ESA Science & Technology - Instruments

Introduction

To gather as much science as possible during its historic mission to the Saturnian region, the Cassini-Huygens spacecraft is equipped with eighteen instruments, twelve on the Cassini orbiter and six on the Huygens descent probe. Many of these sophisticated instruments are capable of multiple functions, and the data that they gather will be studied by scientists worldwide.

Many of the instruments have multiple functions, equipped to thoroughly investigate all the important elements that the Saturn system may reveal.

Instruments in Brief

Cassini Plasma Spectrometer

The Cassini Plasma Spectrometer (CAPS) will measure the flux of ions as a function of mass per charge, and the flux of ions and electrons as a function of energy per charge and angle of arrival relative to the instrument.

CAPS will investigate the molecules originating from Saturn's ionosphere and also determine the configuration of Saturn's magnetic field. CAPS will also investigate plasma in these areas as well as the solar wind within Saturn's magnetosphere.

Cosmic Dust Analyser

The Cosmic Dust Analyser (CDA) is intended to provide direct observations of particulate matter in the Saturnian system, to investigate the physical, chemical, and dynamical properties of these particles, and to study their interactions with the rings, icy satellites, and magnetosphere of Saturn.

Composite Infrared Spectrometer

The Composite Infrared Spectrometer (CIRS) consists of dual interferometers that measure infrared emission from atmospheres, rings, and surfaces over wavelengths from 7 to 1000 micrometers (1400 to 10 cm^-1) to determine their composition and temperatures.

CIRS will address a wide variety of science objectives for the atmospheres of Saturn and Titan, and for Saturn's icy satellites and rings, including composition determination and thermal state measurements.

CIRS will measure infrared emissions from atmospheres, rings and surfaces in the vast Saturn system to determine their composition, temperatures and thermal properties. It will map the atmosphere of Saturn in three dimensions to determine temperature and pressure profiles with altitude, gas composition, and the distribution of aerosols and clouds. This instrument will also measure thermal characteristics and the composition of satellite surfaces and rings.

CIRS Instrument Components

• Far-Infrared Focal Plane [FP1] (16.67 - 1000 µm; 4.3 mrad circular field of view)
• Mid-Infrared Focal Plane [FP3] (9.09 - 16.67 µm; 1 × 10 array of 0.273 mrad squares)
• Mid-Infrared Focal Plane [FP4] (7.16 - 9.09 µm; 1 × 10 array of 0.273 mrad squares)

Ion and Neutral Mass Spectrometer

The Ion and Neutral Mass Spectrometer (INMS) is intended to measure positive ion and neutral species composition and structure in the upper atmosphere of Titan and magnetosphere of Saturn, and to measure the positive ion and neutral environments of Saturn's icy satellites and rings.

INMS will be used to study the neutral gases and positive ions in the atmospheres of Saturn and Titan and gases in the vicinities of the Saturnian rings and the icy satellites. It will also study the magnetosphere of Saturn.

Imaging Science Subsystem

The Imaging Science Subsystem (ISS) is a remote sensing instrument that captures images in visible, infrared and ultraviolet light. The ISS has a camera that can take broad, wide-angle pictures and a camera that can record small areas in fine detail. ISS is expected to return hundreds of thousands of images of Saturn and its rings and moons.

The Cassini orbiter imaging experiments will encompass a wide variety of targets (Saturn, the rings, Titan, the icy satellites, and star fields) and a wide range of observing distances for various scientific purposes. The science objectives include studying the atmospheres of Saturn and Titan, the rings of Saturn and their interactions with the planet's satellites, and the surface characteristics of the satellites, including Titan. Because of these multiple objectives, the ISS has two separate camera designs. The first is a narrow-angle camera (NAC) design that will obtain high-resolution images of the target of interest. The second is a wide-angle camera (WAC) design that provides a different scale of image resolution and more complete coverage spatially. The spacecraft will carry one NAC and one WAC. The NAC is also used to obtain optical navigation images for the mission with the WAC acting as a functionally redundant backup unit for this purpose.

ISS Instrument Components

• Wide Angle Camera (WAC) (20 cm f/3.5 refractor; 380 - 1100 nm; 18 filters; 3.5° × 3.5° field of view; 60 microradians per pixel angular resolution)
• Narrow Angle Camera (NAC) (2 m f/10.5 reflector; 200 - 1100 nm; 24 filters; 0.35° × 0.35° field of view; 6 microradians per pixel angular resolution))

Dual Technique Magnetometer

The primary objective of the Dual Technique Magnetometer (MAG) is to determine the planetary magnetic fields and the dynamic interactions in the planetary environment.

Magnetometers are direct-sensing instruments that detect and measure the strength of magnetic fields in the vicinity of the spacecraft. The Cassini Dual-Technique Magnetometer (MAG) measures magnetic fields during the Titan and Saturn encounters. The MAG consists of a vector/scalar helium magnetometer sensor, a fluxgate magnetometer sensor, a data processing unit, three power supplies, plus operating software and electronics associated with the sensors.

MAG's goals are to develop a three-dimensional model of Saturn's magnetosphere, as well as determine the magnetic state of Titan and its atmosphere, and the icy satellites and their role in the magnetosphere of Saturn.

Magnetospheric Imaging Instrument

The Magnetospheric Imaging Instrument (MIMI) is designed to

• Measure the composition, charge state and energy distribution of energetic ions and electrons
• Detect fast neutral species
• Perform remote imaging of the Saturn's magnetosphere.

This information will be used to study the overall configuration and dynamics of the magnetosphere and its interactions with the solar wind, Saturn's atmosphere, Titan, rings, and icy satellites.

The Magnetospheric Imaging Instrument (MIMI) will provide global images of Saturnian hot plasmas remotely and will perform comprehensive direct measurements of hot plasma, including charge state and elemental composition.

Cassini Radar

The Cassini Radar (RADAR) uses the five-beam Ku-band antenna feed assembly associated with the spacecraft high gain antenna to direct radar transmissions toward targets, and to capture black body radiation and reflected radar signals from targets.

The Cassini Radar will be used to investigate the surface of Saturn's moon Titan by taking four types of observations: imaging, altimetry, backscatter, and radiometry.

RADAR Instrument Components

• Synthetic Aperture Radar Imager [SAR] (13.78 GHz Ku-band; 0.35 - 1.7 km resolution)
• Altimeter (13.78 GHz Ku-band; 24 - 27 km horizontal, 90 - 150 m vertical resolution)
• Radiometer (13.78 GHz passive Ku-band; 7 - 310 km resolution)

Radio and Plasma Wave Science

The major functions of the Radio and Plasma Wave Science (RWPS) instrument are to measure the electric and magnetic fields and electron density and temperature in the interplanetary medium and planetary magnetospheres.

The RPWS instrument will be used to investigate electric and magnetic waves in space plasma at Saturn. Plasma is distributed by the solar wind, and it is also contained by the magnetic fields (the magnetospheres) of bodies such as Saturn and Titan. The Cassini RPWS instrument will measure the AC electric and magnetic fields in the interplanetary medium and planetary magnetospheres and will directly measure the electron density and temperature of the plasma in the vicinity of the spacecraft.

RPWS will study the configuration of Saturn's magnetic field and its relationship to Saturn Kilometric Radiation (SKR), as well as monitoring and mapping Saturn's ionosphere, plasma, and lightning from Saturn's atmosphere.

Radio Science Subsystem

The Radio Science Subsystem (RSS) uses the spacecraft X-band communication link, an S-band downlink and a Ka-band uplink and downlink to study compositions, pressures, and temperatures of atmospheres and ionospheres, radial structure and particle size distribution within rings, body and system masses, and gravitational waves.

Radio science experiments use the spacecraft radio system and ground antennas as the science instrument. These experiments measure the refractions, Doppler shifts, and other modifications to radio signals that occur when the spacecraft is occulted by planets, moons, atmospheres, and physical features such as planetary rings. From these measurements, scientists can derive information about the structures and compositions of the occulting bodies, atmospheres, and rings.

Ultraviolet Imaging Spectrograph

The Ultraviolet Imaging Spectrograph (UVIS) is a set of detectors designed to measure ultraviolet light reflected or emitted from atmospheres, rings, and surfaces over wavelengths from 55.8 to 190 nanometers to determine their compositions, distribution, aerosol content, and temperatures.

UVIS is a set of detectors designed to measure ultraviolet light reflected by or emitted from atmospheres, rings, and surfaces to determine their compositions, distributions, aerosol contents, and temperatures. UVIS will measure the fluctuations of starlight and sunlight as the sun and stars move behind the rings and the atmospheres of Titan and Saturn, and it will determine the atmospheric concentrations of hydrogen and deuterium. These data will be used for studies of the atmospheres, the magnetosphere, and the rings of the Saturnian system.

UVIS Instrument Components

• Far Ultraviolet Spectrograph (FUV) (110 to 190 nm; [0.75 | 1.5 | 6] × 64 mrad field of view)
• Extreme Ultraviolet Spectrograph (EUV) (55.8 to 118 nm; [1 | 2 | 6] × 64 mrad field of view)
• High Speed Photometer (HSP) (115 to 185 nm; 6 × 6 mrad field of view, 2 msec time resolution)
• Hydrogen-Deuterium Absorption Cell (HDAC) (121.5 nm; 55.8 mrad circular field of view)

Visible and Infrared Mapping Spectrometer

The Visible and Infrared Mapping Spectrometer (VIMS) is a pair of imaging grating spectrometers designed to measure reflected and emitted radiation from atmospheres, rings, and surfaces over wavelengths from 0.35 to 5.1 microns to determine their compositions, temperatures, and structures.

