L-Band TWTAs for Navigation Satellites

Abstract

Next-generation positioning systems are expected to open a widely extended range of applications. This drives new system designs toward enhanced resolution, increased accuracy, complete coverage, and extremely high reliability. For the radio-frequency (RF) payload, the main demand is on increased output power at a very low level of signal distortion. Other than solid-state power amplifier (SSPAs), traveling wave tube amplifiers (TWTAs) can provide a practically unlimited range of RF power with nearly no reduction of performance. Qualified as highly reliable devices in hundreds of millions of accumulated operating hours in space, TWTAs seem to offer an ideal solution for future systems like Galileo and Global Positioning System III. Thales Electron Devices (TED) began development of 150-W L-Band TWTs for digital radio services in 1996. More than 70 amplifiers were delivered for the WorldStar system; two satellites are meanwhile successfully broadcasting digital radio programs to wide areas of the Southern Hemisphere. Based on the upcoming needs of new navigation systems, TED performed an upgrade program to extend the performance of its L-band TWT family covering the frequency range 1.1-1.6 GHz. In the meantime, up to 250 W of output power can be provided by a single TWT, with efficiencies as high as 65% (60% for TWTA) in saturated operation. The TWTs have been tested against all sensitive parameters and support all modulation schemes, yielding excellent values for group delay and group delay stability. To meet the linearity requirements, the TWT can be operated either in backoff mode, or in combination with a linearizer in order to retain its high efficiency. For a 100-W amplifier the larger size (10%) and mass (65%) of the TWTA compared to an SSPA can be more than compensated by the higher power, efficiency, and bandwidth (more than 200 MHz) of the TWTA, which allows using a minimum number of TWTAs to support redundancy requirements, and thus helps to effectively optimize the satellite power budget. As output power increases to 150 W and beyond, the increasing complexity and mass of the SSPA make the TWTA look even more favorable. This paper will present actual performance data of TEDs improved L-band tubes, and will demonstrate through the obtained results that L-band TWTAs provide attractive solutions, suited for all navigation systems.