The coordination of technical interfaces among subsystems is a major focus area in the NASA-ISRO partnership.
ISRO provides the heritage spacecraft bus, to which the octagonal instrument structure attaches. The bus includes all systems required for central command and data handling, uplink and downlink, propulsion, and attitude control. The large solar arrays attach to the spacecraft bus and are folded for launch and deployed upon the spacecraft reaching orbit. The solar arrays provide power for all subsystems, including the L-band and S-band radars, which require substantial average power for operation on-orbit.
The spacecraft bus’s Attitude and Orbit Control Subsystem (AOCS) is designed to address several critical science-enabling functions:
It must fly along the same orbit to within narrow tolerances (1640 ft/500 m) over the life of the mission.
It must be able to control the attitude of the observatory as a whole to point at a fixed angular location relative to an ideal orbit track and nadir at any given point on orbit.
It must be able to slew and hold attitude to observe Earth from both sides of the orbit plane.
For orbit control, there is sufficient fuel to accommodate at least five years of operations at the chosen orbit altitude. The propulsion system is agile enough to perform the necessary small orbit control maneuvers every few days that are required to maintain the strict orbital tube requirements.
The baseline science observation plan calls for up to 26 terabits (Tb) per day of radar data collection, downlink and processing. This plan drives the spacecraft design to include a Ka-band telecom system to accommodate the high bandwidth requirements. NASA is supplying a high-capacity/high-speed solid-state recorder (9 Tb at end of life) and high-rate Ka-band payload communication subsystem to manage the large amount of data collected.
NASA augments the ISRO spacecraft capability with GPS receivers, providing GPS time message and a 1 pulse per second (PPS) signal to the spacecraft and radar instruments, allowing precision orbit determination and onboard timing references. Also supplied is the pyro firing system for boom and antenna deployments and a payload data system that monitors and controls the NASA systems and handles communications between all of the NASA systems and the ISRO spacecraft bus.