VIMS will be used to map the surface spatial distribution of the mineral and chemical features of a number of primary and secondary targets. These targets include the Saturnian ring and satellite surfaces, the Saturnian atmosphere, and the atmosphere of Titan.

VIMS Instruments Components

• Visible Channel (VIMS-V) (0.35 to 1.07 µm (96 channels); 32 × 32 mrad field of view)
• Infrared Channel (VIMS-IR) (0.85 to 5.1 µm (256 channels); 32 × 32 mrad field of view)

CAPS: Cassini Plasma Spectrometer

The Cassini Plasma Spectrometer (CAPS) will measure the flux of ions as a function of mass per charge, and the flux of ions and electrons as a function of energy per charge and angle of arrival relative to the instrument.

CAPS will investigate the molecules originating from Saturn's ionosphere and also determine the configuration of Saturn's magnetic field. CAPS will also investigate plasma in these areas as well as the solar wind within Saturn's magnetosphere.

CAPS Scientific Objectives

• To measure the composition of ionised molecules originating from Saturn's ionosphere and Titan
• To investigate the sources and sinks of ionospheric plasma: ion inflow/outflow, particle precipitation
• To study the effect of magnetospheric/ionospheric interaction on ionospheric flows
• To investigate auroral phenomena and Saturn Kilometric Radiation (SKR) generation
• To determine the configuration of Saturn's magnetic field
• To investigate the plasma domains and internal boundaries
• Investigate the interaction of the Saturn's magnetosphere with the solar wind and solar-wind driven dynamics within the magnetosphere
• Study the microphysics of the bow shock and magnetosheath
• Investigate rotationally driven dynamics, plasma input from the satellites and rings, and radial transport and angular momentum of the magnetospheric plasma
• Investigate magnetotail dynamics and sub-storm activity
• Study reconnection signatures in the magnetopause and tail
• To characterise the plasma input to the magnetosphere from the rings
• To characterise the role of ring/magnetosphere interaction in ring particle dynamics and erosion
• To study dust-plasma interactions and evaluate the role of the magnetosphere in species transport between Saturn's atmosphere and rings
• To investigate auroral phenomena and Saturn Kilometric Radiation (SKR) generation
• To study the interaction of the magnetosphere with Titan's upper atmosphere and ionosphere
• To evaluate particle precipitation as a source of Titan's ionosphere
• To characterise plasma input to magnetosphere from the icy satellites
• To study the effects of satellite interaction on magnetospheric particle dynamics inside and around the satellite flux tube

CAPS Instrument Description

The Cassini Plasma Spectrometer Subsystem (CAPS) will measure the flux of ions as a function of mass per charge and the flux of ions and electrons as a function of energy per charge and angle of arrival relative to the CAPS instrument. The CAPS instrument consists of six major subassemblies:

• Mass spectrometer
• Ion beam spectrometer
• Electron spectrometer
• Data processing unit
• High-voltage power supply
• Actuator

Ion Mass Spectrometer

The Ion Mass Spectrometer (IMS) provides species-resolved measurements of the flux of positively charged atomic and molecular ions as a function of energy/charge vs aperture entry direction. The IMS uses a toroidal top-hat electrostatic analyser (for energy/charge data and to create a narrow field of view) combined with a linear electric field time-of-flight mass spectrometer (for mass/charge and other species-resolving data). The IMS consists of an aperture cover/actuator, a toroidal analyser, carbon foils, a time-of-flight spectrometer, microchannel plates, amplifier/discriminators, a time-to-digital converter, a spectrum analyser module, and high-voltage power converters contained in high-voltage units 1 and 2. For information on these components, click on their names.

The IMS aperture cover protects the IMS during ground handling and during launch and early flight. The cover is a strip of flexible material that fits over the IMS annular aperture without creating a hermetic seal. To open the cover the strip is reeled into a container by a spring after the strip is released by a wax thermal actuator (WTA). Closing the cover requires ground handling to unreel the cover and re-attach its end to the cover release mechanism. Thus, in flight, the aperture cover is a one-time use device.

The toroidal analyser (toroidal refers to the configuration of the target reflector) consists of a baffled collimator (a device used to create a parallel beam of particles) mounted on a toroidal "top hat" electrostatic analyser. The geometry of the collimator determines the narrow, annular IMS field of view of 12 degrees by 160 degrees, which is divided into eight angular "pixels" of 12 degrees by 20 degrees each. An electric potential between the inner and outer conductors of the electrostatic analyser allows through this device only ions having energies within a range selected by the analyser potential (i.e., only ions with certain energies will have trajectories, at a given analyser potential, that navigate through the analyser without being stopped by hitting a wall). Energy spectra are taken by stepping the analyser potential through a set of 64 spaced values.

Eight thin carbon foils are arranged in an arc along the exit of the toroidal analyser. The foils are the entrance to the time-of-flight spectrometer chamber formed by the linear electric field (LEF) rings. A -15 kV potential accelerates the positive ions exiting the toroidal analyser into the foils, and this permits ions entering the IMS with as little as 1 eV of energy to pass through the foils. Molecular ions traveling through the foils are usually broken into their constituent atoms and/or molecular fragments. Molecular and atomic ions and molecular fragments exit the foils as neutrals or ions, and upon exiting the neutrals and ions eject secondary electrons into the time-of-flight chamber.

The time-of-flight spectrometer is a cylindrical chamber in the IMS, bounded by linear electric field (LEF) rings, where particles exiting the carbon foils encounter an electric field with a strength that increases linearly with distance parallel to the LEF axis of symmetry. The linear electric field is generated by a stack of thirty equally spaced aluminium rings along which a network of gigaohm resistors establishes a quadratic electric potential with a total potential drop across the stack of rings of 30 kV.

The LEF focuses positively charged particles with energies up to approximately 15 keV, independently of the energy and angle with which they exit the carbon foils. Microchannel plate (MCP) multipliers are located at the ends of the stack of LEF rings. Detection by the "start" (ST) MCP of rapidly travelling secondary electrons from the carbon foils is used as a time-of-flight "start" signal and for determining an ion's elevation angle with respect to the IMS aperture. Simple harmonic motion of ions in the LEF, together with knowledge of their energy gained from the toroidal analyser setting, relates their time-of-flight, from carbon foils to the LEF MCP (at the other end of the chamber from the ST MCP), to mass per charge.

The two microchannel plates (MCPs) each consist of three circular plates of lead oxide glass with a multitude of microscopic channels running at an incline through the thickness of each plate. Electrons, ions, and neutrals striking the outer plate cause secondary electrons to cascade down the semiconductive walls of the channels. With a potential drop of about 1 kV across the thickness of a plate, there is a yield of about 300 electrons per incident particle, or a total gain of 10⁶ electrons across the three-plate stack.

Nine anodes under the ST MCP and one anode under the LEF MCP collect the electrons emitted from the MCPs and pass the signals to two high-speed current amplifiers (one "start" amplifier for the eight 20-degree-wide anodes, and one "stop" amplifier for the LEF anode and the centre ST anode). Two constant fraction discriminators accept the amplifiers' signals and send digital timing pulses (independent of the amplifiers' signal amplitude) to the time-to-digital converter (TDC). Also sent to the TDC is the identification of which of the eight annular anodes was the source of the "start" pulse and whether a "stop" pulse came from the LEF anode or the centre ST anode.

The time-to-digital converter (TDC) measures the time interval between the start and stop pulses from the amplifier/discriminators. The TDC outputs an interval length to the spectrum analyser module, along with information identifying the "start" anode and the "stop" anode. The "start" anode identity determines which of the eight 20-degree-wide IMS elevation resolution elements an ion entered through. A device in the TDC can inject simulated MCP signals just downstream of the MCP anodes, via high-voltage isolating capacitors, in order to test the signal handling functions of the IMS and related hardware and software in the CAPS data processing unit.

The spectrum analyser module (SAM) provides the data gathering, sorting, and transfer functions between the TDC and the data processing unit (DPU). Time interval data from the TDC are "binned" or grouped into a pre-selected (by the DPU) set of time channels associated with certain selected ion species, whether atomic or molecular. The SAM processor performs a deconvolution of the time-of-flight spectra to obtain ion identifications, which it passes on to the DPU.

The IMS has four programmable pulse width modulated high-voltage power converters . These converters supply high voltage to the toroidal analyser, the LEF rings, and the LEF MCP.

Ion Beam Spectrometer

The second major CAPS subassembly is the ion beam spectrometer (IBS). The IBS measures the flux of positively charged ions of all species as a function of energy/charge and aperture entry direction. The IBS consists of a hemispherical electrostatic analyser, channel electron multipliers, amplifiers/discriminators, and high-voltage power converters.

The hemispherical electrostatic analyser consists of two conductive hemispheres of slightly different radius, mounted concentrically one within the other in such a way that there is a small gap between the two conductors. The electric field in the gap selects the range of energy per charge and angular direction that ions are allowed to have in order to pass through the analyser. Energy spectra are taken by stepping the analyser potential through a set of spaced values. Three apertures, each defining a field of view of 1.5 degrees by 150 degrees, spaced 30 degrees apart, allow particles to enter the analyser. Particles are "focused" into three channel electron multipliers 180 degrees from each of the entrance apertures.

