Instrument Host Information
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| IDENTIFIER |
urn:nasa:pds:context:instrument_host:spacecraft.co::1.3
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| NAME |
Cassini Orbiter
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| TYPE |
Spacecraft
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| DESCRIPTION |
The majority of the text below was extracted from the Cassini Mission Plan (JPL Document D-5564, David Seal, 2002). For more information on the NASA Deep Space Network, see Asmar and Renzetti (1993). Instrument Host Overview ========================= For most Cassini Orbiter experiments, data were collected by instruments on the spacecraft then relayed via the orbiter telemetry system to stations of the NASA Deep Space Network (DSN). Radio Science required the DSN for its data acquisition on the ground. The following sections provide an overview, first of the orbiter, then the science instruments, and finally the DSN ground system. Instrument Host Overview - Spacecraft ======================================= Cassini was successfully launched on 15 October 1997 from Cape Canaveral, Florida, using a Titan IV/Centaur launch vehicle with Solid Rocket Motor Upgrade (SRMU) strap-ons and a Centaur upper stage. The spacecraft flew a 6.7-year Venus-Venus-Earth-Jupiter Gravity Assist (VVEJGA) trajectory to Saturn, during which cruise operations included checkout, characterization, calibration, and maintenance of the instruments and limited science observations. Until they separated after Saturn orbit insertion, Cassini was a combined Saturn orbiter and Titan atmospheric probe. It was a three-axis stabilized spacecraft equipped for 27 diverse science investigations with 12 orbiter and 6 Huygens probe instruments, one high gain (HGA) and two low gain antennas (LGAs), three Radioisotope Thermoelectric Generators (RTGs), main engines, attitude thrusters, and reaction wheels. This description covers the orbiter portion of Cassini, which will frequently be called "the spacecraft". Orbiter Description =================== The Cassini orbiter was a three-axis-stabilized spacecraft. The origin of the spacecraft coordinate system was located at the center of the plane at the bus/upper shell structure interface (i.e., base of the electronic bays on the upper equipment module). The remote sensing pallet was mounted on the +X side of the spacecraft, the magnetometer boom extended in the +Y direction, and the +Z axis completed the orthogonal body axes in the direction of the main engine. The primary remote sensing boresights viewed in the -Y direction, the probe was ejected in the -X direction, the HGA boresight was in the -Z direction, the main engine exhaust was in the +Z direction, and the main engine thrust was in the -Z direction. The coordinates and some of the larger elements of the spacecraft are shown in the figure below. /\ ---------------------------------- \ / \ / HGA \ / MAG Boom -------------------------- ... =================| | | h | \ ^ / | | | | | | Ysc -------| v <---o | | b, Xsc | | | | | | | | | | | ---------------------- / \ / \ Main Rocket Engine ---------- | | | V Zsc where b and Xsc point out of the screen or page. The main body of the spacecraft was formed by a stack consisting of the lower equipment module, the propulsion module, the upper equipment module, and the HGA. Attached to this stack were the remote sensing pallet, the fields and particles pallet, and the Huygens Probe system. The Huygens Probe was built by the European Space Agency and was deployed into the atmosphere of Titan by the Orbiter. Some instruments such as RADAR and some instrument components such as those of RPWS were attached to the upper equipment module. The two equipment modules were also used for external mounting of the magnetometer boom and the three radioisotope thermoelectric generators (RTGs) which supplied the spacecraft power. The spacecraft electronics bus was part of the upper equipment module and carried the electronics to support the spacecraft data handling, including the command and data subsystem and the radio frequency subsystem. Other electronics to support instruments and other spacecraft functions were also carried in the bus. During the inner cruise, the HGA and two Low Gain Antennas (LGAs) were used to transmit data and receive commands. One of the two