NASA-LLIS-1180

Abstract

<strong>Description of Driving Event:</strong> <br>The TDRS-H was launched on June 30, 2000. The satellite is the first of a new series of Tracking Data Relay Satellites. The satellite incorporated a number of new communication technologies including the Single Access Springback Reflector Antenna, transmitters and receivers operating at Ka-band and the S-band microstrip patch antenna elements used in the Multiple Access (MA) phase arrays. During In-Orbit Test (IOT), performance deficiencies were observed in the MA forward (MAF) and return (MAR) channels. The MA phase array system is complex in design and relies on both hardware and software to perform the on-board beam forming to track user satellites. Based on analysis of test data, a few minor deficiencies were corrected through changes to the software (sign error) and calibration error (phase shift settings). However, the MAR formed beam G/T performance remained significantly below predicted performance. Performance testing of individual return array elements was initiated revealing widely divergent gain and axial ratios for the elements when compared to pre launch factory measurements. In one case, the performance of a single element was seen to degrade significantly in a 12-hour period. BSS initiated a comprehensive anomaly investigation incorporating: 1) On-orbit testing of TDRS-H; 2) Laboratory performance investigation of flight MA antenna elements; and 3) Factory satellite system level MA testing using the TDRS-I then undergoing integration and test. Ultimately, BSS determined that the most probable cause of the observed performance was due to the MA array sunshield (thermal blanket) coming into contact with the antenna array elements. Such contact creates a dielectric loading of the microstrip patch radiators and transmission lines altering the phase relationship of the radiators and shifting the resonant frequency of the elements. Altering the phase relationship causes the element gain pattern to "squint" or move off axis by some 8 degrees. The peak directivity, resistive loss and VSWR (Voltage Standing Wave Ratio) performance of the MA antenna elements degraded as a result of the close proximity of the sunshield. The sunshield is held in contact with the elements by electrostatic force created by deep charging of the dielectric materials used in the construction of the antenna elements. In response, a negative "image" charge appears in the sunshield (since it is conductive and grounded), and the electrostatic attractive force field is created.