Ions entering the channel electron multipliers (CEMs) strike the inner semiconducting surfaces of the devices and spawn secondary electrons, which bounce down the curved channel in the CEMs, spawning more electrons with each bounce. In this way, the entry of an ion into a CEM results in a pulse of approximately 10⁸ electrons being collected by an anode at the exit of the CEM.

The amplifiers/discriminators amplify the electron pulses collected by the CEM anodes and send the signals to the DPU's IBS interface. A device in the IBS can inject simulated CEM signals just downstream of the CEM anodes in order to test the signal handling functions of the IBS and related hardware and software in the DPU.

The IBS has two programmable pulse width modulated high-voltage power converters, which supply high voltage to the hemispherical analyser and the CEMs.

Electron Spectrometer

The third major CAPS subassembly is the electron spectrometer (ELS), which measures the flux of electrons as a function of energy/charge and aperture entry direction. The ELS consists of a spherical analyser, microchannel plates, amplifiers/discriminators, a sensor management unit, and high-voltage power converters. For information on these components, click on their names.

The spherical analyser consists of a baffled collimator mounted on a spherical "top hat" electrostatic analyser. Collimator geometry determines the narrow, annular ELS field of view of 5 degrees by 160 degrees, which is divided into eight 20-degree "pixels." An electric potential between the inner and outer conductors of the electrostatic analyser allows through this device only electrons having energies and angles within a range selected by the analyser potential and top-hat collimation. Energy spectra are taken by stepping the analyser potential through a set of 96 spaced values.

Two 90-degree "long" annular (i.e., curved) microchannel plates (MCPs) are arranged in a 180-degree arc along the exit of the spherical analyser. electrons striking the surface of the MCPs are multiplied into signals collected by an arc of eight 20-degree anodes.

The anode signals are passed into eight amplifiers/discriminators via high-voltage isolating capacitors and are then accumulated by the sensor management unit. The anode identity determines which of the eight 20-degree-wide ELS elevation resolution elements an electron entered through.

The sensor management unit (SMU) controls the voltage level of the ELS high-voltage power converters in accordance with requests from the CAPS data processing unit (DPU) and passes counts of electrons (from each of the eight angular resolution elements) detected by the ELS to the DPU. A device in the SMU can inject simulated MCP signals just downstream of the isolating capacitors in order to test the signal handling functions of the ELS and related hardware and software in the DPU.

The ELS has two programmable pulse width modulated high-voltage power converters that supply high voltage to the spherical analyser and the MCPs.

Data Processing Unit

The Data Processing Unit (DPU) manages the acquisition and onboard data processing of all CAPS data and controls sensor and actuator motor functions. The DPU is designed to use two CPUs, in addition to the processor in the spectrum analyser module. The first CPU accumulates IMS time-of-flight spectra and compresses all IMS data. The other CPU controls the IMS, ELS, IBS, and the actuator and performs onboard data analysis to determine what measurements will be taken and what data will be placed into housekeeping and science packets.

The DPU consists of the sensor and actuator data control interfaces, a housekeeping analog-to-digital converter, a safe/arm control, a wax thermal actuator (WTA) driver, CPUs with memory, a bus interface unit, low-voltage power converters, supplemental and replacement heaters, a radiator, and the principal structure of CAPS.

DPU Links Through its sensor and actuator data control interfaces, the DPU performs several functions. It accumulates IBS ion flux counts; it collects ELS and IMS data products from the SMU, the TDC, and the SAM; and it directly controls the IMS and IBS pulsers and high-voltage converters. The DPU feeds ion energy/charge data to, and controls, the SAM. In addition, the DPU controls the CAPS actuator (ACT) motor stepping and accepts position and status data from the ACT.

The output voltage of all nine CAPS high-voltage converters, CAPS low voltages, the actuator position encoder, and the temperature at six different locations in CAPS are monitored by a housekeeping analog-to-digital converter. In addition, two temperature sensors (one in the DPU, the other in the IMS cover release mechanism) are monitored directly by the spacecraft. These two sensors do not require that CAPS be powered.

When not enabled by the DPU (via CAPS software), all high-voltage power converters have zero output. When high-voltage converters are enabled, a safe/arm connector on the DPU can be used during ground handling to limit high voltages to approximately 3 percent of what the DPU has commanded them to be.

The wax thermal actuators (WTAs) in the IMS cover release mechanism and the scan motor launch latch are driven and switched by a WTA driver in the DPU.

The DPU uses two nearly identical CPU boards, each containing its own RAM, ROM, and PACE 1750a microprocessor.

CAPS communicates with the Command and Data Subsystem (CDS) via a bus interface unit (BIU) that is electrically, mechanically, and thermally accommodated within the DPU.

All CAPS low-voltage power converters are housed in the DPU. Power at various voltages is supplied for use by the electronics boards in the DPU, including the BIU, and by the amplifiers, D/A converters, and other circuitry housed in the IMS, ELS, IBS, and ACT.

Two redundant high-voltage power supplies mount to the top plate of the DPU.

The CAPS supplemental heater (controlled by CAPS) and replacement heater (controlled by the spacecraft) are mounted to the inside of the DPU's top plate.

The CAPS radiator mounts to the back plate of the DPU.

Along with the actuator, the DPU box forms the principal structure of CAPS. The IMS, ELS, IBS, and the high-voltage power supplies mount to the top plate of the DPU. CAPS mounts to the spacecraft via the actuator, which is mounted to the bottom plate of the DPU.

Actuator

The actuator (ACT) will rotate the CAPS instrument at a steady rate over a maximum range of 184 degrees with the acceleration/deceleration over a further 12 degrees (minimum) at either end of this range. The steady rate and acceleration/deceleration range can be adjusted in-flight, and the 216-degree total range of movement will be limited by hard stops to prevent any "wraparound" effects on the CAPS interface cables.

CDA: Cosmic Dust Analyzer

The Cosmic Dust Analyser (CDA) instrument will provide direct observations of dust and ice particles in interplanetary space and in the Jupiter and Saturn systems. It will investigate the physical, chemical, and dynamic properties of these particles matter as functions of the distances to the Sun, to Jupiter, to Saturn, and to Saturn's satellites and rings. Finally, it will study the interaction of the particles with the Saturnian rings, satellites, and magnetosphere.

CDA Scientific Objectives

• To extend studies of interplanetary dust (sizes and orbits) to the orbit of Saturn
• To define dust and meteoroid distribution (sizes, orbits, composition) near the rings
• To map the size distribution of ring material in and near the known rings
• To analyse the chemical compositions of ring particles
• To study processes (erosional and electromagnetic) responsible for E ring structure
• To search for ring particles beyond the known E ring
• To study the effect of Titan on the Saturn dust complex
• To study the chemical composition of icy satellites from studies of ejecta particles
• To determine the role of icy satellites as a source for ring particles
• To determine the role that dust plays as a magnetospheric charged particle source/sink

CDA Instrument Description

The Cosmic Dust Analyser Subsystem (CDA) will provide direct observations of dust and ice particles in interplanetary space and in the Jupiter and Saturn systems. It will investigate the physical, chemical, and dynamical properties of these particles matter as functions of the distances to the Sun, to Jupiter, to Saturn, and to Saturn's satellites and rings. Finally, it will study the interaction of the particles with the Saturnian rings, satellites, and magnetosphere.

The four major functional elements of the CDA are:

• Dust analyser
• Main electronics
• Articulation mechanism
• High-rate detector assembly

Dust Analyser

The dust analyser (DA) consists of the following components: four charge pick-up grids; a hemispherical target, an ion collector, an electron multiplier, and the sensor electronics.

The charge pick-up grids collect the initial impact particles. They are mounted at the entrance of the sensor.

The hemispherical target is divided into two parts - a ring-shaped impact ionisation target and a chemical analyser target in the middle of the ionisation target. The chemical analyser target has an acceleration grid mounted 3 mm in front of it.

The ion collector has a grid that is negatively biased in order to collect the positively charged plasma ions produced at the impact ionisation target.

The electron multiplier is located in the centre of the hemispherical ion collector target. It amplifies the signal produced by ions capable of penetrating the ion collector grid. These ions originate from plasma produced by particle impact either on the impact ionisation target or the chemical analyser target. The output signal from the multiplier differs depending upon the target from which impacts are being measured.

The sensor electronics are contained in an electronics box attached to the DA sensor chassis. Among other components, this box contains charge-sensitive amplifiers (CSAs) that measure the signals from all of the grids in the DA.

Main Electronics

The CDA main electronics includes amplifiers and transient recorders, a control and timing unit, a microprocessor unit, a bus interface unit, a power input circuit, a low-voltage converter, and a housekeeping system.

All CSA and electron multiplier signals are separately amplified by logarithmic amplifiers and then digitised by an analog-to-digital converter. The data are stored on transient recorders. Only the recorder connected to the pick-up grids is operated continuously. All others are activated only by a signal detected at a target or the acceleration grid. The control and timing unit stores and decodes information received from the microprocessor and produces all timing and synchronisation signals required for instrument operation.

The microprocessor samples and collects the buffered measurement data, coordinates the subsystem measurement cycle, controls the instrument operating modes, processes the data according to a program loaded in its memory, and outputs data to the spacecraft upon request through the bus interface unit (BIU). The BIU is the interface circuit between the spacecraft and the microprocessor and is powered by the CDA instrument. The power input circuit is the interface with the spacecraft Power and Pyrotechnics Subsystem (PPS) and contains a filter circuit and a regulator to produce a DC voltage to feed the low-voltage converter.

