**3.1 Cassini orbiter subsystem functional descriptions**

Cassini was a "stacked configuration" containing a lower equipment module, a propulsion module, an upper equipment module, and a High-gain Antenna (HGA). The Huygens Probe, Remote Sensing Pallet, and Fields & Particles Pallet of scientific instruments are attached to the stack within the upper equipment module, which contains the orbiter's 12-bay electronics bus, along with an 11-m magnetometer boom. Several engineering subsystems/devices control the spacecraft's operation as defined below:

*Command and data subsystem* (*CDS*): The CDS consists of two redundant computers that receive ground commands and memory loads through the RFS subsystem, processing and distributing the data to designated instruments and

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**Figure 6.**

*The Cassini orbiter instrument and device suite.*

*Robotic Autonomous Spacecraft Missions: Cassini Mission-To-Saturn Example*

for Flight Software (FSW) loads and captured science data.

subsystems. CDS also receives data from Cassini's various subsystems and instruments, processing and formatting the data into telemetry packets by applying Reed-Solomon encoding, and then delivering the data to earth-based DSN ground stations through the telemetry data stream via the RFS subsystem. CDS also contains two Solid State Recorders (SSRs) with a 2.01-gigabit mass storage capability

*DOI: http://dx.doi.org/10.5772/intechopen.82161*

*Robotic Autonomous Spacecraft Missions: Cassini Mission-To-Saturn Example DOI: http://dx.doi.org/10.5772/intechopen.82161*

subsystems. CDS also receives data from Cassini's various subsystems and instruments, processing and formatting the data into telemetry packets by applying Reed-Solomon encoding, and then delivering the data to earth-based DSN ground stations through the telemetry data stream via the RFS subsystem. CDS also contains two Solid State Recorders (SSRs) with a 2.01-gigabit mass storage capability for Flight Software (FSW) loads and captured science data.

**Figure 6.** *The Cassini orbiter instrument and device suite.*

*Aerospace Engineering*

• CAssini Plasma Spectrometer (CAPS)

• Cosmic Dust Analyzer (CDA)

Huygens probe instruments:

• Aerosol Collector Pyrolyzer (ACP)

• Doppler Wind Experiment (DWE)

plasma, energetic particles, and dust particles.

magnetosphere of the Saturnian system.

**3.1 Cassini orbiter subsystem functional descriptions**

• Surface Science Package (SSP)

• Magnetospheric IMaging Instrument (MIMI)

• Ion and Neutral Mass Spectrometer (INMS)

• Huygens Atmospheric Structure Instrument (HASI)

• Gas Chromatograph/Mass Spectrometer (GCMS)

The instruments on the Cassini orbiter were body-mounted (no scanning platforms), which required the spacecraft to be oriented toward specific science targets for some instruments. Optical instruments provided imagery and spectrometry, while the Radar supplied imaging, altimetry, and radiometry. Radio links contributed information about intervening material and gravity fields. Other instruments on the orbiter were used to measure electromagnetic fields and the properties of

The Huygens probe was spin-stabilized, returning data via an S-band link to the Cassini orbiter. The probe's six instruments included several sensors to determine the atmospheric properties and composition of Titan. The probe's radiometric and optical sensors produced data on thermal balance and captured images of Titan's atmosphere and its surface. Wind profiles were captured by Doppler measurements between the probe and orbiter. Surface sensors on the probe were implemented to measure the surface impact acceleration, in addition to thermal and electrical properties.

The combined Cassini-Huygens instrument suite enabled scientists to determine the composition, physical, morphological, geological nature, and chemical processes of Saturn and Titan's atmospheres, to investigate their surfaces, and the

Cassini was a "stacked configuration" containing a lower equipment module, a propulsion module, an upper equipment module, and a High-gain Antenna (HGA). The Huygens Probe, Remote Sensing Pallet, and Fields & Particles Pallet of scientific instruments are attached to the stack within the upper equipment module, which contains the orbiter's 12-bay electronics bus, along with an 11-m magnetometer boom. Several engineering subsystems/devices control the spacecraft's opera-

*Command and data subsystem* (*CDS*): The CDS consists of two redundant computers that receive ground commands and memory loads through the RFS subsystem, processing and distributing the data to designated instruments and

• Descent Imager/Spectral Radiometer (DISR)

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tion as defined below:

**Figure 7.** *Huygens probe instrument suite.*

*Attitude and articulation control subsystem* (*AACS*): The AACS is comprised of two redundant computers which provide three-axis stabilization attitude control by either reaction wheel assembly (RWA) control or by the reaction control system (RCS) thrusters. Two sun sensor assemblies (SSA) and two stellar reference units (SRU) provide celestial attitude reference. Inertial reference is furnished by vibrating (nonrotating) gyros. An accelerometer on the central z-axis aids in controlling the duration of the engine burns. AACS flight computers receive commands from the CDS by way of a data bus, sending commands over its own data bus to the AACS controlled assemblies.

*Propulsion module subsystem* (*PMS*): The PMS contains two redundant (gimbaled) 445-N engines with a specific impulse of 3020 N-s/kg (308 lbf-s/lbm), respectively. Approximately, 3000 kg of nitrogentetroxide and monomethylhydrazine are housed in the main bipropellent tanks. A retractable cover protects the main engines (ME) from damage by dust and micrometeoroid impacts. Four sets of mono-propellant hydrazine RCS thrusters (0.2–1.0-N thrust) fire in a direction parallel and perpendicular to the HGA (130 kg hydrazine tank capacity). Helium pressurization feeds the ME and RCS liquid propellants.

