On-Orbit Absolute Calibration of the Global Precipitation Measurement Microwave Imager

Abstract

The Global Precipitation Measurement (GPM) Core Observatory was launched on 27 February 2014. One of the principal instruments on the spacecraft is the GPM Microwave Imager (GMI). This paper describes the absolute calibration of the GMI antenna temperature (TA) and the earth brightness temperature (TB). The deep-space observations taken on 20 May 2014, supplemented by nadir-viewing data, are used for the TA calibration. Data from two backlobe maneuvers are used to determine the primary reflector’s cold-space spillover, which is required to convert the TA into TB. The calibrated GMI observations are compared to predictions from an ocean radiative transfer model (RTM) using collocated WindSat ocean retrievals as input. The mean difference when averaged globally over 13 months does not exceed 0.1 K for any of the nine channels from 11 to 89 GHz. The RTM comparisons also show that there are no significant solar intrusion errors in the GMI hot load. The absolute accuracy of the GMI instrument is defined as the average ocean-viewing error of the measured TA or TB relative to the true TA or TB. Based on the analyses herein, the GMI absolute accuracy for TA is estimated to be about 0.1 K rms over all channels and 0.25 K rms over all channels for TB.

The Global Precipitation Measurement (GPM) mission's scientific achievements and societal contributions: reviewing four years of advanced rain and snow observations

“…(; 2015a). Subsequently, Wentz and Draper () concluded that GMI is the most accurate precipitation radiometer currently in space. To provide unified global precipitation estimates a reliable transfer standard of brightness temperatures (Tbs) between the GPM‐CO and the constellation partner precipitation sensors through the inter‐calibration of all the radiometer sensors has been established (Berg et al ., ).…”

Uncertainties in radiometer intercalibration associated with variability in geophysical parameters

“…The noise diode with GMI indicates no issues of radiometer nonlinearity [Draper et al, 2015b]. Deep-space maneuver has been performed to calibrate GMI against the cosmic background [Wentz and Draper, 2016]. As the cosmic background is homogeneous with known temperature, it has been found very useful to examine radiometer and refine APC algorithm with deep-space maneuver [Wentz et al, 2001;Weng et al, 2013a;H.…”

“…The APC algorithm has been examined and refined with comprehensive efforts through prelaunch test, onboard calibration system, deep-space maneuver, and simulation [Draper, 2014;Draper et al, 2015aDraper et al, , 2015bWentz and Draper, 2016]. For instance, results based on the deep-space maneuver estimate an accuracy of 0.1 K for TA and 0.25 K for TB [Wentz and Draper, 2016]. Problems such as the along-scan biases in warm scenes have been identified in early GMI data versions and subsequently corrected [Yang et al, 2015].…”

Atmospheric absorption model for dry air and water vapor at microwave frequencies below 100 GHz derived from spaceborne radiometer observations

“…The GMI T A measurements are denoted by T A,gmi . The values for GMI η and χ are given by Wentz and Draper [2015].…”

“…For example, a change in α D or α V of 0.00005 km −1 (0.0002 dB/km) produces a change of about 0.1 K in the ocean brightness temperature ( T B ) seen by a microwave radiometer onboard a satellite. The 0.1 K precision is typical of todays' advanced satellite microwave radiometers particularly after averaging over millions of space‐based observations [ Wentz and Draper , ]. The sensitivity of 0.00005 km −1 is much higher than is generally reported in the extensive literature on α D and α V [ Liebe , , , ; Clough et al , ; Liebe et al , ; Rosenkranz , ; Tretyakov et al , ; Payne et al , , ].…”