Use of Receiver as a Calibration Target
Kangas, V.; Colliander, A.
ESA-ESTEC

Traditionally spaceborne microwave radiometers are calibrated using two point method. Depending on radiometer type and application, calibration methods vary significantly. External or internal calibration targets may be used, depending on radiometer type. During last 10 years, use of Active Cold Load (ACL) as an internal cold calibration target has become viable. ACL’s are based on the measurement of an input port of an amplifier. This port appears colder in noise temperature than its physical temperature.

ACL requires a dedicated circuit and careful design. What is proposed in this paper as a new, novel idea is to use the input port of a complete radiometer receiver as a calibration target instead of a dedicated ACL. Modern LNA technology has made very low noise figures and return losses possible; both key parameters for a cold and stable ACL. This combined with a typical high isolation of an amplifier makes it possible to use a receiver itself as a cold calibration target. This is especially beneficial in instruments already having two receivers (polarimetric radiometer for example). What is also proposed is to use this receiver input port as a phase calibration target on top of traditional power calibration target. A phase calibration target is beneficial in systems, where the relative phase of the receivers is of importance (polarimetric radiometers and synthetic aperture interferometric radiometers).