PRECISION DSN RADIOMETER SYSTEMS - IMPACT ON MICROWAVE CALIBRATIONS

The NASA Deep Space Network (DSN) has a long history of providing larg e parabolic dish antennas with precision surfaces, low-loss feeds, and ultra-low noise amplifiers for deep-space telecommunications. To real ize the benefits of high sensitivity, it is important that receiving s ystems are accurately calibrated and monitored to maintain peak perfor mance. A method is described to measure system performance and to cali brate the receiving system using procedures, software, and commercial instruments that are easy to implement and efficient to use. The utili ty of the measurement procedures and the precision of the receiver cal ibration technique were demonstrated by performing tests at K(a)-band (32 and 33.68 GHz) frequencies at Goldstone on a 34-m beam-waveguide a ntenna. Observations of multiple calibration radio sources are used to measure the dependence of antenna gain and system noise temperature o n source elevation and derive the performance parameters. Receiving sy stem nonlinearities are frequently overlooked as an error source in th e calibration of microwave radiometers. The experimental results descr ibed in this paper illustrate some of the ways that receiving system n onlinearity can negatively impact system performance. A simple radiome ter calibration technique and analysis provide quantitative informatio n that enables the system engineer to adjust and linearize the receivi ng system. When that is not practical, the experimenter or the operato r can apply correction coefficients to the measured values of system n oise temperature and thereby compensate for the receiving system nonli nearity. The high-performance antennas and the sensitive receiving sys tems of the DSN are valuable resources for scientific research in addi tion to the primary telecommunication tasks that support space mission s. The antenna gain and system noise temperature measurements and the radiometer calibration method described in this paper are also useful to perform precision research experiments.