Novel Architectures for Orbital SAR Sensors

Advanced semiconductor and antenna technologies are at the heart of any space based sensor system - where size, weight and performance play a critical role. In this paper we present novel single chip front end transceivers and metasurface antennas that deliver superior performance in small form factor, compared to existing technologies. An evaluation of commercial foundry Gallium Nitride (GaN) RF HEMTs as power switches in high frequency switching circuits is described. The switches are used within the multi-functional blocks for a system-on-chip architecture of the front-end of a pulsed VHF radar for planetary applications. The GaN power switches are evaluated in a voltage mode Class-D (VMCD) RF amplifier and as a DC-DC converter. The power switches and its associated drivers are designed in a 0.25 $\mu$m commercially available RF GaN process technology. The efficiency of the circuit when evaluated as a Class D power amplifier (PA) is 74.5% at 250 MHz and delivers an output power of 39.5 dBm. The measured DC-DC converter circuit has an efficiency of over 92.4% at 100 MHz, delivering 40.8 dBm. These are the highest reported efficiencies for the RF power amplifier and the switching modulator at its respective operating frequencies. As part of the front-end, we also present a flat-panel holographic metasurface antenna concept that can achieve selective stripmap and spotlight SAR scanning. The metasurface aperture layout can substantially simplify the deployment architecture of airborne and spaceborne instruments for SAR applications.