LGAs was selected when operational constraints prevented pointing the HGA towards the Earth. The spacecraft stood 6.8 meters (22.3 ft) high. Its maximum diameter, the diameter of the HGA, was 4 meters (13.1 ft). Therefore, the HGA could fully shield the rest of the spacecraft (except the deployed MAG boom and RPWS antennas) from sunlight when the HGA was pointed within 2.5 degrees of the Sun. The dry mass of the spacecraft was 2523 kg, including the Huygens Probe system and the science instruments. The best estimate of the actual spacecraft mass at separation from the Centaur was 5573.8 kg. Future estimates of spacecraft mass were maintained by the Spacecraft Operations team (SCO). SPACECRAFT SUBSYSTEMS --------------------- The spacecraft comprised several subsystems, which are described briefly below. For more detailed information, see JPL Document D-5564. Structure Subsystem ------------------- The Structure Subsystem (STRU) provided mechanical support and alignment for all flight equipment including the Huygens Probe. It also served as a local thermal reservoir and provided an equipotential container, an electrical grounding reference, RFI shielding, and protection from radiation and meteoroids. The STRU consisted of the Upper Equipment Module (UEM) which contained the 12-bay electronics bus assembly, the instrument pallets, and the MAG boom, and the Lower Equipment Module (LEM), plus all the brackets and structure for integrating the Huygens Probe, the HGA, LGAs, RTGs, reaction wheels, the main rocket engines, the four RCS thruster clusters, and other equipment. The STRU also included an adapter which supported the spacecraft on the Centaur during launch. Radio Frequency Subsystem ------------------------- The Radio Frequency Subsystem (RFS) provided the telecommunications facilities for the spacecraft and was used as part of the radio science instrument. For telecommunications, it produced an X-band carrier at 8.4 GHz, modulated it with data received from the CDS, amplified the X-band carrier power to produce 20 W from the Traveling Wave Tube Amplifiers (TWTA), and delivered it to the Antenna Subsystem (ANT). From ANT, RFS accepted X-band ground command/data signals at 7.2 GHz, demodulated them, and delivered the commands/data to CDS for storage and/or execution. The Ultra Stable Oscillator (USO), the Deep Space Transponder (DST), the X- band Traveling Wave Tube Amplifier (TWTA), and the X-band Diplexer were elements of the RFS which were used as part of the radio science instrument. The DST could phase-lock to an X-band uplink and generate a coherent downlink carrier with a frequency translation adequate for transmission at X-, S-, or Ka-band. The DST had the capability of detecting ranging modulation and of modulating the X-band downlink carrier with the detected ranging modulation. Differenced one-way ranging (DOR) tones could also be modulated onto the downlink. The DST could also accept the reference signal from the USO and generate a non-coherent downlink carrier. Propulsion Module Subsystem --------------------------- The Propulsion Module Subsystem (PMS) provided thrust and torque to the spacecraft. Under command from AACS, the thrust and torque established the spacecraft attitude, pointing, and the amount of velocity vector change. For attitude control, the PMS had a Reaction Control Subsystem (RCS) consisting of four thruster clusters mounted off the PMS core structure adjacent to the LEM at the base of the spacecraft. Each of the clusters contained 4 hydrazine thrusters. The thrusters were oriented to provide thrust along the spacecraft +/-Y and -Z axes. RCS thrusters also provide DV for small maneuvers. For larger DVs, the PMS had a primary and redundant pressure-regulated main rocket engine. Each engine was capable of a thrust of approximately 445 N when regulated. The bipropellant main engines burned nitrogen tetroxide (N2O4) and monomethylhydrazine (N2H3CH3) producing an expected specific impulse of up to 308s. These engines were gimbaled so when under AACS control during burns the thrust vector could be maintained through the shifting center of mass of the spacecraft. AACS-provided valve drivers for all the engines/thrusters operated in response to commands received from AACS via the CDS data bus. Power and Pyrotechnics Subsystem -------------------------------- The Power and Pyrotechnics Subsystem (PPS) provided regulated electrical power from three RTGs on command from CDS to spacecraft users at 30 Volts DC, distributed over a power bus. In addition, PPS provided power to the various pyrotechnic devices on command from CDS. PPS disposed of excess power by heat radiation to space via a resistance shunt radiator. Measurements of the output of the radioisotope thermoelectric generators indicated a beginning-of- life power of 876 +/- 6 Watts, 740 Watts at SOI, and 692 Watts at end of mission. These estimates were at least 30 Watts above pre-launch predictions. Command and Data Subsystem -------------------------- The Command and Data Subsystem (CDS) received the uplink command stream via the RFS and decoded it. The stream included timing (immediate or sequence), routing, action, and parameter information. The CDS then distributed commands designated for other subsystems or instruments, executed those commands which were decoded as CDS commands, and stored sequence commands for later execution. The Cassini spacecraft included two identical Solid State Recorders (SSRs). Each CDS (A and B) was attached to the two SSRs such that each CDS could communicate (read, write) with only one SSR at any one time. The Mission and Science Operations Office had the capability to control how the SSR attachments were configured via immediate command or a stored sequence. Under fault response conditions flight software (FSW) could switch an SSR attachment from CDS A to CDS B. The CDS received data from other on-board subsystems via the data bus, then processed and formatted them for telemetry and delivered them to RFS for transmission to Earth. Each subsystem interfaced with the data bus through a standard Bus Interface Unit (BIU) or a Remote Engineering Unit (REU). Data were collected in 8800 bit frames, and Reed-Solomon Encoded on downlink. A 32 framesync marker along with the encoding increased these frames to 10,112 bits. CDS software contained algorithms that provided protection for the spacecraft and the mission in the event of a fault. Fault protection software ensured that, in the case of a serious fault, the spacecraft would be placed into a safe, stable, commandable state (without ground intervention) for a period of at least two weeks to give the mission operations team time to solve the problem and send the spacecraft a new command sequence. It was also capable of autonomously responding to a predefined set of faults needing immediate action. Attitude and Articulation Control Subsystem ------------------------------------------- The Attitude and Articulation Control Subsystem (AACS) provided dynamic control of the spacecraft in rotation and translation. It provided fixed- target staring for HGA and remote sensing pointing and performed target relative pointing using inertial vector propagation as well as repetitive subroutines such as scans and mosaics. AACS also controlled actuators for the main rocket engine gimbals. Rotational motion during the Saturn tour that required high pointing stability was normally controlled by the three main Reaction Wheel Assemblies (RWAs), although modes requiring faster rates or accelerations may have used thrusters. The additional fourth reaction wheel could articulate to replace any single failed wheel. AACS contained a suite of sensors that included redundant Sun Sensor Assemblies (SSA), redundant Stellar Reference Units (SRU, also called star trackers), a Z-axis accelerometer, and two 3-axis gyro Inertial Reference Units (IRU). Each IRU consisted of four gyros, three orthogonal to each other and the fourth skewed equidistant to the other three. Temperature Control Subsystem ----------------------------- The TEMPerature control subsystem (TEMP) allowed operations over the expected solar ranges (0.61 to 10.1 AU) with some operational constraints. Temperatures of the various parts of the spacecraft were kept within allowable limits by a large number of local TEMP thermal control techniques, many of which were passive. The 12-bay electronics bus had automatically positioned reflective louvers. Radio Isotope Heater Units (RHU) were used where constant heat input rates were needed and where radiation was not a problem. Multilayer insulation blankets covered much of the spacecraft and its equipment. Electric heaters were used in different locations and operated by CDS and instruments. Temperature