The low-voltage converter is a DC/DC converter that provides different regulated low voltages for the electronics circuits and the supply voltage for the high-voltage converters. The converters are synchronised to the 100 kHz clock provided through the BIU from the Command and Data Subsystem (CDS).

The CDA housekeeping system is a data system that multiplexes, digitises, and stores information on the instrument current, the low voltages, the high voltages, and temperature measurements.

Articulation Mechanism

The articulation mechanism (AM) allows the entire CDA instrument, including the high-rate detectors, the dust analyser, the main electronics, and the articulation mechanism electronics, to be rotated or repositioned with respect to the spacecraft coordinate system.

High-Rate Detector Assembly

The high-rate detectors (HRDs) are two redundant independent sensors. The electronics for the sensors are contained in the HRD electronics box, and each sensor has its own electronics, independent of the other sensor. The HRD will be operated in two modes: normal and calibrate. In normal mode, the operational HRD continuously collects dust particle data. In calibrate mode, a calibration cycle is initiated, which consists of a sequence of pulses sent to the HRD by the in-flight calibrator (IFC) to verify the stability of the electronics.

CIRS: Composite Infrared Spectrometer

The Composite Infrared Spectrometer (CIRS) consists of dual interferometers that measure infrared emission from atmospheres, rings, and surfaces over wavelengths from 7 to 1000 microns (1400 to 10 cm^-1) to determine their composition and temperatures.

CIRS will address a wide variety of science objectives for the atmospheres of Saturn and Titan, and for Saturn's icy satellites and rings, including composition determination and thermal state measurements.

The Composite Infrared Spectrometer (CIRS) will measure infrared emissions from atmospheres, rings and surfaces in the Saturn system to determine their composition, temperatures and thermal properties. It will map the atmosphere of Saturn in three dimensions to determine temperature and pressure profiles with altitude, gas composition, and the distribution of aerosols and clouds. This instrument will also measure thermal characteristics and the composition of satellite surfaces and rings.

CIRS Scientific Objectives

• To map the global temperature structure within Titan's and Saturn's atmospheres
• To map the global gas composition within Titan's and Saturn's atmospheres
• To map global information on hazes and clouds within Titan's and Saturn's atmospheres
• To collect information on energetic processes within Titan's and Saturn's atmospheres
• To search for new molecular species within Titan's and Saturn's atmospheres
• To map the global surface temperatures at Titan's surface
• To map the composition and thermal characteristics of Saturn's rings and icy satellites

CIRS Instrument Description

The CIRS instrument consists of two assemblies: optics and electronics.

CIRS Optics

The CIRS optics assembly consists of a telescope, a far infrared interferometer, a mid infrared interferometer, a reference interferometer, a moving scan mechanism, a cooler, thermal control equipment, an optics assembly mount, covers for the telescope and cooler, and a calibration shutter mechanism.

The telescope is composed of a 50.8 cm diameter paraboloid primary mirror and a hyperboloid secondary mirror. A sun shade will be present around the primary mirror. This shade will also serve as the primary mirror radiator. A cylindrical tube extends from the central portion of the primary mirror to support the secondary mirror, which has its own radiator.

The CIRS science data will be collected by two of the instrument's three interferometers. Interferometers are instruments designed to make precise measurements of wavelength within some range of the electromagnetic spectrum. For example, the CIRS far infrared (FIR) interferometer covers a spectral range of 10 to 600 cm^-1. The FIR instrument is a polarising interferometer that uses substrate-mounted wire grid polarisers to polarise and analyse the radiation. The interferometer operates by first polarising the radiation and then modulating its polarisation. The FIR interferometer has its moving mirror mounted on one end of the moving scan mechanism, which it shares with the mid infrared interferometer. The fixed and moving mirrors are roof mirrors, and the FIR focal plane consists of a matched pair of thermopile detectors, each with a concentrator.

The mid-infrared interferometer (MIR), which is a conventional Michelson instrument, covers a spectral range of 600 to 1500 cm^-1. The MIR interferometer has its moving mirror mounted on the opposite end of the moving scan mechanism from the FIR mirror. The fixed and moving mirrors are cube corners, and the MIR uses a germanium lens to focus the interferometer output onto focal planes FP3 and FP4.

The reference interferometer will provide timing correlation of the science data sampling to the scan mechanism position. Specifically, the scan mechanism's motion will produce a variable reference interferometer signal that will be used to generate the timing signals necessary to time the recording of science data. The reference interferometer is a Michelson instrument, used on-axis at the optic centre of the MIR interferometer. It includes laser diode and LED sources, a quartz beam splitter/compensator, optics, and a silicon detector. The reference interferometer uses cube corners for the fixed and moving mirrors.

The moving scan mechanism subassembly includes the optical and mechanical components in the optics assembly required for moving the interferometer mirrors to permit controlled sampling in the optical path difference. This subassembly consists of a common carriage with a moving shaft for the three interferometer mirrors, a motor to drive the shaft, a cantilever-spring motor mount, and a velocity transducer. The scan mechanism includes a launch lock that can be locked and unlocked repeatedly without refurbishment by remote command through the onboard computer.

A single-stage, passive cooler, radiating to space, provides a 70 to 80 K cold finger with four discrete, commandable set points within that range. The nominal set point temperature will be 80 K. The cold finger has heaters for decontamination and detector annealing.

A system of thermal control equipment, including temperature sensors, electrical heaters, proportional heater controllers, and radiators, is used to maintain the thermal control of the optics assembly. The temperature of the instrument is monitored by sensors located at appropriate places in the instrument, including, but not limited to, the telescope mirrors, the interferometers, the detectors, the optics housing, the radiating surface, and the Michelson motor.

The optics assembly mount thermally decouples the optics assembly from the remote sensing pallet (RSP).

The telescope and 80 K cooler will be protected with covers until the orbiter leaves the inner solar system, at which point the covers will be separately jettisoned by wax thermal actuators (WTAs). Each cover has two redundant paraffin actuators, either one of which can initiate the action of a pin-puller assembly, which in turn initiates the action of an ejection spring assembly.

The calibration shutter mechanism will be used to interrupt the MIR beam, causing the MIR detectors to view a controlled 170 K black surface inside the instrument. The shutter is commandable through the onboard computer.

CIRS Electronics

The CIRS electronics assembly includes front-end electronics, scan mechanism electronics, reference interferometer electronics, temperature control and monitor electronics, the instrument data system, and power converter electronics for conditioning the spacecraft power as required by the instrument.

The front-end electronics provides analog and digital processing of the detector signals. This includes filtering, multiplexing, amplification, and digitisation to achieve the science data requirements.

The scan mechanism electronics provides the operation and control of the linear moving scan mechanism. The control system provides a constant velocity travel during the forward (scan) direction and a fast flyback during the reverse (flyback) direction. The control is a function of the reference interferometer timing signals and other sensors, as needed. The scan mechanism electronics also includes circuitry to actuate the calibration shutter.

The reference interferometer electronics provides timing signals for accurate sampling of the science data and for accurate control of the scan mechanism velocity.

The CIRS instrument has four zones that are independently controlled by the temperature control and monitor electronics. These zones are the primary mirrors, the secondary mirrors, the interferometers, and the 80 K cooler. The temperature is monitored by temperature sensors located on the four zones, which provide accurate temperature values at the cooled temperatures. The sensors located on the optics assembly provide course values at the decontamination temperature. The monitored temperatures will be transmitted to the spacecraft.

The instrument data system (IDS) is a microprocessor circuit that provides the instrument data processing function and communication with the spacecraft. The IDS receives and processes commands, data, and timing information from the spacecraft and configures and controls the instrument's operational states. It processes the science data from the front-end electronics and the housekeeping data from all of the subassemblies, and then it transmits these data to the spacecraft.

The power converter electronics (PCE) conditions the regulated power received from the spacecraft and provides direct power to the instrument data system and the temperature controller and monitor subassemblies. The PCE also provides, on command, power to other subassemblies and release of the scan mechanism launch lock, the 80 K cooler cover, and the telescope cover.

INMS: Ion and Neutral Mass Spectrometer

The Ion and Neutral Mass Spectrometer (INMS) is intended to measure positive ion and neutral species composition and structure in the upper atmosphere of Titan and magnetosphere of Saturn, and to measure the positive ion and neutral environments of Saturn's icy satellites and rings.

INMS will be used to study the neutral gases and positive ions in the atmospheres of Saturn and Titan and gases in the vicinities of the Saturnian rings and the icy satellites. It will also study the magnetosphere of Saturn.

INMS Scientific Objectives

• To measure ion and neutral species composition and structure in the upper atmosphere of Titan
• To study Titan atmospheric chemistry
• To investigate the interaction of Titan upper atmosphere with the magnetosphere and solar wind
• To measure ion and neutral species compositions during ring plane crossings and icy satellite flybys

INMS Instrument Description

The major functional components of the INMS Subsystem are an open ion source, a closed ion source, a quadrupole deflector and lens system, a quadrupole mass analyser, and a dual detector system.

The open ion source produces ions by ionising neutral gases. It includes an ion trap/deflector that forms trapped ions into a beam. This minimises interaction effects between the gas environment and the open source surface as the source directly samples the gaseous species.

