Microwave antennas deployed for spaceborne radiometers are diffraction-limited, with typical beam efficiency of about 90%, and provide relatively poor selectivity against radiation from celestial bodies (Moon, Sun, galaxy) when they are at the side or back of the instrument. Traditionally, this contamination has been removed by preparing multiple data files that represent the relative position of the spacecraft and the celestial sources throughout the mission lifetime.
The data files are dynamic with time, and this approach is time-consuming as it introduces an additional source of uncertainty when maps calculated for one orbit at the beginning of the mission are applied to the same orbit many months after launch, and need to be recalculated if the equatorial crossing time differs from the planned one. This work proposes that the correction be modeled with only a handful of static files that are dependent only upon a few a priori parameters: the ephemeris of the Sun and the Moon, the map of galactic emission, the gain pattern of the instrument's antenna, and the altitude of the orbit.
The secular variation of the correction is controlled by the pointing of the antenna main beam in Right Ascension and Declination coordinates, which is easily calculated at any moment in time from the spacecraft and instrument ephemeris. Only a few files are needed under the ideal conditions of a nadir-looking instrument, a circularly symmetric antenna gain pattern, and a circular orbit of constant altitude. Under conditions typical for most Earth-observing satellites, less than 10 static data files would be needed to correct for radiation contamination regarding microwave antennas during the entire lifespan of on orbital mission.