sensors were located at many sites on the spacecraft, and their measurements were used by CDS to command the TEMP heaters. Shading was executed by pointing the HGA (-Z axis) towards the sun; the HGA was large enough to provide shade for the entire spacecraft body including the Huygens Probe. Mechanical Devices Subsystem ---------------------------- The mechanical devices subsystem provided a pyrotechnic separation device used to separate the spacecraft from the launch vehicle adapter. Springs provided the impulse to separate the spacecraft from the adapter. The mechanical devices subsystem also provided a self-deploying 10.5 meter coiled longeron mast stored in a canister for the two magnetometers, electrostatic discharge covers over inflight separation connectors, an articulation system for the backup reaction wheel assembly, a "pinpuller" for the RPWS Langmuir Probe, and louvers and variable RHUs for temperature control. Electronic Packaging Subsystem ------------------------------ The Electronic Packaging Subsystem (EPS) consisted of the electronics packaging for most of the spacecraft in the form of the 12-bay electronics bus. The bus was made up of bays containing standardized, dual-shear plate electronics modules. Solid State Recorder Subsystem ------------------------------ The two Solid State Recorders (SSRs) were the primary memory storage and retrieval devices used on the orbiter. Each SSR contained 128 submodules, of which 8 were used for flight software and 120 were used for telemetry. Each submodule could hold 16,777,200 bits for data, so the total data storage for telemetry on each SSR was 2.013 Gbits. Expressed in terms of 8800-bit telemetry frames, this was 228,780 frames per SSR. Spacecraft telemetry and AACS, CDS, and instrument memory loads were stored in separate files called partitions. All data recorded to and played back from the SSR was handled by the CDS. There were three different SSR functional modes: Read-Write to End, Circular FIFO, and Ring Buffer. There was also a record pointer and a playback pointer, which marked the memory addresses at which the SSR could write or read. In Read-Write to End, there was a logical beginning and end to the SSR. Recording began at this logical beginning and continued until either the SSR was reset (the record and playback pointers were returned to the logical beginning) or until the record pointer reached the end. If the record pointer did not reach the end, recording was halted until the SSR was reset. In Circular FIFO, there was no logical end to the SSR. The data was continuously recorded until the record pointer reached the playback pointer. The Ring Buffer mode behavior was similar to the Circular FIFO except that recording did not stop if the record pointer reached the playback pointer. Antenna Subsystem ----------------- The ANTenna subsystem (ANT) provided a directional high gain antenna (HGA) with X-, Ka-, S and Ku-band for transmitting and receiving on all four bands. Because of its narrow halfpower beam width of 0.14 deg for Ka-band, it had to be accurately pointed. The HGA, and the low gain antenna 1 (LGA1) located on the HGA feed structure, were provided by the Italian Space Agency. Another LGA (LGA2) was located below the Probe pointing in the -X direction. During the inner solar system cruise, the HGA was Sun- pointed to provide shade for the spacecraft. ANT provided two LGAs which allowed one or the other to receive/transmit X-band from/to the Earth when the spacecraft was Sun-pointed. The LGAs also provided an emergency uplink/downlink capability while Cassini was at Saturn. The HGA downlink gain at X-band was 47dBi and the LGA1 peak downlink gain was 8.9 dBi. The X-band TWTA power was 20 watts. ORBITER SCIENCE INSTRUMENTS --------------------------- There were 12 science instrument subsystems on the Cassini spacecraft, listed immediately below with their acronyms, then described in more detail in the following paragraphs. Three of the instruments (CAPS, CDA, and MIMI/LEMMS) were capable of commanded articulation relative to the spacecraft. Cassini Plasma Spectrometer CAPS Cosmic Dust Analyzer CDA Composite Infrared Spectrometer CIRS Ion and Neutral Mass Spectrometer INMS Imaging Science Subsystem ISS Magnetometer MAG Magnetospheric Imaging Instrument MIMI Cassini RADAR RADAR Radio and Plasma Wave Science RPWS Radio Science Subsystem RSS Ultraviolet Imaging Spectrograph UVIS Visible and Infrared Mapping Spectrometer VIMS Cassini Plasma Spectrometer (CAPS): The CAPS instrument was designed to perform an in-situ study of plasma within and near the magnetosphere of Saturn. Specific science and measurement objectives were: 1) Orbital Tour Observing Objectives: a) Near continuous survey. b) MAPS Campaigns. c) SOI, targeted Titan and icy satellite observations. d) CAPS Magnetospheric Survey. 