SPACECRAFT
Structure	Sandwich CFRP
Booms	1.8 m, CFRP
Kick-Motor	Star 13A
At'tude Cntrl.	Magnetorquers
At'tude Dterm.	Sun-Sensor and Magnetometer Data
Solar Arrays	16 Panels, 70 W BoL
Batteries	4 Ni-Cd, 1 Ah each
System Unit	Spacecraft Computers Mass Memory Attitude Control Power Distribution Pyro-Conrol
Communication	S-Band 500 bps receiver 32 - 262 kbps transmitter 1-W output power CRC-code
Total Mass	230 kg

Table 1

For scientific reasons the satellite has to be spin stabilized with a comparatively high spin rate of 40 rpm. This fact and the field-of-view requirements of the scientific instruments determined the spacecraft design. The spacecraft structure has two main decks, the lower one for the scientific instruments, the upper platform for the spacecraft subsystems. They are connected by a central tube, which houses the solid propellant kick motor, and by diagonal struts, conducting the loads to the interface adapter. The satellite is covered by 16 side panels and a self carying hood of sixteen solar panels with a top plate. All decks and panels are made from carbon-fiber or aluminum sandwich material with rigid foam cores, while the tube and the struts are fabricated from kevlar- or carbon-fiber prepregs. Two 1.8 m carbon-fiber magnetometer booms are deployed radially and three of the six antennas axially.

The spacecraft mass is 230 kg, the maximum width 1.65 m and the height 1.26 m. The spin axix will be oriented perpendicular to the earth-sun-line so that the solar panels are properly illuminated by the sun.

Spin-rate and spin-axis orientation are controlled by magnetorquers: two 100-Am² coils and four 140-Am² rods. Attitude is determined from sun-sensor and magnetometer data.

Power is generated by 16 planar solar panels mounted around the circumference of the spacecraft. Each panel is covered by two strings of conductively coated Ga-As cells, one high-power string consisting of 36 large-area cells, and one low-power string consisting of 36 small-area cells. Begin-of-life power is 70 W. Four Ni-Cd batteries, with 1 Ah capacity each, provide power during eclipses. They are charged by the current from the low-power strings.

All spacecraft functions are controlled from a single box, the Flight System Unit (FSU). It contains redundant spacecraft computers, based on NSC-800 micro-processors, the 1.5 Gbit mass memory for data storage, the power-distribution unit, the magnetorquer electronics, and the pyro-system that supplies the currents for firing the kick-motor and some electro-explosive devices. Concentration of all these functions in a single unit reduces the number of interfaces, improves reliability, and simplifies testing. The software for the FSU enjoys a significant degree of heritage from AMPTE/IRM, where the same processors were employed.

A set of four antennas, two of them on deployable booms, supports ground communications. Commands are uplinked at 500 bps in the S-band. Data are telemetered to the ground via an S-band transmitter with 1-W output power, at rates that vary between 32 and 262 kbps. A CRC-code is used for error detection and correction. Receivers and transmitters are redundant.

Table 1 lists some key spacecraft features.