NASA's high-rate Ka-band downlink system for the NISAR mission

Abstract

This paper provides a description and analyses of NASA's (National Aeronautics and Space Administration) high-rate Ka-band downlink system for the upcoming NISAR (NASA-ISRO Synthetic Aperture Radar) mission developed at the Jet Propulsion Laboratory (JPL). NISAR is a collaborative Earth-Science mission between NASA and the Indian Space Research Organisation (ISRO), which features an L-band SAR instrument and an S-band SAR instrument. The simultaneous dual-frequency radar system at peak rates will produce data at gigabit-per-second speeds, which drives the data-volume requirements. The key driving requirement for the Payload Communication Subsystem is to provide a minimum of 35 Terabits per day of radar science data to the ground. NASA's high-rate transmitter on the flight system is a software-defined radio based on the Universal Space Transponder platform, providing an offset quadrature phase shift key modulated waveform with Low-Density Parity Check encoding of the data transfer frames. Two transmitters used in a dual-polarization configuration with each transmitter providing two giga-symbols per second (Gsps) of coded data provides an aggregate rate of four Gsps. In this system, using the ISRO-provided 70-cm high-gain antenna, only 1 W of signal power is necessary on each polarization to overcome propagation losses and achieve a successful RF link. Several Near Earth Network (NEN) ground station sites (Alaska in the United States, Svalbard in Norway, and Punta Arenas in Chile) are baselined for the space-to-Earth communications link. Each ground station will also feature multiple upgrades to support NISAR's transmission starting with new Ka-band antennas, and new electronic units such as wideband downconverters and high-rate receivers. In addition, a baseband data processor called Data Acquisition Processor and Handling Network Environment (DAPHNE), newly developed by the NEN, provides data storage and connectivity to backhaul networks. With NISAR's unprecedented large volume of data (nearly 5 Petabytes over the mission duration), a cloud system enables science data processing cost reduction to the mission. The NASA downlink system described herein will be the first operational use of Gsps-class downlink rates on an Earth-Science mission.