2) Solar Wind/Aurora Campaign Objectives: a) Measure solar wind while ORS observed aurora. b) Unambiguous measurements of unperturbed solar wind, correlation with Earth based and RPW auroral data. 3) Study microphysical and rapidly varying processes near the bow shock and magnetopause. 4) Observe particle acceleration, particle injection, and dynamical events (e.g. substorms) in the magnetotail. 5) Measure vertical (field aligned) structure of plasma in the inner magnetosphere. 6) Observe the dynamics and microphysics of the auroral and Saturn Kilometric Radiation (SKR) source regions. 7) Study the Titan plasma torus and distant signatures of the interaction of Titan with the magnetosphere. 8) Study the distant signatures of satellites and ring interactions with the magnetosphere. Cosmic Dust Analyzer (CDA): The CDA instrument was designed to perform an in- situ study of dust grains in the Saturn system. Specific science and measurement objectives were: 1) Study interplanetary and interstellar dust at Saturn. 2) Saturn Rings Objectives: a) Map size distribution. b) Search for particles in the so-called clear zone (F/G ring). c) Determine orbits of particles to identify their possible parents. d) Study the interaction between the E ring and magnetosphere of Saturn e) Distinguish temporal and spatial effects. f) Analyze eccentricity and inclination of dust orbits independently. 3) Icy Satellites Objectives: a) Interaction with the ring system. b) Role of satellites as a source and sink for ring particles. c) Chemical composition of satellites (dust atmospheres). Composite Infrared Spectrometer (CIRS): The CIRS instrument was designed to perform spectral mapping to study temperature and composition of surfaces, atmospheres, and rings within the Saturn system. Specific science and measurement objectives were: 1) Thermal Structure Objectives: a) Vertical profiles of atmospheric temperature. b) Maps of atmospheric and surface temperatures. c) Aerosol opacities. d) Thermal inertia of surfaces. e) Subsurface regolith structure. f) Ring particle sizes. g) Ring thermal structure. 2) Composition Objectives: a) Spatial distribution of atmospheric gases. b) Surfaces. c) Ring material. 3) Atmosphere Objectives: a) Circulation: Zonal jets, Meridional motion, vortices, wave, convection. b) Composition: Dis-equilibrium species, elemental and isotope abundances and distribution, ortho/para ratio, condensable gases, external sources (e.g., rings). c) Clouds/Aerosols: Composition, microphysical properties, spatial and temporal distribution. d) Atmospheric. Structure: Temperature, pressure, density, vertical distribution of major constituents. e) Internal Structure: He abundance, internal heat, gravity. f) Aurora, lighting, airglow: Spatial and temporal distribution, special properties. g) Titan: Aerosols and clouds, Titan winds. 4) Rings Objectives: a) Vertical structure and thermal gradient. b) Vertical Dynamics. c) Particle Surface Properties. d) Particle Composition. e) Radial Structure. 5) Non-Targeted Icy Satellites Objectives: a) Determine surface composition. b) Determine vertical thermal structure (Greenhouse). c) Determine thermophysical properties (Thermal Inertia). d) Search for active thermal sources (space and time). Ion and Neutral Mass Spectrometer (INMS): The INMS instrument was designed to perform an in-situ study of the compositions of neutral and charged particles within the Saturn magnetosphere. Specific science and measurement objectives were: 1) Outer Magnetosphere: Science Objectives: a) Neutral and ion composition of the magnetosphere. b) Composition of the Titan plasma torus. c) Additional low energy ion distribution function information to complement CAPS. 2) Inner Magnetosphere: Science Objectives: a) Studies of solar system formation. Plasma sources derived from the rings and icy satellites - composition and isotopic ratio. b) Studies of plasma transport. Determination of plasma transport velocities and determination of momentum transfer from charge exchange chemistry - water products. Imaging Science Subsystem (ISS): The ISS instrument was designed to perform multispectral imaging of Saturn, Titan, rings, and icy satellites to observe their properties. Specific science and measurement objectives were: 1) Motions and Dynamics: a) Basic flow regime (Titan). b) Poleward flux of momentum. c) Poleward flux of heat (with CIRS). d) Life cycles and small-scale dynamics of eddies. e) Radiative heating for dynamical studies. 