Power & Thermal Subsystem
The Radioisotope Thermoelectric Generator and Power Regulation
The RTG currently provides 202W to power the spacecraft. The amount of power provided by the RTG slowly reduces as its nuclear fuel decays. The RTG powers the spacecraft Main Bus to which all the spacecraft electrical units are connected. The spacecraft Main Bus Voltage is maintained within a 2% tolerance of 28V under all operational conditions by the Main Bus control circuitry, consisting of three error amplifiers each of which feeds redundant Majority Voting Logic units. The MVL determine how much power must be routed to the Internal Power Dumpers in order to maintain the Bus.
Under certain curcumstances, there may not be enough power to dump to the IPDs and to maintain the Main Bus Voltage; this is a potentially serious situation as:
If heaters warming components containing fuel are not powered, the fuel could freeze.
Many of the critical spacecraft units rely on consistent, regulated 28V power to function correctly.
To prevent the first of these eventualities, a Saturation Detector monitors the power level of the Hot Case Heater, a critical heating curcuit for fuel-bearing components. If this curcuit power drops below rated power for more than 24 seconds, the Saturation Detector disconnects the payload from the Main Bus by opening the Main Switch. Since the payload science instruments consume about 55W, this action, termed a "Disconnect Non-Essential Loads" (DNEL), should restore sufficient power to ensure heater saturation.
Main Bus Under Voltage (MUV)
Should a more serious failure cause a drop in the regulated voltage, the Main Bus Undervoltage Detector will switch off the spacecraft payload, and swich off the Electronic Power Converters (EPCs) that power the amplifiers for the X-band downlink. These two actions should make about 130W temporarily available to support the Main Bus. An MUV will result in temporary loss of Ulysses' downlink signal, which will be automatically restored after about 5 minutes when the EPC logic powers up the redundant power converter and amplifier.
Subsystem Reconnect Logic (SRL)
MUV is a dramatic action to support the Main Bus Voltage and thereby return the spacecraft to a functional state, however it leaves the spacecraft in a hazardous, exposed condition in which it may not be able to survive for long. SRL is hardwired to switch the spacecraft units into a state that ensures the long-term support of the Main Bus. SRL switches on Data Handling Converter 1 and both Receivers, to ensure the spacecraft can receive and process telecommands. SRL diverts all remaining power to the External Linear Dumpers by switching on the Cold Case Heater, Hot Case Heater, and the 100W and 50W External Power Dumpers.