Command and Data Subsystem
The Command and Data Subsystem was the "brain" of the spacecraft. It stored and processed data from all of the subsystems, sensors and science instruments. It also provided commands to all of the subsystems and instruments. Commands could be either issued from the ground or through on-board fault protection software which placed the spacecraft in a safe, stable, state and able to receive commands should an equipment failure occur. The software also responded automatically to faults requiring immediate action. The heart of this subsystem was the Engineering Flight Computer. Designed and fabricated by IBM, this computer interfaced with all the spacecraft components through a bus interface system.
Solid State Recorder
The Solid State Recorder Subsystem recorded science data and information on the spacecraft's health and status. The words "solid state" mean the recorder had no moving parts, and Cassini-Huygens was the first deep space mission to use this technology. Older missions had to rely on flight tape recorders to store the data collected. In addition to its recording and playback functions, the recorder was used to store critical flight programs. Science data was periodically sent to Earth and then erased from the recorder in order to free space on the disk to collect new data.
Propulsion Module Subsystem
The Propulsion Module Subsystem provided thrust, also called "directed impulse," for spacecraft trajectory and orbit changes, and for attitude control. The main engine was used for spacecraft velocity and trajectory correction changes. To be on the safe side, the spacecraft had two identical main engines: One was in use with the other as a backup. There were also 16 monopropellant hydrazine thrusters arranged in four groups of four. The thruster engines were used for attitude control and also for small velocity-change maneuvers.
Attitude and Articulation Control Subsystem (AACS)
The Attitude and Articulation Control Subsystem was responsible for three functions, with varying abilities. Its first responsibility was to maintain attitude control of the spacecraft, which means its position along three axes. The second, to a much lesser degree, was articulation, and the third function was pointing control of the main propulsion engines relative to the spacecraft.
In order to alter its position, it must first know where it is and its orientation. This is called attitude determination and was achieved in the Attitude and Articulation Control Subsystem by three Inertial Reference Units (IRUSs) and a Stellar Reference Unit (SRU), or star tracker. The inertial reference units used solid-state gyroscopes developed by the Delco Division of the Hughes Aircraft Company. The stellar reference unit essentially navigated by the stars. It detected stars in its field of view and it compared the view with its onboard catalog of 5,000 stars. Finally, Reaction Wheel Assemblies (RWAs) were one of the two systems used to provide pointing control of the spacecraft in flight (with the thrusters of the Propulsion Module Subsystem as the other). The reaction wheel assemblies contained electrically powered wheels. They were mounted along three orthogonal axes aboard the spacecraft.
Power and Pyrotechnics Subsystem (PPS)
The Power and Pyrotechnics Subsystem provided regulated 30 Volts DC electrical power to the spacecraft. The power was derived from the three Radioisotope Thermoelectric Generators (RTGs) onboard. It was then conditioned and distributed to the powered spacecraft components. This subsystem also initiated electro-explosive, or pyrotechnic, devices. These devices were used throughout the spacecraft to initiate one-time events such as separating the spacecraft from the Centaur launch vehicle.
Radio Frequency Subsystem (RFS)
The Radio Frequency Subsystem, together with the antenna subsystem, provided communication functions for the spacecraft to and from Earth. Part of the radio frequency subsystem was also used by the Radio Science Instrument. For telecommunications, the radio frequency subsystem produced an X-band carrier at 8.4 Ghz, modulated it with data received from CDS, amplified the X-band carrier power to produce 20 Watts from the Traveling Wave Tube Amplifiers (TWTA), and delivered it to the antenna subsystem.
The parts of this subsystem used for the radio science instruments were: The High-gain Antenna (ANT), the Ultra Stable Oscillator (USO), the Deep Space Transponders (DSTs), the X-band Traveling Wave Tube Amplifiers (X-TWTAs), and the X-band Traveling Wave Tube Amplifier. The other acronym shown in the photo is the Microwave Components (MW).
