested at Ball and Goddard over all on-orbit and launch environments. The key results of the ground performance and environmental testing are reported as well as projections for on-orbit performance based on the ground measurements. We describe the on-orbit performance. The areas discussed include the NEDT performance for each channel, the number of counts from each channel when viewing the warm target, the stability of each channel, the temperature stability of the instrument and the resulting stability of each channel, spin rate stability, and early indicators of absolute calibration performance. [1] Hou, A and Kirshbaum, D, “At the core, Global Precipitation Measurement (GPM) Mission,” Meteorological Technology International, pp. 6-10, Nov 2010. [2] Draper, D. and Newell, D, “Global Precipitation Measurement (GPM) Microwave Imager (GMI) calibration features and predicted performance,” MicroRad Conference Publication, pp. 236-240, March 2010. [3] D. W. Draper, D.A. Newell, D.A. Teusch, P.K. Yoho, “Global Precipitation Measurement Microwave Imager (GMI) hot load calibration,” IEEE Trans. Geosci. Rem. Sens., vol. 51, no. 9, Sep. 2013