C-band FET power amplifier for TWTA replacement

The technology of gallium arsenide field-effect transistors has advanced to the point where these devices can serve to replace traveling wave tubes in spaceborne communication transponders.This paper describes interim results of a C-band FET amplifier which is currently being developed by RCA for INTELSAT. The FETA is designed to provide a 6-W output with an efficiency greater than 25%, and a backed-off 1.5-W output level with low distortion and efficiency of 13%.The third order intermodulation ($\text{C}\text{I}$) of a FETA at saturation is typically 15 dB as compared to 10 dB for a TWTA. At 10 dB input power back-off the $\text{C}\text{I}$ improves to 24 dB while that of a TWTA is only 16 dB.We fabricated a breadboard 5-W FETA which demonstrates that a linear operation can be achieved at an output level of 1.5 W with 10% efficiency. By contrast the efficiency of a typical 5 W TWTA in the same linear region is no greater than 3–5%.     

VI: FUTURE CONCEPTS: Attitude and Drag Control: An Application to the GOCE Satellite

The Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite, currently planned to he launched in the course of 2006, will require a precise drag compensation and a fine attitude control along the Local Orbiting Reference Frame (LORF) of a polar Sun-synchronous low orbit, allowing the Earth gravity field to be recovered with unprecedented accuracy by post-processing the scientific telemetry. To this aim, the spectral density of the spacecraft linear and angular accelerations must be limited below 0.025 respectively, in the frequency range from 5 mHz to 0.1 Hz, the gradiometer measurement bandwidth. In the same range, the orientation errors of the spacecraft in the LORF and of the LORF in the inertial frame must be kept below 10 . The Drag-Free Mode, encharged of drag-free and attitude control (DFAC) during measurement phases, determines the spacecraft state vector using a very precise gradiometer, one large Field-of-View Star Tracker and a Satellite-to-Satellite Tracking Instrument. Force and torque commands are actuated by two assemblies of thrusters: a single ion-thruster acting along the orbital direction, a set of eight micro-thrusters acting along the other five degrees of freedom. To cover every mission scenario, other control modes have been studied and designed: the Coarse Pointing Mode dedicated to rate damping and Sun acquisition, the Fine Pointing Mode handling the transition to Drag-Free Mode and the Ultimate Safe Mode, a survival operative mode to improve mission reliability. Results presented in this paper give a positive perspective on the solidity of the current DFAC design. This revised version was published online in August 2006 with corrections to the Cover Date.