INTERCOMPARISON OF MICROWAVE BLACKBODY CALIBRATION TARGETS USING A RADIOMETRIC MILLIMETRE-WAVE TESTBED

M. Kotiranta¹, M. Brandt², A. Murk¹

¹University of Bern, ²RPG Radiometer physics GmbH

The brightness temperatures of different microwave blackbody calibration targets are determined with a temperature stabilised radiometric testbed that is equipped with a W-band heterodyne test receiver and a backend allowing simultaneous measurements with a total power detector and a FFT spectrometer. The testbed was built in the framework of a technology development project of the European Space Agency to characterise internal calibration subsystems. Using focusing optics and a rotating planar mirror, the receiver beam is pointed sequentially to three targets that are mounted next to the testbed. A breadboard of the MetOp-SG MWS on-board calibration target is used as an ambient target. It is a pyramidal target with an aluminium kernel and a multi-layer epoxy composite coating. A conical cavity target that is made of carbon-loaded polypropylene is used as a second ambient target. The third target is a cold target based on a pyramidal foam absorber that is immersed in liquid nitrogen (LN2). A reflective baffle with a low density foam window, through which the receiver beam enters the target, stands on top of the LN2 tub. The rotating mirror allows the target to be viewed at different incidence angles. When viewed at an angle of about 20 degrees, the reflected power from the LN2 surface hits the surrounding baffle and is sent back towards the foam absorber. This not only reduces the bias caused by the emission of the warm testbed but should also lead to suppression of standing waves between the target and the receiver when compared to the situation where the target is viewed at a normal incidence angle. The testbed allows the standing waves to be determined: the rotating mirror is mounted on a linear stage, and FFT spectra at two different linear stage positions that are separated by a quarter of a wavelength are obtained. The spectra are subtracted from each other to obtain the standing wave pattern. We compare the standing wave patterns of the ambient targets with each other and the brightness temperatures of the ambient targets to their physical temperatures that are measured with platinum resistance thermometers Also, the brightness temperatures and standing wave patterns of the cold target obtained at different viewing angles are compared with each other.