2) Clouds and Aerosols: a) Could and haze stratigraphy (strongly couples with wind studies). b) Particle optical properties. c) Particle physical properties. d) Auroral processes and particle formation. e) Haze microphysical models. 3) Lightning (related to water clouds on Saturn; we do not know what to expect for Titan). 4) Auroras (H and H2 emissions on Saturn, N and N2 emissions on Titan). 5) ISS High Priority Rings Goals: a) Ring Architecture/Evolution: Azimuthal, radial, temporal variations across tour. b) New satellites: orbits, masses/densities, effects on rings; complete inventory of inner moons of Saturn. c) Search and characterize material potentially hazardous to Cassini: diffuse rings, arcs, Hill sphere material, etc. d) Orbit refinement of known satellites; temporal variations; resonant effects. e) Particle/Disk properties: vertical disk structure; particle physical properties and size distribution; variations across disk. f) Spokes: Formation timescales/process; periodic variations. g) Diffuse Rings (E, G): Structure, characterize particle properties. Magnetometer (MAG): The MAG instrument was designed to study the magnetic field of Saturn and its interactions with the solar wind. Specific science and measurement objectives were: 1) Intrinsic magnetic fields of Saturn and its moons: a) Determine the multiple moments of dynamo-driven magnetic field of Saturn b) Determine weather Titan has an internal field due to dynamo action, electromagnetic induction or even remnant magnetization in a so-called dirty ice crust. c) Search for possible evidence of ancient dynamos and crustal remnants in the icy satellites. 2) Derive a 3-D global model of the magnetospheric magnetic field. 3) Establish the relative contributions to electromagnetic and mechanical stress balance. 4) Identify the energy source for dynamical processes (rotationally driven, solar wind driven, or other). 5) Characterize the phenomena of the distant dayside/flank planetary environment. 6) Survey satellite/dust/ring/torus electromagnetic interactions. 7) Determine tail structure and dynamic processes therein. 8) Establish nature and source of all ULF wave sources. 9) Magnetosphere/ionosphere coupling. 10) Titan: a) Determine the internal magnetic field sources of Titan as well as the sources external to I I - thereby determining the interaction type. b) Determine all Titan-plasma flow interactions (magnetosphere, magnetosheath, solar wind). c) Determine the variation of the Titan-magnetosphere interaction with respect to Titan orbital phase. d) Determine the nature of the low frequency wave (ion cyclotron/hydromagnetic) spectrum of the near-Titan plasma environment. 11) Icy Satellites: a) Search for possible evidence of ancient dynamos and crustal remanence in the icy satellites b) Investigate icy satellite plasma environments. Magnetospheric Imaging Instrument (MIMI): MIMI was designed for global magnetospheric imaging and in-situ measurements of the magnetosphere of Saturn and solar wind interactions. Specific science and measurement objectives were: 1) MIMI Survey: a) What is the source of energetic particles in the magnetosphere of Saturn and how are they energized? b) To what extent does the solar wind and rotation regulate the size, shape and dynamics of the magnetosphere of Saturn; are there Earth-like storms and substorms? c) How does the interaction between the magnetospheric particle population and Saturn cause the aurora, and affect magnetospheric and upper atmospheric processes? d) How does the distribution of satellites affect global magnetospheric morphology and processes? 