Effect of the Lunar Radiation on the Cold Sky Horn Antennas of the Chang'E-1 and -2 Microwave Radiometers

To determine the likelihood that the surface contamination effects can explain the offsets between Chang’E-1 (CE-1) and Chang’E-2 (CE-2) microwave radiometers’ (MRMs) calibrated antenna temperatures (TAs), we quantitatively estimate the effects of the lunar contamination of the CE MRMs’ cold sky horn antennas on TA. We calculated the lunar radiation contributions to the cold sky horn ( ${ {\Delta T}}_{\text {cosmic}}$ ) values from the Apollo model brightness temperatures (TBs) convolved with the CE-1 and CE-2 cold sky horn antenna patterns for the circular orbit geometries of CE-1 and CE-2. The calculations were done for all four channels and estimated 20°–40° range of the full width half maximum (FWHM) Gaussian beam sizes over a latitude range of 0°–64°. Small differences between the predawn and noon results illustrate that the local time effects are negligible. Most noteworthy is the result that the largest plausible “ ${ {\Delta T}}_{\text {cosmic}}$ ” are less than 10.8 K for CE-1 and less than 23.6 K for CE-2 at the largest assumed beam size. The latitude effect is small, although significant (~5–6 K difference), from the equator to 64°. Most importantly, the cold sky horn contamination effects are shown to produce target TA errors of only 2–5 K for CE-1 and 4–10 K for CE-2 with the largest errors occurring at the lowest TAs. These errors are inadequate to resolve much larger relative offsets reported in the CE-1 and CE-2 TA data. We thus propose an alternative recalibration scheme that focuses on the role of uncertainties in the preflight derivation of hardware loss coefficients.