The closed ion source also produces ions by ionising neutral gases. It uses ram density enhancement to provide measurements of higher accuracy and sensitivity for the more inert atomic and molecular species than provided by the open ion source. This is achieved by maintaining a high input flux to an enclosed antechamber and then limiting the gas conductance or output from the antechamber by the use of an orifice.

Ions are directed to the mass analyser from the selected ion source by changing the potentials on a 90 degree quadrupole deflector. This electrostatic device allows both sources of ions to be sequentially switched into a common exit lens system.

The quadrupole mass analyser consists of four precision ground hyperbolic rods mounted in a rigid mechanical assembly. The transmitted mass, the resolution, and the ion transmission are controlled by variations in RF and DC electric fields between adjacent rod pairs, while opposite rod pairs are kept at the same potential.

The ion dual detector system amplifies and detects the input from the mass analyser by the use of two continuous dynode multipliers.

ISS: Imaging Science Subsystem

The Imaging Science Subsystem (ISS) is a remote sensing instrument that captures images in visible, infrared and ultraviolet light. The ISS has a camera that can take broad, wide-angle pictures and a camera that can record small areas in fine detail.

ISS Scientific Objectives

• To map the three dimensional structure and motions within the Saturn/Titan atmospheres
• To study the composition, distribution, and physical properties of clouds and aerosols
• To investigate scattering, absorption, and solar heating within the S/T atmospheres
• To search for evidence of lightning, aurorae, airglow, and planetary oscillations
• To study the gravitational interactions between the rings and Saturn's satellites
• To determine the rate and nature of energy and momentum transfer within the rings
• To determine ring thickness and sizes, composition, and physical nature of ring particles
• To map the surfaces of the satellites (including Titan) to study their geological histories
• To determine the nature and composition of the icy satellite surface materials.
• To determine the rotation states of the icy satellites.

ISS Instrument Description

The Cassini orbiter imaging experiments will encompass a wide variety of targets (Saturn, the rings, Titan, the icy satellites, and star fields) and a wide range of observing distances for various scientific purposes. The science objectives include studying the atmospheres of Saturn and Titan, the rings of Saturn and their interactions with the planet's satellites, and the surface characteristics of the satellites, including Titan. Because of these multiple objectives, the ISS has two separate camera designs. The first is a narrow-angle camera (NAC) design that will obtain high-resolution images of the target of interest. The second is a wide-angle camera (WAC) design that provides a different scale of image resolution and more complete coverage spatially. The spacecraft will carry one NAC and one WAC. The NAC is also used to obtain optical navigation images for the mission with the WAC acting as a functionally redundant backup unit for this purpose.

The Cassini imagers differ primarily in the design of the optics. The NAC has a focal length of 2000 mm, and the WAC , which uses optics inherited from the Voyager mission, has a focal length of 200 mm. The cameras each have a focal plane shutter of the same type as used on both Voyager and Galileo, and they have a two-wheel filter-changing mechanism derived from the Hubble Space Telescope Wide Field/Planetary Camera (WF/PC) design. The CCD detector is cooled to suppress dark current (residual current in the CCD beyond that released by incident light), which is dependent upon temperature. It is also shielded from ionising radiation.

The CCD detector design is a square array of 1024² pixels, each pixel measuring 12 microns on a side. The CCD uses a three-phase, front-side-illuminated architecture, with a coating of lumogen phosphor to provide ultraviolet response. The detector is passively cooled by a radiator to approximately 10 K below its nominal operating temperature (approximately 180 K), and then it is controlled to the operating temperature by a proportional control heater. To minimise radiator size and heater power, the detector/radiator combination is thermally isolated from the rest of the camera head assembly (CHA).

The entire NAC is thermally isolated from the remote sensing pallet (RSP) on which it is mounted in order to minimise the effects of RSP thermal variations on NAC image quality. The WAC, being an inherited design with less stringent imaging requirements, is not thermally isolated.

The electronics for each camera are identical. All ISS command and telemetry functions will be handled by the electronics, including receipt of commands from the Command and Data Subsystem, expansion of commands, and collection and transmission of imaging data and telemetry to the CDS.

The ISS controls the amount of power it draws from the spacecraft during operations. To accomplish this, the profile of ISS command timing is structured to reduce the power the ISS requires for certain internal functions (e.g., shutter or filter wheel movement). When the filter is moving, the power from the optical heater (if present) in the active camera is turned off. When the movement is complete, the optical heater is turned on (if needed). In addition, simultaneous filter positioning within a single camera, either the WAC or NAC, is not permitted.

During the cruise phase of the mission, the cameras will periodically be turned on for maintenance, calibration, and monitoring of instrument health and performance. Other than these specified times, the ISS will be off and replacement heaters will be on. In addition, decontamination/radiation heater 1 will be on throughout most of the cruise.

Upon arrival at the Saturnian system, the cameras will be on most of the time. Spacecraft power limitations will be the controlling parameter determining whether the ISS will be turned off or put into a low-power state. During the Saturn tour, high-activity periods for Saturn and its rings will be clustered around periapsis; for the satellites, the high-activity periods will be when the spacecraft is closest to them. At these times, high-resolution images of all targets will be acquired through various camera filters, and the data will be stored in the spacecraft solid-state recorder (SSR). During lower activity periods (i.e., when the spacecraft is orbiting farther from the targets), long-term atmospheric and ring monitoring will take place, and ISS calibrations will be performed.

MAG: Dual Technique Magnetometer

The primary objective of the Dual Technique Magnetometer (MAG) is to determine the planetary magnetic fields and the dynamic interactions in the planetary environment.

Magnetometers are direct-sensing instruments that detect and measure the strength of magnetic fields in the vicinity of the spacecraft. The Cassini Dual Technique Magnetometer (MAG) measures magnetic fields during the Titan and Saturn encounters.

MAG Scientific Objectives

• To determine the internal magnetic field of Saturn
• To develop a three-dimensional model of Saturn's magnetosphere
• To determine the magnetic state of Titan and its atmosphere
• To derive an empirical model of the Titan electromagnetic environment
• To investigate the interactions of Titan with the magnetosphere, magnetosheath, and solar wind
• To survey the ring and dust interactions with the electromagnetic environment
• To study the interactions of the icy satellites with the magnetosphere of Saturn
• To investigate the structure of the magnetotail and the dynamic processes therein

MAG Instrument Description

The MAG consists of a vector/scalar helium magnetometer sensor, a fluxgate magnetometer sensor, a data processing unit, three power supplies, plus operating software and electronics associated with the sensors.

The vector/scalar helium magnetometer (V/SHM) sensor is used to make both vector (magnitude and direction) and scalar (magnitude only) measurements of magnetic fields. The V/SHM and its electronics are being supplied by the Jet Propulsion Laboratory (JPL). The fluxgate magnetometer (FGM) sensor is used to make vector field measurements. This sensor and its electronics are being provided by Imperial College, London.

The instrument data processing unit (DPU) is the responsibility of the Technical University of Braunschweig. The DPU interfaces with the spacecraft Command and Data Subsystem through the JPL designed bus interface unit (BIU). All commands, data, and processor program changes are received or transmitted through the BIU. The MAG components are powered by three power supplies plus the 30 volt spacecraft bus. Power supply 0 powers the BIU and the DPU. Power supplies 1 and 2 are redundant and power the V/SHM electronics. The FGM electronics are powered by the spacecraft bus.

Since magnetometers are sensitive to electric currents and ferrous components on the spacecraft, they are generally placed on an extended boom, as far from the spacecraft as possible. In this case, the FGM sensor is located midway out on the Cassini magnetometer boom, and the V/SHM sensor is located at the end of the boom. The boom itself, composed of thin, nonmetallic rods, will be collapsed very compactly during launch and deployed only after the spacecraft has separated from the launch vehicle.

MIMI: Magnetospheric Imaging Instrument

The Magnetospheric Imaging Instrument (MIMI) is designed to: (1) measure the composition, charge state and energy distribution of energetic ions and electrons; (2) detect fast neutral species; and, (3) conduct remote imaging of the Saturn's magnetosphere. This information will be used to study the overall configuration and dynamics of the magnetosphere and its interactions with the solar wind, Saturn's atmosphere, Titan, rings, and icy satellites.

The Magnetospheric Imaging Instrument (MIMI) will provide global images of Saturnian hot plasmas remotely and will perform comprehensive direct measurements of hot plasma, including charge state and elemental composition.

MIMI Scientific Objectives

• To determine the global configuration and dynamics of hot plasma in the magnetosphere of Saturn
• To monitor and model magnetospheric sub-storm-like activity and correlate this activity with Saturn Kilometric Radiation (SKR) observations
• To study magnetosphere/ionosphere coupling through remote sensing of aurora and measurements of energetic ions and electrons
• To investigate plasma energisation and circulation processes in the magnetotail of Saturn
• To determine through imaging and composition studies the magnetosphere/satellite interactions at Saturn and understand the formation of clouds of neutral hydrogen, nitrogen, and water products
• To measure electron losses due to interactions with whistler waves
• To study the global structure and temporal variability of Titan's atmosphere
• Monitor the loss rate and composition of particles lost from Titan's atmosphere due to ionisation and pickup
• To study Titan's interaction with the magnetosphere of Saturn and the solar wind
• To determine the importance of Titan's exosphere as a source for the atomic hydrogen torus in Saturn's outer magnetosphere
• To investigate the absorption of energetic ions and electrons by Saturn's rings and icy satellites
• To analyse Dione's exosphere

MIMI Instrument Description

The MIMI instrument consists of one set of electronics, the MIMI electronics box, servicing three detector heads that perform the various measurements:

• Low-Energy Magnetospheric Measurements System (LEMMS)
• Charge-Energy-Mass Spectrometer (CHEMS)
• Ion and Neutral Camera (INCA)

Low-Energy Magnetospheric Measurements System

The Low-Energy Magnetospheric Measurements System (LEMMS) detector head will measure low- and high-energy proton, ion, and electron angular distributions. The LEMMS head is mounted on a scan platform capable of 180-degree rotations. The platform is mounted so that the rotation axis is oriented perpendicular to the spacecraft X axis and so that its extrapolation intersects the spacecraft Z axis.

