Terra
The First EOS Morning Satellite

The Terra spacecraft supports the operation of the five instruments on the mission. All instruments are mounted on the nadir-facing deck of the spacecraft for a clear view of the Earth.

To support the cooling requirements of ASTER and MOPITT, the spacecraft provides an advanced technology capillary-pumped heat transport system that transports heat from the instruments to the passive radiators mounted on the spacecraft. This advanced heat transport system, developed at NASA's Goddard Space Flight Center, was tested on the Space Shuttle and is expected to yield significant benefits to this and future spacecraft.

The science data generated by each instrument are sent to the spacecraft over high-bandwidth communication lines. These data are multiplexed and recorded in a solid-state recorder, then sent to the communication subsystem for transmission to the ground. This state-of-art solid-state recorder is designed to hold approximately two orbits of data, up to 140 Gigabytes, using advanced technology solid-state memory devices.

For transmission to the ground, two communications capabilities will be available. First, data can be transmitted via the Tracking and Data Relay Satellite System (TDRSS) on Ku band to the TDRSS Ground Station, White Sands, NM. Second, data can be transmitted directly to the ground on X band. In both cases, transmission is scheduled to occur at specific times based on the availability of TDRSS, or the proximity of a ground station. Continuous direct broadcast of MODIS data will be available on X band and can be received by individual users around the world.

The orientation of the spacecraft is maintained by the guidance, navigation, and control subsystem. This complex and precise subsystem maintains spacecraft pointing accuracy to within 150 arc-seconds of the desired pointing direction, and determines pointing to within 90 arc-seconds using star trackers. Additionally, this subsystem provides safe hold control in the event of a spacecraft operational anomaly.

Electrical power to operate all the instruments and spacecraft subsystems is provided by the electrical subsystem. Power is generated by an advanced, light-weight solar array using gallium arsenide solar cells. This power is then converted into operating voltages, regulated, and distributed to all subsystems and instruments. Electrical energy is stored in nickel hydrogen batteries to enable the spacecraft to operate on the dark portion of the orbit.