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Cassini Orbiter Engineering Subsystems

Cassini Orbiter Engineering Subsystems

The Cassini Orbiter is made up of many subsystems, each designed for specific functions. Choose from the list below to learn more about how this amazing spacecraft works.

Command and Data Subsystem
Solid State Recorder
Propulsion Module Subsystem
Attitude and Articulation Control Subsystem (AACS)
Power and Pyrotechnic Subsystem (PPS)
Radio Frequency Subsystem (RFS)
Antenna Subsystem (ATM)
Structure Subsystem
Mechanical Device Subsystem (DEV)
Electronic Packaging Subsystem (EPS)
Cabling Subsystem
Temperature Control Subsystem (TEMP)


Command and Data Subsystem
Command and Data Subsystem
The Command and Data Subsystem is the "brain" of the spacecraft. It stores and processes data from all of the subsystems, sensors and science instruments. It also provides commands to all of the subsystems and instruments. Commands can either be issued from the ground or through on-board fault protection software which places the spacecraft in a safe, stable, state and able to receive commands should an equipment failure occur. The software also responds automatically to faults requiring immediate action. The heart of this subsystem is the Engineering Flight Computer. Designed and fabricated by IBM, this computer interfaces with all the spacecraft components through
a bus interface system.

Solid State Recorder

Solid State Recorder
The Solid State Recorder Subsystem records science data and information on the spacecraft's health and status. The words "solid state" mean the recorder has no moving parts, and Cassini-Huygens is 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 is used to store critical flight programs. Science data is 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
Attitude and Articulation Control Subsystem (AACS)
The Propulsion Module Subsystem provides thrust, also called "directed impulse," for spacecraft trajectory and orbit changes, and for attitude control. The main engine is used for spacecraft velocity and trajectory correction changes. To be on the safe side, there are two identical main engines: One is in use and the other is a backup. There are also 16 monopropellant hydrazine thrusters arranged in four groups of four. The thruster engines are used for attitude control and also for small velocity-change maneuvers.
Attitude and Articulation Control Subsystem (AACS)

Attitude and Articulation Control Subsystem (AACS)
The Attitude and Articulation Control Subsystem is responsible for three functions, with varying abilities. Its first responsibility is to maintain attitude control of the spacecraft, which means its position along three axes. The second, to a much lesser degree, is articulation, and the third function is 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 is 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 use solid-state gyroscopes developed by the Delco Division of the Hughes Aircraft Company. The stellar reference unit essentially navigates by the stars. It detects stars in its field of view and it compares the view with its onboard catalog of 5,000 stars. Finally, Reaction Wheel Assemblies (RWAs) are 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 contain electrically powered wheels. They are mounted along three orthogonal axes aboard the spacecraft.

Power and Pyrotechnic Subsystem (PPS)
Power and Pyrotechnic Subsystem (PPS)
The Power and Pyrotechnics Subsystem provides regulated 30 Volts DC electrical power to the spacecraft. The power is derived from the three Radioisotope Thermoelectric Generators (RTGs) onboard. It is then conditioned and distributed to the powered spacecraft components. This subsystem also initiates electro-explosive, or pyrotechnic, devices. These devices are used throughout the spacecraft to initiate one-time events such as separating the spacecraft from the Centaur launch vehicle.
Radio Frequency Subsystem (RFS)
Radio Frequency Subsystem (RFS)
The Radio Frequency Subsystem ,together with the antenna subsystem, provides communication functions for the spacecraft to and from Earth. Part of the radio frequency subsystem is also used by the Radio Science Instrument. For telecommunications, the radio frequency subsystem produces an X-band carrier at 8.4 Ghz, modulates it with data received with data received from CDS, amplifies the X-band carrier band-carrier power to produce 20Watts from the Traveling Wave Tube Amplifiers (TWTA), and delivers it to the antenna subsystem.

The parts of this subsystem used for the radio science instruments are: 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)

Antenna Subsystem (ATM)
The Antenna Subsystem consists of the High-Gain Antenna (HGA) and two Low-Gain Antennas (LGA-1 and LGA-2). The primary function of the high-gain antenna is to support communication with Earth. It is also used for S-band Huygens Probe Science, Ku-band RADAR, and Ka-band Radio Science. The high-gain antenna is 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 fee 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 must 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.

Structure Subsystem
Mechanical Device Subsystem (DEV)
The Structure Subsystem provides mechanical support and alignment for all flight equipment, including the Huygens probe. In addition to its skeletal function, it provides thermal conductivity, serves as an equipotential (i.e. not preserving any unbalanced electrical field)and is an electrical grounding reference. It is also used as shielding from radio frequency interference and protects other spacecraft equipment from radiation and micrometeoroids. Before launch, it provided attachment points for ground handling.
Mechanical Device Subsystem (DEV)

Mechanical Device Subsystem (DEV)
The Mechanical Device Subsystems supply equipment to the spacecraft that provides non-feedback controlled motion. These subsystems supply 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 provide 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)
Electronic Packaging Subsystem (EPS)
The Electronic Packaging Subsystem contains almost all of the electronic equipment for the orbiter. This subsystem consists 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 are shielded from electromagnetic interference and electric cross-coupling.

Cabling Subsystem

Cabling Subsystem

The Cabling Subsystem provides system wiring for all of the other subsystems. Interconnections are required for power, instrumentation, command, data, signal and pyrotechnic device actuations. The Cassini Cabling System is a passive system -- it contains no active electronic components, generated no signals of its own, and requires no power. Its sole function is to transfer electrical signals from one subsystem to another, ideally without changing the signals in the transfer process.

Temperature Control Subsystem (TEMP)
Temperature Control Subsystem (TEMP)
As its name implies, the Temperature Control Subsystem is 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 is at Saturn, temperatures are nearly 100 times colder than on Earth. Temperature on the Cassini-Huygens spacecraft is 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 includes thermal blankets, shades, thermal shields, louvers and heaters. Thermal blankets provide insulation. Thermal shields shade components from the sun. Louvers dissipate heat from electronics bays. Each instrument has an electrical heater, but they are used sparingly, to bring equipment up to operating temperature. However, because of clever design techniques, few other electrical heaters are needed, as waste heat from the Radioisotope Thermoelectric Generators (RTGs) is used to heat electronic equipment.



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