Charge-Energy-Mass Spectrometer

The Charge-Energy-Mass Spectrometer (CHEMS) head will measure the charge state and composition of ions in the most energetically important portion of the Saturnian magnetospheric plasma.

Ion and Neutral Camera

The Ion and Neutral Camera (INCA) will make two different types of measurements. It will obtain with very high sensitivity the three-dimensional distribution, velocities, and rough composition of magnetospheric and interplanetary ions for those regions in which the energetic ion fluxes are very low. The INCA instrument will also obtain remote images of the global distribution of the energetic neutral emission of hot plasmas in the Saturnian magnetosphere, measuring the composition and velocities of those energetic neutrals for each image pixel.

RADAR: Cassini Radar

The Cassini Radar (RADAR) uses the five-beam Ku-band antenna feed assembly associated with the spacecraft high gain antenna to direct radar transmissions toward targets, and to capture black body radiation and reflected radar signals from targets.

RADAR Scientific Objectives

• To determine whether oceans exist on Titan, and, if so, to determine their distribution
• To investigate the geologic features and topography of the solid surface of Titan
• To acquire data on non-Titan targets (rings, icy satellites) as conditions permit

RADAR Instrument Description

The Cassini Radar will be used to investigate the surface of Saturn's moon Titan by taking four types of observations: imaging, altimetry, backscatter, and radiometry.

In the imaging mode of operation, the RADAR instrument will bounce pulses of microwave energy off the surface of Titan from different incidence angles and record the time it takes the pulses to return to the spacecraft. These measurements, when converted to distances (by dividing by the speed of light), will allow the construction of visual images of the target surface. Radar will be used to image Titan because the moon's surface is hidden from optical view by a thick, cloud-infested atmosphere: radar can "see" through such cloud cover.

Radar altimetry similarly involves bouncing microwave pulses off the surface of the target body and measuring the time it takes the "echo" to return to the spacecraft. In this case, however, the goal will not be to create visual images but rather to obtain numerical data on the precise altitude of the surface features of Titan.

In the backscatter mode of operation, the RADAR will act as a scatterometer. That is, it will bounce pulses off Titan's surface and then measure the intensity of the energy returning. This returning energy or backscatter, is always less than the original pulse, because surface features inevitably reflect the pulse in more than one direction. From the backscatter measurements, scientists can infer the composition of the surface of Titan.

Finally, in the radiometry mode, the RADAR will operate as a passive instrument, simply recording the energy emanating from the surface of Titan. This information will tell scientists the amount of latent heat (i.e.. moisture) in the moon's atmosphere, a factor that has an impact on the precision of the other measurements taken by the instrument.

During imaging, altimetry, and backscatter operations, the RADAR instrument will transmit linear frequency-modulated Ku-band pulsed signals toward the surface of Titan using the high-gain antenna (HGA). These signals, after reflection from the surface, will be captured by the same antenna and detected by the RADAR Radio Frequency Electronics Subsystem. During radiometry operations, the instrument will not transmit any radar signals, but the HGA will again be used for radiometric observations.

To improve the surface coverage by radar imaging, a switched, multiple Ku-band antenna feed array structure is part of the HGA and permits the formation of five antenna beam patterns. Each of these beams will have a different pointing angle relative to the antenna reflector's focal axis.

The major functional components of the RADAR Subsystem are the Radio Frequency Electronics Subsystem, the Digital Subsystem, and the Energy Storage Subsystem.

Radio Frequency Electronics Subsystem

The Radio Frequency Electronics Subsystem (RFES) has three principal functions: the transmission of high-power frequency-modulated and unmodulated pulses, the reception of both reflected energy from the target and passive radiometric data, and the routing of calibration signals. The RFES has a fully enclosed structural housing and Faraday cage (i.e., an electrostatic shield). The RFES electronics units are individually enclosed and are mounted to the RFES housing wall opposite the wall that mounts to the spacecraft. For thermal control, heat flows conductively from the units to the housing wall and is then radiated away from the RFES.

The RFES consists of the following components: a frequency generator, a digital chirp generator, a chirp up-converter and amplifier, a high-power amplifier, front-end electronics, a microwave receiver unit, and an RFES power supply.

The frequency generator (FG) contains an ultra-stable oscillator that is the system timing source for the RADAR instrument.

The digital chirp generator (DCG) generates the low-power, baseband frequency, modulated pulse upon request from the RADAR Digital Subsystem. Both the bandwidth and the pulse width of this pulse can be varied in accordance with the parameters received from the Digital Subsystem.

The chirp up-converter and amplifier (CUCA) converts the baseband chirp pulse to Ku band and provides the up-converted pulse to the high-power amplifier.

The high-power amplifier (HPA) receives a low-power Ku-band chirp pulse from the CUCA and amplifies that pulse to the required power level for transmission.

The purpose of the front-end electronics (FEE) is to route the high-power transmission pulses, the returning low-energy echoes and radiometric signals, and the calibration signals. The FEE receives the high-power pulse from the HPA and routes the signal to one of five different antenna ports on the RFES via an antenna switch module. The echo returns and radiometric signals are routed from one of the five antenna ports to the RFES microwave receiver. The FEE also steers the selected calibration signal to the microwave receiver during periods of calibration mode operation.

The microwave receiver (MR) receives signals at Ku band and down-converts these to baseband so that they can be properly sampled. The sources of these signals are the echo returns, radiometric signals, and calibration signals routed through the FEE. The MR receives the re-routed chirp calibration signal from the CUCA and passes that signal to the FEE for proper routing. The MR is also the source of the noise diode calibration signal that is provided to the FEE for routing. MR gain and bandwidth information is provided to the MR from the DSS.

The RFES power supply converts the (approximately) 30 Volt DC input from the Power and Pyrotechnic Subsystem to the required voltages for the RFES.

Digital Subsystem

The RADAR Digital Subsystem (DSS) performs three principal functions: reception and depacketisation of RADAR commands from the Command and Data Subsystem (CDS), configuration control and timing signal generation for RADAR, and the packetisation of RADAR housekeeping (i.e., hardware status) data and science data for transfer to the CDS.

DSS subassemblies are contained within a spacecraft bay and will be supported in shear by shear plates and the top and bottom rings of the Cassini spacecraft bus. Electronic harnesses, which face inboard on the spacecraft and be supported by the inboard shear plate, are used to provide interconnections between the RADAR subassemblies and the spacecraft.

The DSS uses two primary modes of heat transfer in its design. These are (1) the conduction of heat from the electronic components to the subchassis and the outboard shear plate, and (2) the radiation of heat from the outboard shear plate to the space environment. High-power heat dissipation components are mounted on a special heatsink bracket, which is bolted directly to the outboard shear plate to optimise heat transfer. Thermal compounds were applied between the components and the heatsink to minimize contact thermal resistance.

The DSS consists of the following components: a bus interface unit, a flight computer unit, a control and timing unit, a signal conditioner unit, and a DSS power supply.

The bus interface unit (BUI) is the interface between RADAR and the CDS. On the RADAR side, the BIU interfaces to the flight computer unit for command, software, and data transfers.

The flight computer unit (FCU) receives commands and software from the CDS and sends data and status to CDS by way of the BIU. It depacketises the commands and provides the RADAR configuration and timing information to the control and timing unit. It also receives housekeeping values in a predetermined order from the low-speed A/D converter and packetises the housekeeping and science data to be passed to the CDS by way of the BIU. In addition, the FCU receives spacecraft time broadcasts and RADAR software uploads from CDS by way of the BIU. The FCU is built around an engineering flight computer (EFC) with additional banks of ROM and internal interface circuitry.

The purpose of the control and timing unit (CTU) is to control the hardware configuration and the timing of control signals within RADAR. The parameters for determining RADAR configuration and timing are passed to the CTU from the FCU. The CTU provides the configuration and timing control signals to the RFES and to other portions of the DSS. In addition, the CTU is responsible for updating to millisecond resolution the spacecraft time received from the CDS.

The signal conditioner unit (SCU) consists of a science data buffer and high- and low-speed analog-to-digital (A/D) converters. The science data buffer (SDB) is the digital data rate buffer for RADAR. The sole purpose of the SDB is to receive and store the high-rate digital science data from the high-speed A/D converter during the proper receive window period (as determined by the CTU) and then to provide this data upon request to the FCU at a slower rate. The high-speed A/D converter digitises the imaging data output from the RFES microwave receiver and provides the data to the SDB for buffering. The low-speed A/D converter performs two tasks. It digitises the analog housekeeping telemetry values from throughout RADAR at predetermined times and provides these digitised values to the FCU upon request. It also digitises the radiometer output from the RFES microwave receiver and provides those values to the FCU upon request.