Antenna Subsystem (ATM)
The Antenna Subsystem consisted of the High-Gain Antenna (HGA) and two Low-Gain Antennas (LGA-1 and LGA-2). The primary function of the high-gain antenna was to support communication with Earth. It was also used for S-band Huygens Probe Science, Ku-band RADAR, and Ka-band Radio Science. The high-gain antenna was a Cassegrain antenna consisting of a 4-meter (13.1-foot) parabolic primary reflecto, a sub-refractor mounted in front of the focal point of the primary reflector and the feed horn between the two.
To shield the harmful hot rays of the sun from the spacecraft's instruments during most of the early portion of the long journey to Saturn, the high-gain antenna was positioned toward the sun, functioning as an umbrella. With its most powerful antenna not pointed toward Earth, the spacecraft used the low-gain antennas to exchange information with ground controllers. Low-gain antennas also have the added bonus to provide omni directional coverage, as opposed to the high-gain antenna, which had to be accurately pointed. Once Cassini-Huygens was far enough from the Sun, it finally began using the high-gain antenna for communicating with Earth, thus achieving much faster transmission rates.
The Structure Subsystem provided mechanical support and alignment for all flight equipment, including the Huygens probe. In addition to its skeletal function, it provided thermal conductivity, served as an equipotential (i.e. not preserving any unbalanced electrical field) and was an electrical grounding reference. It was also used as shielding from radio frequency interference and protected other spacecraft equipment from radiation and micrometeoroids. Before launch, it provided attachment points for ground handling.
Mechanical Device Subsystems (DEV)
The Mechanical Device Subsystems supplied equipment to the spacecraft that provided non-feedback controlled motion. These subsystems supplied a number of mechanisms for separating Cassini from the Centaur launch vehicle, as well as all of the required pyrotechnic devices and initiators. They also provided the deployable Magnetometer Science Boom Assembly (MAG); an articulated platform for the redundant reaction wheel; Thermal Louver Assemblies (TLAs) for passive heat transfer; and Variable Radioisotope Heater Units (VRHUs).
The other acronyms included in the photo are: Sun Sensor Head Integration Structures (SSHISs), Articulated Reaction Wheel Mechanism (ARWM), and Dual Drive Actuator (DDA).
Electronic Packaging Subsystem (EPS)
The Electronic Packaging Subsystem contained almost all of the electronic equipment for the orbiter. This subsystem consisted of a circular electronics bus made up of 12 standardized bays containing the electronics modules. The packaging of all electronic assemblies was designed with attention to functional, cabling, temperature control, radiation, magnetic and center-of-gravity considerations. In addition, the electronics assemblies were shielded from electromagnetic interference and electric cross-coupling.
The Cabling Subsystem provided system wiring for all of the other subsystems. Interconnections were required for power, instrumentation, command, data, signal and pyrotechnic device actuations. The Cassini Cabling System was a passive system -- it contained no active electronic components, generated no signals of its own, and required no power. Its sole function was to transfer electrical signals from one subsystem to another, ideally without changing the signals in the transfer process.
Temperature Control Subsystem (TEMP)
As its name implies, the Temperature Control Subsystem was responsible for maintaining the temperature of the spacecraft within an acceptable range. Cassini's circuitous route to Saturn took it through several temperature extremes far greater than those on Earth. For example, when flying by Venus, the sun's warmth was nearly three times hotter than it is at the Earth's distance from the sun. To the other extreme, when Cassini was at Saturn, temperatures were nearly 100 times colder than on Earth. Temperature on the Cassini-Huygens spacecraft was maintained through a combination of special hardware and special handling procedures. For example, during the cruise to Saturn, the high-gain antenna was oriented toward the sun inside 2.7 Astronomical Units to shield most of the other spacecraft components. Special temperature control hardware included thermal blankets, shades, thermal shields, louvers and heaters. Thermal blankets provided insulation. Thermal shields shaded components from the sun. Louvers dissipated heat from electronics bays. Each instrument had an electrical heater, but they were used sparingly, to bring equipment up to operating temperature. However, because of clever design techniques, few other electrical heaters were needed, as waste heat from the Radioisotope Thermoelectric Generators (RTGs) was used to heat electronic equipment.