Abstract
This study presents a correction algorithm to remove the backlobe intrusion from the hot-load reflector of microwave radiation imager (MWRI) on-board China FengYun-3C (FY-3C) meteorological satellite. Discontinuities in the radiometric gain of MWRI, due to inaccurate backlobe spillover of the hot-load reflector, have been diagnosed. A physical correction method, relating the radiometric gain difference between two distinct backlobe scenes to the spillover of the hot-load reflector, is thus established to estimate the in-orbit spillover factor of the MWRI hot-load reflector. Meanwhile, it is found that brightness temperature (TB) at each 1° × 1° grid could not address the backlobe pattern appropriately, leading to abrupt changes in the radiometric gain occurring near the coastlines. To better represent the backlobe spillover, the effective backlobe TB is now averaged over a 4° × 4° area from advanced microwave scanning radiometer-observed TBs, which is now in a finer resolution of 0.25° × 0.25°. With the adjusted spillover for the hot-load reflector, anomalous behaviors of the radiometric gain are mitigated effectively and the improvement at the coastlines is pronounced, inducing TB variation up to 2 K in magnitude at the 10.65 GHz channel. Although the algorithm presented in this study is only restricted to channel frequencies up to 23.8 GHz due to its accuracy limitation, it could tackle similar issues for those microwave radiometers not capable of in-orbit backlobe maneuvers and receiving the radiances inevitably from the outer edge of the hot-load or cold-sky reflectors.
Highlights
M ICROWAVE radiation imager (MWRI) is a conically scanning microwave radiometer on-board ChinaManuscript received December 8, 2020; revised March 21, 2021; accepted April 21, 2021
Speculated that the hot view is contaminated by the Earth scene from the outer edge of the hot-load reflector and the backlobe intrusion has not been completely removed by the calibration process, resulting in gain anomalies correlating to the backlobe scene at the 10.65 H channel
The backlobe spillover factor of the microwave radiation imager (MWRI) hot-load reflector decreases with increasing channel frequency, from ∼3% at 10.65 GHz channels to less than 1% at 23.8 GHz channels
Summary
M ICROWAVE radiation imager (MWRI) is a conically scanning microwave radiometer on-board China. To ensure accurate estimates of the backlobe intrusion, we establish the relation between the spillover factor of the hot-load reflector and the radiometric gain when the satellite is flying across the coastlines. This method is independent of the RT reference standard and enables in-orbit adjustment of reflector spillover for those microwave imagers, which suffer from the severe backlobe spillover of the hot-load reflector or cold-sky reflector from the Earth.
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