Evaluation of in-orbit temperature variation on performance of MIRAS prototype noise injection radiometer

As the purpose of the noise injection radiometer (NIR) is to work as a reference for the MIRAS (Microwave Imaging Radiometer by Aperture Synthesis) measurements its performance and, especially, its stability is critical. The noise injection method, known from its inherent stability, was selected in order to meet the challenging requirement. An additional challenge is the fact that in orbit the NIR has only one really well known external target, namely the deep space. With the noise injection method one-point calibration is in principle possible, since the noise injection level can be determined with one known target. A series of measurements over relevant temperature ranges were carried out with the prototype in order to establish the temperature dependence, which is a critical factor of the overall stability. The results show that after the losses of the antenna and its connecting network are accounted for, due to the one-point calibration, an additional correction algorithm is needed to compensate for the thermal variations. Various correction algorithms were utilized and evaluated. It was shown that by using a well-designed correction algorithm the required performance is indeed achievable