The DSS power supply converts the (approximately) 30 Volt DC input from the Power and Pyrotechnic Subsystem to the voltages required for the DSS.

Energy Storage Subsystem

The RADAR Energy Storage Subsystem (ESS) converts the (approximately) 30 Volt DC input from the PPS to a higher voltage, stores energy in a capacitor bank, and provides a regulated voltage to the high-power amplifier (HPA) of the RFES. The ESS subassemblies are contained within a spacecraft bay and are supported in shear by shear plates and the top and bottom rings of the Cassini spacecraft bus. High-strength fasteners will be used to tie the electronics assemblies to the spacecraft. Electronic harnesses, which face inboard on the spacecraft and be supported by the inboard shear plate, are used to provide interconnections between the RADAR subassemblies and the spacecraft.

The ESS uses two primary modes of heat transfer in its design. These are (1) the conduction of heat from the electronic components to the subchassis and the outboard shear plate, and (2) the radiation of heat from the outboard shear plate to the space environment. High-power heat dissipation components will be mounted on a special heatsink bracket, which will be bolted directly to the outboard shear plate to optimise heat transfer. Thermal compounds will be applied between the components and the heatsink to minimize contact thermal resistance.

The ESS consists of boost circuitry, the capacitor bank, and a buck regulator.

The boost circuitry increases the (approximately) 30 Volt DC input power to approximately 85 Volts DC for more efficient energy storage by the capacitor bank. Soft-start circuitry limits the current draw from the power source, and an input voltage filter prevents electromagnetic interference (EMI) from being conducted back into the source.

The capacitor bank stores energy to supply to the buck regulator (and the HPA) during RADAR pulse bursts. The capacitor bank voltage drops during each burst but returns to normal before the next burst.

The buck regulator regulates the varying capacitor bank voltage for the HPA.

RPWS: Radio and Plasma Wave Science

The major functions of the Radio and Plasma Wave Science (RWPS) instrument are to measure the electric and magnetic fields and electron density and temperature in the interplanetary medium and planetary magnetospheres.

The RPWS instrument will be used to investigate electric and magnetic waves in space plasma at Saturn. Plasma is distributed by the solar wind, and it is also contained by the magnetic fields (the magnetospheres) of bodies such as Saturn and Titan. The Cassini RPWS instrument will measure the AC electric and magnetic fields in the interplanetary medium and planetary magnetospheres and will directly measure the electron density and temperature of the plasma in the vicinity of the spacecraft.

RPWS will study the configuration of Saturn's magnetic field and its relationship to Saturn Kilometric Radiation (SKR), as well as monitoring and mapping Saturn's ionosphere, plasma, and lightning from Saturn's atmosphere.

RPWS Scientific Objectives

• To study the configuration of Saturn's magnetic field and its relationship to Saturn Kilometric Radiation (SKR)
• To monitor and map the sources of SKR
• To study daily variations in Saturn's ionosphere and search for outflowing plasma in the magnetic cusp region
• To study radio signals from lightning in Saturn's atmosphere
• To investigate Saturn Electric Discharges (SED)
• To determine the current systems in Saturn's magnetosphere and study the composition, sources, and sinks of magnetospheric plasma
• To investigate the dynamics of the magnetosphere with the solar wind, satellites, and rings
• To study the rings as a source of magnetospheric plasma
• To look for plasma waves associated with ring spoke phenomena
• To determine the dust and meteoroid distributions throughout the Saturnian system and interplanetary space
• To study waves and turbulence generated by the interaction of charged dust grains with the magnetospheric plasma
• To investigate the interactions of the icy satellites and the ring systems
• To measure electron density and temperature in the vicinity of Titan
• To study the ionisation of Titan's upper atmosphere and ionosphere and the interactions of the atmosphere and exosphere with the surrounding plasma
• To investigate the production, transport, and loss of plasma from Titan's upper atmosphere and ionosphere
• To search for radio signals from lightning in Titan's atmosphere, a possible source for atmospheric chemistry
• To study the interaction of Titan with the solar wind and magnetospheric plasma
• To study Titan's vast hydrogen torus as a source of magnetospheric plasma
• To study Titan's induced magnetosphere

RPWS Instrument Description

The major components of the RPWS instrument are the electric field sensor, the magnetic search coil sensor assembly, the Langmuir probe sensor assembly, and the instrument main electronics.

The electric field sensor is made up of three deployable antenna elements, an associated preamplifier, and antenna deployment mechanism drive electronics. The antennas are composed of interlocking sections made from beryllium copper, and each antenna element is deployable separately to 10 meters with its own 400 Hz AC. motor. The electric field preamplifier is used to add gain to the output signals from the antennas. The antenna deployment mechanism electronics convert ±15 volt primary power to 400 Hz AC power for the antenna drive motors.

The magnetic search coil sensor assembly is composed of a triaxial sensor assembly and an associated preamplifier. The triaxial sensor consists of three orthogonal (i.e., perpendicular) metallic alloy cores with two sets of windings each, one to produce flux in the core and another to detect the flux. The magnetic search coil preamplifier adds gain to the output signal from the sensor assembly.

The Langmuir probe sensor assembly consists of a sensor, a preamplifier, and associated control electronics. The Langmuir probe sensor is a 5 cm diameter sphere located at the end of a rod approximately one meter in length. The sensor rod is folded in a stowed state until deployed in flight. The probe sensor preamplifier adds gain to the output from the probe.

The RPWS main electronics includes a digital data processing unit, a high-frequency receiver, a wideband receiver, a medium-frequency receiver, a low-frequency five-channel waveform receiver, the Langmuir probe bias circuitry, and a power converter.

The data processing unit (DPU) will control all instrument functions and will handle all communications with the orbiter. It will contain a large block of RAM to be used as waveform storage for the five-channel waveform receiver. Software in the DPU will be used to enhance the scientific return of the instrument by performing various analysis and data compression operations.

The high-frequency receiver is a digital waveform processor that operates by digitizing a portion of the bandwidth received from the electric field sensor antennas and deriving spectral and waveform vector information from the waveforms using digital signal processing techniques.

The wideband receiver will obtain very high-resolution electric or magnetic field waveforms for selected time intervals that vary from under a minute to as much as an hour or more. The receiver has two selectable passbands: 50 Hz to approximately 10 kHz and 10 kHz to approximately 80 kHz. The wideband receiver uses high-rate telemetry to transfer waveform information in a given bandwidth from a selected sensor directly. The input signal is selectable from one of five inputs: two electric, one magnetic, a frequency-converted output from the high-frequency receiver, and the Langmuir probe.

The medium-frequency receiver provides spectrum measurements over the frequency range from 25 Hz to 12.6 kHz. This receiver is attached to one of four sensor inputs (two electric and two magnetic) and uses double frequency conversion to convert the input bandwidth down to a low-frequency constant frequency band, where it is detected by an amplitude detector.

The low-frequency five-channel waveform receiver provides high-resolution spectral measurements of electric and magnetic fields over the frequency range from 0.1 Hz to 2.5 kHz. It provides simultaneous waveforms from all five antennas (three magnetic axes and two electric axes). This receiver captures blocks of waveform data simultaneously from the five sensors and maintains a high-degree of phase and amplitude accuracy. The data is processed through five parallel amplifier and filter channels.

The power converter converts DC power from the spacecraft power supplies to AC power to operate the instrument. The conversion frequency will be fixed at 100 kHz by locking onto a signal from the spacecraft bus interface unit.

RSS: Radio Science Subsystem

The Radio Science Subsystem (RSS) uses the spacecraft X-band communication link, an S-band downlink and a Ka-band uplink and downlink to study compositions, pressures, and temperatures of atmospheres and ionospheres, radial structure and particle size distribution within rings, body and system masses, and gravitational waves.

Radio science experiments use the spacecraft radio system and ground antennas as the science instrument. These experiments measure the refractions, Doppler shifts, and other modifications to radio signals that occur when the spacecraft is occulted by planets, moons, atmospheres, and physical features such as planetary rings. From these measurements, scientists can derive information about the structures and compositions of the occulting bodies, atmospheres, and rings.

RSS Scientific Objectives

• To search for and characterise gravitational waves coming from beyond the solar system
• To study the solar corona and general relativity when Cassini passes behind the Sun
• To improve estimates of the masses and ephemerides of Saturn and its satellites
• To study the radial structure and particle size distribution within Saturn's rings
• To determine temperature and composition profiles within Saturn's/Titan's atmospheres
• To determine temperatures and electron densities within Saturn's/Titan's ionospheres

RSS Instrument Description

The RSS consists of a Ka-band traveling wave tube amplifier, a translator, an exciter; an S-band transmitter; and various microwave components.

The purpose of the Ka-band traveling wave tube amplifier (K-TWTA) subassembly is to amplify the signals going to the high-gain antenna to the power level necessary for them to be received by the Deep Space Network. The K-TWTA subassembly consists of the a traveling wave tube (TWT) and an electronic power conditioner (EPC). The non-redundant TWT is the signal amplifier. It can be commanded into a standby mode for low d.c. power consumption. The EPC converts d.c. power from the Power and Pyrotechnic Subsystem (PPS) to the voltages required to operate the TWT. It can power the TWT in the standby mode or power down the TWT in case of TWT or EPC fault detection. The EPC also supplies engineering telemetry to the RFS and provides direct-access signals.

