CALIBRATION ISSUES FOR MILLIMETER WAVE EARTH TERMINALS

Abstract

The National Bureau of Standards (NBS) has investigated the calibration and measurement support requirements of millimeter wave satellite systems such as MILSTAR. Essentially three new measurement problems arise because of operating in the upper SHF and EHF frequency ranges. First, without adequate methods to measure the atmospheric loss, the accuracy of EIRP measurements in the 20 GHz to 45 GHz range can be no better than 0.5 dB to 3 dB (depending on frequency and antenna elevation angle). The atmosphere absorbs and scatters radiation traveling through it, both reducing the magnitude of and depolarizing a received signal. Second, standards, measurement support services, and some measurement techniques are not presently available from NBS and they are needed to support millimeter wave antenna gain and thermal noise measurements. Of special concern are the effects of connectors and adapters, since they can introduce significant errors into mm-wave measurements. Theird, if the sun and/or moon are to be used for measuring earth terminal G/T, earth terminal antenna gain, or satellite effective isotropic radiated power in the millimeter region, they need to be appropriately characterized at those frequencies. The sun and moon are only useful as measurement sources for antenna systems with gains less than about 50 dB, but most MILSTAR systems are expected to fall in this category.