2) MIMI Campaigns: a) How do satellites and their exospheres affect local magnetospheric plasma flow and contribute to energetic particle populations? b) What particles (species, energy) cause Saturnian aurora; what processes accelerate them and what is the exospheric response? c) What unique role do the rings of Saturn play in controlling the structure, composition, and transport of the inner magnetosphere? Cassini RADAR (RADAR): The RADAR instrument was designed for synthetic aperture RADAR (SAR) imaging, altimetry, and radiometry of the surface of Titan. Specific science objectives for the Cassini mission were as follows. 1) Rings: a) Determine scattering properties of rings. b) Determine ring global properties. c) Determine additional thermal and compositional properties of rings. d) Extended ring global properties: low-elevation measurements. e) Radial scans through optically thin rings (E, F and G). f) Identify thermal component. 2) Catalog of base radar/radiometric properties each satellite and their degree of global variation. Radio and Plasma Wave Science (RPWS): The RPWS instrument was designed to study plasma waves, radio emissions, and dust in the Saturn system. Specific science and measurement objectives were: 1) Aurora and SKR: Obtain radio and plasma wave data which provide information on the SKR source and plasma waves on auroral field lines. 2) Satellite and ring interactions: Measure dust flux, look for effects of selective absorption of electrons and ions near rings (thermal anisotropy), multi-ion wave particle interactions, satellite torii. 3) Inner Magnetosphere: Wave particle interactions via ULF waves; Stability of trapped electrons and relation to whistler-mode emissions; ECH (N+ 1/2)fce waves trapped near the equatorial region and heating of cool electrons. 4) Titan Interactions: Multi-ion species wave-particle interactions; Evidence of Titan plumes/detached plasma blobs. 5) Magnetospheric Boundaries: Nature of the Saturnian Bow shock: Look for the signatures of waves accelerating electrons. 6) What is the nature of the magnetotail of Saturn? Are there substorms or other dynamical processes there? 7) Observe lightning via SED and whistlers from the atmosphere of Saturn (and possibly Titan). 8) Determine the equatorial dust flux and scale height as a function of radial distance. 9) Provides for mapping and synoptic measurements required for the RPWS portion of the magnetospheric survey. 10) Search for electromagnetic phenomena which may be triggers of ring spokes. Radio Science Subsystem (RSS): The RSS was designed to study atmospheres and ionospheres of Saturn, Titan, rings, and gravity fields of Saturn and its satellites (also, search for gravitational waves during cruise). Specific science and measurement objectives were: 1) Ring Occultations: a) To profile radial ring structure with resolution less than or equal to 100m; characterize structure variability with azimuth, wavelength, ring-opening-angle, and time. b) To determine the physical particle properties (size distribution, bulk density, surface density, thickness, viscosity). c) To study ring kinematics and dynamics (morphology, interaction with embedded and exterior satellites), and to investigate ring origin and evolution. 2) Atmospheric Occultations: a) To determine the global fields of temperature, pressure, and zonal winds in the stratosphere and troposphere of Saturn. b) To determine the small scale structure due to eddies and waves. c) To determine the latitudinal variations of NH3 abundance in the atmosphere of Saturn. d) To improve the knowledge of H2/He ratio in the troposphere of Saturn (RSS+CIRS). 3) Ionospheric Occultations: a) To determine the vertical profiles of the electron density in the terminator ionosphere of Saturn, and its variability with latitude. b) To investigate interaction of the ionosphere with the magnetosphere and rings of Saturn. 4) Gravity Field of Saturn: a) To determine the mass of Saturn and zonal harmonic coefficients of its gravity field to at least degree 6 (J2, J4, J6). b) To constrain models of the interior of Saturn based on the results 5) Gravity Field and Occultation of Untargeted Satellites: a) To determine the masses of Mimas, Tethys, Dione, Hyperion, and Phoebe. b) To search for a possible tenuous ionosphere around any occulted satellites (a la Europa and Callisto). Ultraviolet Imaging Spectrograph (UVIS): The UVIS instrument was designed to produce spatial UV maps, map ring radial structure, and to determine hydrogen/deuterium ratios. Specific science and measurement objectives were: 1) Saturn System Scans: a) EUV and FUV low resolution spectra of magnetosphere neutral and ion emissions. b) System scans at every apoapsis. 