Sensitivity of XCAL double difference approach to ocean surface emissivity and its impact on inter-calibration in GPM constellation

Abstract

A robust XCAL double difference (DD) approach for radiometric calibration has been successfully applied between the TRMM Microwave Imager, TMI, (previous calibration transfer standard for NASA's Precipitation Measuring Mission) and a number of precipitation measuring radiometers in polar sun-synchronous orbits. Now that the TRMM Mission has ended (April 2015), the radiometric transfer standard was changed from TMI to the current GPM Microwave Imager (GMI). The use of an ocean radiative transfer model (RTM) is an integral part of our XCAL DD approach. Because the RTM physics is imperfect and because the environmental parameters are likewise only estimates of their true values, it is important to assess the sensitivity of the derived brightness temperature biases to these limitations. Therefore, in this paper, we conduct the inter-calibration between GMI and other constellation satellite microwave radiometers using the XCAL DD approach with two different ocean surface emissivity models. Results are presented that demonstrate the robustness of the XCAL DD approach for multiple instruments over a wide range of channel frequencies.