Specification of channel filters for an interferometric radiometer

Differences among the channels that form an interferometric radiometer result in amplitude and phase errors in the cross correlations measured from the signals collected by each pair of antennas (visibility samples). Since point source calibration of a satellite-embarked instrument operating in a radio astronomy protected band is, in principle, not possible, and although separable phase errors can be corrected, the nonseparable ones produce an it-recoverable loss of information and have to be kept below specified limits. Preliminary work performed by the authors established basic requirements on the channel transfer function so as to meet these strict specifications imposed by proper calibration and inversion procedures. These requirements were set, from a simple filter model, in terms of three basic parameters: center frequency, bandwidth, and overall group delay. The purpose of this work is, for any kind of filter, to translate these requirements into a given insertion loss mask that must be satisfied by all filters so as to keep nonseparable error terms under certain specified bounds. The numerical results and computer simulations presented are based on the specifications of MIRAS, a two-dimensional aperture synthesis radiometer currently under study by the European Space Agency, and its proposed in-orbit calibration procedure.