2) Satellites: a) Latitude, longitude and phase coverage coordinated through SSWG. b) Distant stellar occultations to determine satellite orbits and Saturn reference frame. 3) Atmosphere: a) Vertical profiles of H, H2, hydrocarbons, temp in exo, thermosphere. b) Long integrations map of hydrocarbons, airglow. c) Map emissions with highest resolution at the limb. d) Auroral Map: H and H2 emissions over several rotations. 4) Ring Stellar Occultation Objectives: a) Highest Radial resolution (20m) structure of rings b) Discovery and precise characterization of dynamical features generated by ring-satellite interactions. - Density waves and bending waves. - Edge waves and ring shepherding. - Embedded moonlets and discovery of new moons from dynamical response in rings. c) Discovery and precise characterization of azimuthal structure in rings. - Eccentric rings. - Density waves and edge waves. - Small-scale self-gravitational clumping in rings. d) Measure temporal variability in ring structure. e) Simultaneously measure UV reflectance spectrum of rings. - Determine microstructure on particle surfaces. - Compositional information on ring particles. f) Measure size distribution of large particles through occultation statistics. g) Measure dust abundance in diffraction aureole. h) Simultaneously search for flashes from 0.1 m - 1.0 m meteoroid impacts. Visible and Infrared Mapping Spectrometer (VIMS): The VIMS instrument was designed to produce spectral maps to study the composition and structure of surfaces, atmospheres, and rings. Specific science and measurement objectives were: 1) Ring Observation: a) Determine ring composition and its spatial variations. b) Determine light scattering behavior of rings as a function of I, e, and alpha. c) Constrain sizes and surface textures of ring particles. d) Establish optical depth profile of rings as a function of wavelength, incidence angle, and longitude. e) Characterize variable features such as non-circular ringlets, F ring, spokes, etc., and their evolution. f) Ring moon compositions. 2) Icy Satellite Observation: a) Measure UV and NIR spectra to: - Identify and map surface materials at the highest spatial resolution. - Determine microphysical surface properties. - Provide data on energy balance. Instrument Host Overview - DSN ================================ Radio Science investigations utilized instrumentation with elements both on the spacecraft and at the NASA Deep Space Network (DSN). Much of this was shared equipment, being used for routine telecommunications as well as for Radio Science. The Deep Space Network was a telecommunications facility managed by the Jet Propulsion Laboratory of the California Institute of Technology for the U.S. National Aeronautics and Space Administration. The primary function of the DSN was to provide two-way communications between the Earth and spacecraft exploring the solar system. To carry out this function the DSN was equipped with high-power transmitters, low-noise amplifiers and receivers, and appropriate monitoring and control systems. The DSN consisted of three complexes situated at approximately equally spaced longitudinal intervals around the globe at Goldstone (near Barstow, California), Robledo (near Madrid, Spain), and Tidbinbilla (near Canberra, Australia). Two of the complexes were located in the northern hemisphere while the third was in the southern hemisphere. The network comprised four subnets, each of which included one antenna at each complex. The four subnets were defined according to the properties of their respective antennas: 70-m diameter, standard 34-m diameter, high-efficiency 34-m diameter, and 26-m diameter. These DSN complexes, in conjunction with telecommunications subsystems onboard planetary spacecraft, constituted the major elements of instrumentation for radio science investigations.
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| REFERENCES |
Asmar, S. W., N. A. Renzetti, The Deep Space Network as an Instrument for Radio Science Research, JPL Publication 80-93, Rev. 1, April 15, 1993.
Cassini Mission Plan, Revision N (PD 699-100), JPL Document D-5564, Jet Propulsion Laboratory, Pasadena, CA, 2002.
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