Baseline Issues in an Airborne 650 GHz Radiometer

Remote sensing with millimeter and submillimeter wave radiometers is an important tool in atmospheric research. Since the errors from the radiometric noise of the receiver can be reduced with low noise receivers and long integration times, the measurements are often limited by the systematic baseline errors. These are all deviations of the measured power spectra from the true atmospheric signal, which can be caused by different instrumental effects. Among these are the nonlinearities in any of the components of the receiver and the differential nonlinearities of the spectrometer. These errors can be reduced by using a balanced calibration scheme instead of a total power calibration of the radiometer [14]. Another possible error source is the response of the mixer to harmonic sidebands [10]. In this paper we will focus only on the baseline caused by standing waves in the quasi-optics