Antenna temperature (Ta). A weighted average of the brightness temperature incident on a radiometer from all directions, where the weighting is the antenna radiation pattern.
Receiver noise temperature (Trec). The equivalent brightness temperature that would produce the same signal strength as the noise generated by a radiometer’s electronics.
System noise temperature (Tsys). The sum of the antenna temperature entering a radiometer plus its receiver noise temperature.
Antenna radiation pattern. The sensitivity of an antenna to incoming radiation, usually expressed as a function of angle of arrival in polar coordinates and normalized so that the integral over all angles is unity.
Antenna mainbeam. The portion of an antenna radiation pattern localized near the direction of maximum sensitivity.
Antenna sidelobes. The antenna radiation pattern away from its mainbeam.