*Power and pyrotechnic subsystem* (*PPS*): Power was provided by three radioisotope thermoelectric generators (RTG). At the beginning of Cassini's mission, an allocation of 882 W of power was available, declining to 600 W by the end of the Solstice mission. The PPS distributes regulated 30-V dc power to orbiter instruments and subsystems by way of a power bus and 192 solid-state power switches (SSPS). Firing of pyrotechnic devices is supplied by the PPS once commands are received by the

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*Robotic Autonomous Spacecraft Missions: Cassini Mission-To-Saturn Example*

antennas for uplink commanding, and to capture downlinked telemetry.

CDS subsystem. A shunt radiator disposes all unused heat energy from the RTGs by

*Radio frequency subsystem* (*RFS*): An X-band link (7.2–8.4 GHz) provides communication between the ground and Cassini orbiter. Contained in this subsystem are redundant deep space transponders (DST; each includes a receiver and an exciter), and redundant traveling-wave tube power amplifiers which provide a 20 W radio frequency output [3]. The RFS also includes two command detector units (TCU), an ultra-stable oscillator (USO) for the radio science investigation, as well as an auxiliary oscillator. Telemetry modulation units and interface control units are also included. DSN station support is supplied through the use of 70- and 34-m ground

Cassini's antenna suite consists of a 4-m parabolic HGA and two low-gain antennas (LGA) fixed to the structure of the vehicle. Communication is accomplished through an X-band feed. To receive telemetry from the Huygens probe after Cassini/ probe separation, an S-band feed was used. A Ka-feed and 5 Ku-feeds supplied

*Thermal subsystem*: The thermal subsystem provides control of vehicle temperatures by the application of reflective multilayer insulating blankets, radiators, reflective and absorptive paints, louvers, shades, radioisotope heater units, and electrical heaters. For selected devices, autonomous thermal control (ATC) is applied.

In general, redundancy was applied to devices whose failure could cause loss of the mission, or loss of data from more than one scientific instrument. Onboard fault protection (FP) was designed into the system to safeguard against many possible fault conditions. Most electronic parts were radiation hardened and designed to be

Before spacecraft like Cassini can be launched, designers must consider external and internal influences on all devices and instruments. These components must be monitored, regulated, and controlled on a continuous basis during the entire

Temperature conditions internal and external to the spacecraft must be monitored constantly. The vacuum of space exposes the spacecraft to intense heat from the sun when the spacecraft is in close proximity, causing its surfaces to superheat. Shadowed surfaces are subject to extremely low temperatures which can cause onboard propellants to freeze. Once frozen, the spacecraft will be rendered inoperative, since it inhibits the spacecraft's ability to maneuver, so that it will eventually become misaligned with the earth (and unable to receive ground commands). Material stresses are also a concern with these temperature extremes, since thermal expansion-contraction can introduce camera distortion, breakage of components, and warpage. Also, computers and spacecraft components will cease to work if temperatures become too extreme. Instruments can fall out of operating limits, since many devices only function properly within a narrow range of temperatures. Heat build-up can also occur from the spacecraft's own systems. For Cassini, several protective measures were applied to control these hazardous conditions: the application of reflective multilayer insulating blankets to reflect the sun's heat, radiators were added, reflective/absorptive paints applied, louvers and shades installed, radioisotope heater units added, in addition to the inclusion of electrical heaters and

*DOI: http://dx.doi.org/10.5772/intechopen.82161*

additional beams for radar experiments.

resistant to single-event upsets (SEU).

**4. Cassini's mission challenges**

lifetime of the mission.

**4.1 Flight environment**

radiating the excess into space.

#### *Robotic Autonomous Spacecraft Missions: Cassini Mission-To-Saturn Example DOI: http://dx.doi.org/10.5772/intechopen.82161*

CDS subsystem. A shunt radiator disposes all unused heat energy from the RTGs by radiating the excess into space.

*Radio frequency subsystem* (*RFS*): An X-band link (7.2–8.4 GHz) provides communication between the ground and Cassini orbiter. Contained in this subsystem are redundant deep space transponders (DST; each includes a receiver and an exciter), and redundant traveling-wave tube power amplifiers which provide a 20 W radio frequency output [3]. The RFS also includes two command detector units (TCU), an ultra-stable oscillator (USO) for the radio science investigation, as well as an auxiliary oscillator. Telemetry modulation units and interface control units are also included. DSN station support is supplied through the use of 70- and 34-m ground antennas for uplink commanding, and to capture downlinked telemetry.

Cassini's antenna suite consists of a 4-m parabolic HGA and two low-gain antennas (LGA) fixed to the structure of the vehicle. Communication is accomplished through an X-band feed. To receive telemetry from the Huygens probe after Cassini/ probe separation, an S-band feed was used. A Ka-feed and 5 Ku-feeds supplied additional beams for radar experiments.

*Thermal subsystem*: The thermal subsystem provides control of vehicle temperatures by the application of reflective multilayer insulating blankets, radiators, reflective and absorptive paints, louvers, shades, radioisotope heater units, and electrical heaters. For selected devices, autonomous thermal control (ATC) is applied.

In general, redundancy was applied to devices whose failure could cause loss of the mission, or loss of data from more than one scientific instrument. Onboard fault protection (FP) was designed into the system to safeguard against many possible fault conditions. Most electronic parts were radiation hardened and designed to be resistant to single-event upsets (SEU).