The Ka-band translator (KAT) subassembly receives the 34 GHz uplink carrier from the high-gain antenna and translates it by a factor of 14/15 for retransmission back to the DSN. The phase and phase-shift of the signal are used for the actual science observations and measurements. The KAT contains a power converter that allows it to operate from the 30 Volt DC power bus. It also supplies engineering telemetry data to the RFS and provides for direct access.

The Ka-band exciter (KEX) generates a stable 32 GHz signal and provides an RF power combiner to combine the RF signal generated by the KAT with its own signal. It is powered by the 30 Volt spacecraft bus, and it supplies telemetry data to the RFS and provides direct access.

The S-band transmitter (SBT) receives a 115 MHz signal from the RFS, multiplies it by 20, amplifies it to 10 watts, and supplies the resultant signal at approximately 2290 MHz to the high-gain antenna. This carrier signal is used for radio science experiments. The transmitter contains a power converter to allow operation from the 30 volt power bus, and it supplies telemetry data to the RFS and provides direct access.

The microwave components consist of two band pass filters (BPFs) and waveguide components. BPFs are filters that allow only certain wavelengths of microwave energy to pass, with all other wavelengths being blocked. In this case, the BPFs permit reception and transmission of the Ka-band signals using different antenna feed polarizations and provide isolation between the transmit and receive frequencies. Waveguide is essentially tubing of precise dimensions that provides a path for microwave energy of a certain wavelength. In this subsystem it is used for all Ka-band microwave component interconnections.

UVIS: Ultraviolet Imaging Spectrograph

The Ultraviolet Imaging Spectrograph (UVIS) is a set of detectors designed to measure ultraviolet light reflected or emitted from atmospheres, rings, and surfaces over wavelengths from 55.8 to 190 nanometers to determine their compositions, distribution, aerosol content, and temperatures.

UVIS is a set of detectors designed to measure ultraviolet light reflected by or emitted from atmospheres, rings, and surfaces to determine their compositions, distributions, aerosol contents, and temperatures. UVIS will measure the fluctuations of starlight and sunlight as the sun and stars move behind the rings and the atmospheres of Titan and Saturn, and it will determine the atmospheric concentrations of hydrogen and deuterium. These data will be used for studies of the atmospheres, the magnetosphere, and the rings of the Saturnian system.

UVIS Scientific Objectives

• To map the vertical/horizontal composition of Titan's and Saturn's upper atmospheres
• To determine the atmospheric chemistry occurring in Titan's and Saturn's atmospheres
• To map the distribution and properties of aerosols in Titan's and Saturn's atmospheres
• To infer the nature and characteristics of circulation in Titan's and Saturn's atmospheres
• To map the distribution of neutrals and ions within Saturn's magnetosphere
• To study the radial structure of Saturn's rings by means of stellar occultations
• To study surface ices and tenuous atmospheres associated with the icy satellites

UVIS Instrument Description

UVIS has two channels: the extreme ultraviolet channel and the far ultraviolet channel. The ultraviolet channels are built into weight-relieved aluminium cases, and each contains a reflecting telescope, a concave grating spectrometer, and an imaging, pulse-counting detector. UVIS also includes a high-speed photometer channel, a hydrogen-deuterium absorption cell channel, and an electronics and control subassembly.

The extreme ultraviolet channel (EUV) will be used for imaging spectroscopy and spectroscopic measurements of the structure and composition of the atmospheres of Titan and Saturn. The EUV consists of a telescope with a three-position slit changer, a baffle system, and a spectrograph with a CODACON microchannel plate detector and associated electronics. The telescope consists of an off-axis parabolic section with a focal length of 100 mm, a 22 mm by 30 mm aperture, and a baffle with a field of view of 3.67 degrees by 0.34 degrees. A precision mechanism positions one of the three entrance slits at the focal plane of the telescope, each translating to a different spectral resolution.

The spectrograph uses an aberration-corrected toroidal grating that focuses the spectrum onto an imaging microchannel plate detector to achieve both high sensitivity and spatial resolution along the entrance slit. The microchannel plate detector electronics consists of a low-voltage power supply, a programmable high-voltage power supply, charge-sensitive amplifiers, and associated logic.

The EUV channel also contains a solar occultation mechanism to allow solar flux to enter the telescope when the sun is still 20 degrees off-axis from the primary telescope.

The far ultraviolet channel (FUV) will be used for imaging spectroscopy and spectroscopic measurements of the structure and composition of the atmospheres of Titan and Saturn and of the rings. The FUV is similar to the EUV channel except for the grating ruling density, optical coatings, and detector details. The FUV electronics are similar to those for the EUV except for the addition of a high-voltage power supply for the ion pump.

The high-speed photometer channel (HSP) will perform stellar occultation measurements of the structure and density of material in the rings. The HSP resides in its own module and measures undispersed (zero-order) light from its own parabolic mirror with a photomultiplier tube detector. The electronics consists of a pulse-amplifier-discriminator and a fixed-level high-voltage power supply.

The hydrogen-deuterium absorption cell channel (HDAC) will be used to measure hydrogen and deuterium in the Saturn system using a hydrogen cell, an oxygen cell, a deuterium cell, and a channel electron multiplier (CEM) detector to record photons not absorbed in the cells. The hydrogen and deuterium cells are resonance absorption cells filled with pure molecular hydrogen and deuterium, respectively. They are located between an objective lens and a detector. Both cells are made of stainless steel coated with teflon and are sealed at each end with MgF₂ windows. The electronics consists of a pulse-amplifier-discriminator, a fixed-level high-voltage power supply, and two filament current controllers.

The UVIS microprocessor electronics and control subassembly consists of input-output elements, power conditioning, science data and housekeeping data collection electronics, and microprocessor control elements.

VIMS: Visible and Infrared Mapping Spectrometer

The Visible and Infrared Mapping Spectrometer (VIMS) is a pair of imaging grating spectrometers designed to measure reflected and emitted radiation from atmospheres, rings, and surfaces over wavelengths from 0.35 to 5.1 microns to determine their compositions, temperatures, and structures.

VIMS will be used to map the surface spatial distribution of the mineral and chemical features of a number of primary and secondary targets. These targets include the Saturnian ring and satellite surfaces, the Saturnian atmosphere, and the atmosphere of Titan.

VIMS Scientific Objectives

• To map the temporal behaviour of winds, eddies, and other features on Saturn/Titan
• To study the composition and distribution of atmospheric and cloud species on S/T
• To determine the composition and distribution of the icy satellite surface materials
• To determine temperatures, internal structure, and rotation of Saturn's deep atmosphere
• To study the structure and composition of Saturn's rings
• To search for lightning on Saturn and Titan and for active volcanism on Titan
• To observe Titan's surface

VIMS Instrument Description

The VIMS Subsystem is organized into two assemblies: the optical pallet assembly and the main electronics assembly.

Optical Pallet Assembly

The optical pallet assembly consists of the following elements: the infrared channel, the visible channel, the visible channel electronics, and the signal processing electronics. The optical pallet has one mechanical interface with the spacecraft, and all electrical interfaces are via the main VIMS electronics. The pallet maintains all alignments internal to the VIMS instrument relative to the spacecraft mounting surface.

The infrared channel (VIMS-IR) is an opto-mechanical subassembly designed to produce multi-spectral images in the IR range. It consists of a Cassegrain telescope, a conventionally ruled spectrometer grating, and a 256-element linear array focal plane assembly cooled to its required operating temperature by a passive radiator. The VIMS-IR will be configured as a "whiskbroom" scanning imager, which means that the optical instrument's instantaneous field of view (IFOV) is a single pixel. A two-dimensional image is created by scanning along a row of pixels, dropping down a row, scanning that row, etc., using a two-axis scanning mirror.

The visible channel (VIMS-V) is an opto-mechanical assembly designed to produce multi-spectral images in the visible range. It consists of a Shafer telescope, a holographic spectrometer grating, and a silicon CCD area array focal plane detector cooled to its required temperature by a passive radiator. The VIMS-V will be configured as a "pushbroom" imager, which means that the optical instrument's IFOV is an entire line of pixels. This is scanned over the scene with a single-axis scanning mirror to produce a series of contiguous rows, which together form a two-dimensional image.

The visible channel electronics (VCE) support the operation of the VIMS-V and the preprocessing of its data for relay to the signal processing electronics. The signal processing electronics (SPE) support the operation of the VIMS-IR and the preprocessing of its data for relay to the main electronics.

Main Electronics Assembly

The VIMS main electronics (ME) is a single assembly that synchronizes the visible and IR channel mirror scanning mechanisms and controls the acquisition of data according to the selected operational hardware configuration. The ME serves as the only electronic interface between all elements of the VIMS instrument and the spacecraft, including power, data, command, and telemetry.

During flight operations, the ME will serve as the interface between ground operations (via the spacecraft) and the VIMS Subsystem. During the first 180 days after launch, all VIMS decontamination heaters will be on to prevent contamination from outgassing products (i.e., gasses liberated from nearby components). During the cruise phase, VIMS will be turned on once for monitoring of instrument health and performance. At other times, the instrument be off. During these times, decontamination heaters will be used periodically to keep the optics and radiator surfaces clean. At Saturn, VIMS will be used as determined by spacecraft command sequencing to acquire the desired science data.