Spacecraft
The spacecraft body is roughly a box, approximately 3.2 × 3.3 × 2.1 m in size. The box mounted the 1.65 m dish antenna and the GPHS-RTG radioisotope thermoelectric generator (RTG) power source. The box was divided into noisy and quiet sections. The noisy section abutted the RTG; the quiet section housed the instrument electronics. Particularly "loud" components, such as the preamps for the radio dipole, were mounted outside the structure entirely, and the box acted as a Faraday cage.
Ulysses is spin-stabilised about its z-axis which roughly coincides with the axis of the dish antenna. The RTG, whip antennas, and instrument boom are placed to stabilize this axis. Spin is nominally 5 rpm. Inside the body is a hydrazine fuel tank. Hydrazine monopropellant was used for course corrections inbound to Jupiter, and later used exclusively to repoint the spin axis (and thus, the antenna) at Earth. The spacecraft is controlled by eight thrusters, in two blocks. Thrusters are pulsed in the time domain to perform rotation or translation. Four Sun sensors detected orientation. For fine attitude control, the S-band antenna feed is mounted slightly off-axis. This offset feed combined with the spacecraft spin introduces an oscillation to an S-band radio signal transmitted from Earth when received on board the spacecraft. The amplitude and phase of this oscillation is proportional to the orientation of the spin axis relative to the Earth direction. This method of determining the relative orientation is called CONSCAN and was widely employed in early infra-red guided missiles.
The spacecraft uses S-band for uplinked commands and downlinked telemetry, through dual redundant 5-watt transceivers. The spacecraft used X-band for science return (downlink only), using dual 20 W TWTAs until the failure of the last remaining TWTA in January 2008. Both bands use the dish antenna with prime-focus feeds, unlike the Cassegrain feeds of most other spacecraft dishes.
Dual tape recorders, each of approximately 45-megabit capacity, store science data between the nominal eight-hour communications sessions during the prime and extended mission phases.
The spacecraft is designed to withstand both the heat of the inner Solar System and the cold at Jupiter distance. Extensive blanketing and electric heaters protect against cold.
Total mass at launch was 366.7 kg (808 lb), of which 33.5 kg was hydrazine (used for attitude control and orbit correction).
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