Figure 1. The SOHO spacecraft, showing the Payload Module (PLM) and Service Module (SVM) elements
The SOHO spacecraft consists of two major elements (Fig. 1), the Service Module (SVM) and the Payload Module (PLM).
The Service Module is a box-shaped structure made of aluminium honeycomb panels attached to a corrugated aluminium cylinder by four shear webs. The four lateral panels carry the data-handling, communication, attitude and orbit control, and power subsystems. The box's upper floor houses the propulsion subsystem, tank and thruster masts. The high-gain antenna is mated into the aft part of the central cylinder.
The Payload Module provides an optical bench for the experiments and is composed of four upper lateral panels and a top panel connected to a central cylinder by a number of shear webs. The bottom section, consisting of three lower lateral panels connected to the central cylindrical tube by a number of shear webs and floors, houses the PLM electronics.
SOHO's nominal operational lifetime is 29 months, including the four months of the transfer phase and one month of in-orbit commissioning. It carries sufficient onboard propellant for six years of operation. The main spacecraft performance parameters are given in Table 1.
Figure 1. The SOHO spacecraft, showing the Payload Module (PLM) and Service Module (SVM) elements
The PLM's thermal-control subsystem maintains all equipment mounted on the PLM structure within acceptable temperature limits and provides a stable thermal environment to meet all pointing requirements. Temperature control for both the PLM and SVM is provided with dedicated adjustable heaters. Substitu-tion heaters replace the heat inputs of the experiments at times when the latter are switched off.
Critical spacecraft elements are temperature-stabilised by adjusting the heaters to compensate for seasonal variations and ageing. The temperature of the front sunshield assembly, for example, will be maintained to within +-1 deg C for two months.
The PLM/SVM interface will be maintained at 20 deg C +- 2.5 deg C in order to limit any deformation of the PLM structure.
The heart of this subsystem is a Central Data Management Unit(CDMU) responsible for:
The packet telemetry is used in conjunction with convolutional and Reed-Solomon encoding.
Three remote terminal units under the supervision of the CDMU provide the necessary interfaces to the users. Universal time (UTC) is generated on board by an ultrastable oscillator, and distributed to the experimenters.
Automatic failure detection and reconfiguration is provided and vital spacecraft parameters are retained in a special 'context memory'.
Figure 2. The CDMU's solid-state memory
The Attitude and Orbit Control Subsystem's (AOCS) overall task is to provide the SOHO spacecraft with the requisite pointing performance during the various spacecraft activities as the mission evolves from SOHO's separation from the Centaur upper stage until and throughout its mission as an operational space solar observatory in a halo orbit around the L1 Lagrangian point, 1.5 million kilometres from Earth, for up to six years.
Immediately after SOHO's separation from the Centaur upper stage, the AOCS will cancel the angular rates imparted by the separation and will coarsely align the spacecraft towards the Sun using hydrazine thruster and Sun-acquisition sensors arranged in a configuration giving omni-directional coverage. The control algorithms reside in the memory of an onboard microprocessor that processes the sensor data and issues commands to the thrusters.
SOHO's solar panels (Fig. 3) will subsequently be deployed with the AOCS in stand-by configuration. After full solar-panel deployment has been achieved, the AOCS will again coarsely point the spacecraft at the Sun and perform the transition to fine Sun pointing and roll-angle control using a fine-pointing Sun sensor (Fig. 4), a star tracker and three reaction wheels. It is in this configuration that the payload instruments will make their scientific observations, when the spacecraft's pointing towards the Sun is stabilised to within a few tenths of an arcsecond under quiescent conditions, i.e. when no spacecraft or experiment mechanism is being operated, or within about one arcsecond when some mechanisms are active to, for example, realign the spacecraft's high-gain antenna or to adjust an experiment's line of sight.
Figure 3. Deployment testing of SOHO's solar arrays, at Intespace in Toulouse (F)
Figure 4. One of the AOCS fine-pointing Sun sensors
The power subsystem provides regulation, protection and distribution of 1500 W of solar-array power, supported by two 20 Ah nickel-cadmium batteries. The main spacecraft power bus is regulated to 28 V +-1% with a three-domain regulation of solar-array shunt mode, batterydischarge and battery charge mode. All power lines to users are protected by latching-current limiters.
Decentralised undervoltage protection is provided in each Latching Current Limiter (LCL), which provides automatic switch-off if the input voltage falls below 23.5 V +1V.In the event of a main-bus undervoltage, the system automatically enters a safe mode by switching off all LCLs until the main bus recovers. In addition, foldback current limiters are provided for some essential loads, as well as regulated battery power for the Kevlar cable cutters.
All electrical power subsystem functions are redundant, including connector redundancy all the way from the solar-array and battery inputs to the power distribution outputs.
Table 1. SOHO facts and figures
The SOHO Launch Scenario
The SOHO spacecraft will be tracked with the 26 m antenna of NASA's Deep-Space Network. The links between the spacecraft and ground will be provided by an S-band RF system, with duplicated receivers and transmitters. RF coverage is provided by a pointable high-gain antenna and two low-gain antennas, to ensure the full coverage.
SOHO's High-Gain Antenna (HGA) assembly is composed of a 0.8 m-diameter antenna and associated mechanisms (Fig. 5). During launch, the HGA is rigidly attached to the spacecraft and is released only after spacecraft separation. The halo orbit requires that the HGA be capable of pointing in all directions within a +-32 degrees cone.
Figure 5. The High-Gain Antenna (HGA) assembly
The data-handling central onboard software (written in ADA) resides in a 16-bit computer (MAS 281) that uses the 1750A MIL Standard Instruction Set and contains a set of programs controlling the data handling. The central on-board software will play a pivotal role in performing a variety of functions, including telecommand and telemetry management and numerous application programme tasks. It manages and distributes the onboard time, performs the thermal monitoring and thermal control of the satellite, manages the antenna pointing and monitoring, handles the inter- instrument data exchange, performs various other monitoring functions, and executes the initial Sun-acquisition sequence.
The attitude and orbit control software resides in a similar 16-bit computer and obtains outputs from the spacecraft's attitude sensors in order to provide commands to the attitude-control actuators, after processing the digitally implemented control algorithms. In addition, it organises the data for telemetry communication and processes the telecommands.
At lift-off, the spacecraft will be powered by the batteries, the data handling will be powered, and the Service Module will be thermally controlled. Both gyroscopes will be on and their temperatures controlled.
After SOHO's separation from the launcher, the spacecraft's attitude-control and communications subsystems will be activated. The low-gain antenna will be used for communication with the NASA Deep-Space Network (DSN). The initial Sun acquisition will be performed.
After solar-array deployment, the data-handling subsystems will re-initialise the attitude control subsystem. The roll attitude acquired by the Centaur prior to separation will be maintained. The transfer-orbit correction manoeuvres, halo-orbit station-keeping manoeuvres, and momentum-wheel off-loading will be initiated as necessary.
The routine operating scenario during the halo-orbit phase will be as follows:
SOHO will be in continuous contact with the DSN for helioseismology data return for a period of two months per year.
As their contribution to this ESA/NASA collaborative mission, NASA is supplying:
The author would like to acknowledge the contributions made by the following spacecraft team members: J. Candé, G. Coupé, P. Rumler, P. Strada and F. Teston.
ESA Bulletin Nr. 84.