The Advanced Communication Technology Satellite (ACTS) is a key to reaching NASA's goal of developing high-risk, advanced communications technology using multiple frequency bands to support the nation's future communication needs. Using the multiple, dynamic hopping spot beams and advanced on board switching and processing systems, ACTS will open a new era in communications satel]ite technology. One of the key technologies to be validated as part of the ACTS program is the multibeam antenna

more »

... rapidly reconfigurable hopping and fixed spot beam to serve users equipped with small-aperture terminals within the coverage areas. The proposed antenna technology experiments are designed to evaluate in-orbit ACTS multibeam antenna performance (radiation pattern, gain, cross pol levels, etc.) Introduction-Experiments Description and Goals The ACTS multibeam antenna technology verification experiments are divided in two general categories, direct and indirect performance evaluations. The direct antenna performance evaluation consist of in-orbit antenna pattern measurements. The indirect antenna performance evaluations consist of inferring antenna performance by simulating the spacecraft thermal/RF environment (Ref.l), measurement of the reflector surface and structures temperature and continuously monitoring the downlink C/N power ratios. The measurements of in-orbit antenna patterns of the ACTS spacecraft will characterize (Ref.2-3) the antenna system performance (gain,pointing, sidelobe level,etc.). This measurements will also be used to verify the thermal/RF analysis computer codes. The direct identification of individual causes for antenna performance degradation from thermal effects may not be possible to determine from the measured antenna patterns. These effects of thermal distortions on the performance of ACTS antenna system may be indirectly assessed from the measured reflector surface and structures temperatures. This data can be used in conjunction with thermal/RF analysis program for estimating ACTS antenna performance. Also by continuously monitoring the downlink C/N power ratios at the NASA Lewis ground station it is possible to determine and correlate the changes in received power to changes in antenna performance due to thermal effects, spacecraft drift, autotrack drift, etc.