Power System Performance

The TRACE Power subsystem of 200 Watts on average per orbit comes from four deployed panels of gallium arsenide solar cells totalling 2.0 m2. The power supply accommodates the degradation of the solar arrays over the course of the mission. During TRACE's first year, the actual production was about 280 W of which TRACE only consumes 115 to 125 W. A good size margin meets peak consumption needs and provides quick battery recharge. The power that is not used is dissipated through a series of resistors or shunts in the form of heat which is radiated into space.

Electrical Power Distribution

Virtually every electrical or electronic component on a spacecraft may be switched on or off via command. This is accomplished using solid-state or mechanical relays that connect or disconnect the component from the common distribution circuit, called a main bus. On some spacecraft, it is necessary to power off some set of components before switching others on in order to keep the electrical load within the limits of the supply. Voltages are measured and telemetered from the main bus and a few other points in the electrical system, and currents are measured and telemetered for many individual spacecraft components and instruments to show their consumption. Typically, a shunt-type regulator maintains a constant voltage from the power source. The voltage applied as input to the shunt regulator is generally variable but higher than the spacecraft's required constant bus voltage. The shunt regulator converts excess electrical energy into heat, which is radiated away into space via a radiating plate.

Electrical Power Storage

Spacecraft which use photovoltaics are usually equipped with rechargeable batteries, which receive a charge from the main bus when the solar panels are in the sunlight, and discharge into the bus to maintain its voltage whenever the solar panels are shadowed by the planet or off-pointed during spacecraft maneuvers. TRACE uses a 9 Amp-hr nickel-cadium battery. Due to the TRACE's Sun-synchronous orbit, there is only one eclipse season of 3 months per year during which TRACE must rely on the battery for power during the night portion of the day/night cycle.

Sun-Synchronous Orbit