Spacecraft

The spacecraft bus, mounted below the payload module that carries the science instruments, houses most of the subsystems needed to fly the mission. These include:

This subsystem generates, stores, and distributes the power needed to operate the spacecraft. The source of power is sunlight collected by SWOT’s two solar arrays, which are 48.8 feet (14.9 meters) in length with an area of 335 square feet (31 square meters). Deployed from opposite sides of the spacecraft bus, the two arrays remain pointed at the Sun via small motors.

The KaRIn instrument produces about one kilowatt of heat as it operates, and its temperature must remain extremely stable, changing less than 0.09 degrees Fahrenheit (0.05 Celsius) per minute. To maintain allowable flight temperatures, the thermal subsystem uses multiple tools, including mechanical-thermostat and flight-software-controlled heating elements, multilayer insulation blankets, thermal-control surface finishes, high-conductivity interface materials, and materials for heat spreading.

To determine SWOT’s precise orientation in space, or attitude, SWOT will use star trackers. These navigation devices collect images of stars around the spacecraft and compare them to patterns formed by stars with well-documented locations and brightness. By comparing the observed star locations with the known patterns, the trackers can calculate exactly where the spacecraft is. Adjustments in the spacecraft’s orientation will be carried out as needed using electromagnets (magnetic torquers) and reaction wheels (spinning wheels used on many missions that help a spacecraft adjust and maintain its orientation) mounted on the bus.

SWOT will adjust its orbit when necessary by firing its thrusters – small rocket engines used only for in-flight corrections. This subsystem also includes a propellant tank.

Operational communications between SWOT and Earth will take place four times a day on S-band microwave wavelengths (2-4 GHz), which are commonly used for satellite communications. Science data will be transmitted to the ground 21 times a day by an antenna using X-band microwave wavelengths (8-12 GHz).

A centralized processor will manage the observatory’s communication links and perform “housekeeping” tasks such as sending and receiving commands and storing data.

CNES’ Satellite Control Center in Toulouse, France, will oversee satellite operations. Four times a day, CNES’ S-band telemetry network will uplink commands and other communications to some of its ground stations in Canada, Sweden, South Africa, French Guiana, and Aussaguel (near Toulouse) in France. NASA instruments will be monitored from the instrument control center at JPL.

The SWOT payload module will downlink science data about 21 times a day using X-band radar wavelengths. About 1 terabyte of data will be downlinked from SWOT every day, with KaRIn data representing about 99% of the total. The spacecraft will process some ocean data onboard to reduce the volume that must be downlinked, but higher-resolution data on areas on freshwater bodies will be downlinked directly and processed on the ground.

CNES will process the entire set of daily operational data to create various science and research products. Routine products will be generated within a few days of data acquisition. JPL reprocesses all needed ocean and hydrology science data products during the prime mission. Data products will be distributed via both NASA’s Physical Oceanography Distributed Active Archive Center and CNES distribution portals.