Spacecraft

The complexity of the NISAR spacecraft has required close collaboration between U.S. and Indian teams. NASA provided the L-band radar, one of the observatory’s two science instruments. NASA also provided a key engineering payload, which includes a high-rate communication subsystem for science data, GPS receivers, a solid-state recorder, a power distribution unit, and a payload data subsystem. NASA’s Jet Propulsion Laboratory leads the U.S. component of the project on behalf of the agency.

The Indian Space Research Organisation (ISRO) provided the spacecraft bus and most of the engineering subsystems, including the power systems, thermal management systems, onboard computer, actuators, and sensors. The other science instrument, the S-band radar, was also contributed by ISRO. Mission operations are collaborative, with some activities led by ISRO and others by NASA JPL, depending on the subsystem. Highlights of key spacecraft components and engineering activities are described below.

The spacecraft body, or bus, includes the command and communications systems for the instrument payload and was provided by ISRO. The bus hosts systems such as:

Digital rendering of the NISAR satellite with its boom and radar antenna reflector deployed. Credit: NASA/JPL-Caltech | Full Image

Nearly 40 feet (12 meters) in diameter and resembling a snare drum, the radar antenna reflector is mounted at the end of a 30-foot-long (9-meter-long) boom, which extends from the bus. Pre-deployment checkouts of the radar antenna reflector and boom, led by NASA JPL, are scheduled for 10 days after launch. The boom deployment commences 11 days after launch and takes four days to complete. The reflector deployment is planned to occur 17 days after launch, and the mission team has set aside a full day to complete this operation.

This subsystem generates, stores, and distributes the power needed to operate the satellite, including the L-band and S-band radar systems. The power source is sunlight collected by the two solar arrays provided by ISRO. Measuring 6 feet (1.8 meters) wide and 7 feet (2.2 meters) long, each array consists of three panels mounted on opposite sides of the spacecraft bus. Combined, the arrays supply approximately 5 kilowatts of power when fully illuminated. The spacecraft bus also houses a 180 amp-hour lithium ion battery that can provide about 4 kW of power. The power, at 70 volts, is converted to 28 volts by a JPL-provided power distribution unit, which drives the JPL engineering payload.

To ensure NISAR’s precise orientation in space, or attitude, the satellite will use star sensors, Sun sensors, and magnetometers. In addition, the satellite has gyroscopes and accelerometers for velocity and acceleration measurements. The ISRO-built attitude control system uses reaction wheels to control the satellite’s attitude. Thrusters — small rocket engines used for in-flight corrections — of 1-newton and 11-newton magnitude provide needed changes to the satellite’s orientation and trajectory. There is enough fuel aboard for five years of operations.

Mission operations will be a joint NASA-ISRO effort, with both teams jointly developing the commands that will be sent to the spacecraft via the ISRO Telemetry Tracking and Command Network (ISTRAC) center in Bengaluru. ISTRAC will also monitor the spacecraft and downlink housekeeping telemetry.

For more information on ISTRAC, visit istrac.gov.in.

All science data will be written to the onboard solid-state recorder. From there, the data can travel down one of the two downlink paths.

All L-band science data will be downlinked via the onboard JPL-built Ka-band telecom system. The NASA downlink path will flow through one of four of the agency’s Near Space Network Ka-band ground stations capable of receiving the data. They include stations located in Svalbard, Norway; Punta Arenas, Chile; Fairbanks, Alaska; and Wallops Island, Virginia. NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the Near Space Network. The agency’s science data will be processed by the JPL Science Data System in the cloud and shared via NASA’s Alaska Satellite Facility Distributed Active Archive Center.

All S-band science data and a subset of the L-band science data will be downlinked directly to ISRO’s ground stations via the onboard ISRO Ka-band telecom system. There are two Ka-band ground stations capable of receiving the data: one in Shadnagar, India, and the other in Antarctica. The ISRO science data will be processed by the National Remote Sensing Centre in Hyderabad, India, and distributed through the Bhoonidhi portal.

For more information on ISRO’s National Remote Sensing Centre, visit nrsc.gov.in. For more information on the Bhoonidhi portal, visit bhoonidhi.nrsc.gov.in.