|INSTRUMENT_HOST_NAME|| "CASSINI ORBITER"|
The majority of the text in this file was extracted from the Cassini
Mission Plan Document, D. Seal, 2003. [JPLD-5564]
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
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
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
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 Titan's
atmosphere 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 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
The spacecraft comprised several subsystems, which are described briefly
below. For more detailed information, see JPLD-5564.
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
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 are 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 & 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
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
Solid State Recorder Subsystem
Cassini's 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.
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
Cassini Plasma Spectrometer CAPS
Cosmic Dust Analyzer CDA
Composite Infrared Spectrometer CIRS
Ion and Neutral Mass Spectrometer INMS
Imaging Science Subsystem ISS
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 Saturn's magnetosphere.
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
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
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 Titan's
interaction with the magnetosphere.
8) Study the distant signatures of satellites and ring interactions with
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 'clear zone' (F/G ring).
c) Determine orbits of particles to identify their possible parents.
d) Study the interaction between E ring and Saturn's magnetosphere.
e) Distinguish temporal and spatial effects.
f) Analyze eccentricity and inclination of dust orbits
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.
c) Ring material.
3) Atmosphere Objectives:
a) Circulation: Zonal jets, Meridional motion, vortices, wave,
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,
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
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
1) Motions and Dynamics:
a) Basic flow regime (Titan).
b) Poleward flux of momentum (u'v').
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, don't know what to expect
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 Saturn's inner moons.
c) Search and characterize material potentially hazardous to Cassini:
diffuse rings, arcs, Hill's sphere material, etc.
d) Orbit refinement of known satellites; temporal variations;
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 Saturn's
magnetic field and 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 Saturn's dynamo-driven
b) Determine weather Titan has an internal field due to dynamo action,
electromagnetic induction or even remnant magnetization in a 'dirty
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
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
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.
a) Determine the internal magnetic field sources of Titan as well as
the sources external to I I - thereby determining the interaction
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
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 Saturn's magnetosphere
and solar wind interactions. Specific science and measurement objectives
1) MIMI Survey:
a) What is the source of energetic particles in Saturn's magnetosphere
and how are they energized?
b) To what extent does the solar wind and rotation regulate the size,
shape and dynamics of Saturn's magnetosphere; 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 Saturn's rings 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 Titan's surface.
Specific science objectives for the Cassini mission are as follows.
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 each satellite's base radar/radiometric properties 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
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 Saturn's magnetotail? Are there substorms or
other dynamical processes there?
7) Observe lightning via SED and whistlers from Saturn's atmosphere and
8) Determine the equatorial dust flux and scale height as a function of
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
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 <= 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
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
d) To improve the knowledge of H2/He ratio in Saturn's troposphere
3) Ionospheric Occultations:
a) To determine the vertical profiles of the electron density in
Saturn's terminator ionosphere, and its variability with latitude.
b) To investigate interaction of the ionosphere with Saturn's
magnetosphere and Saturn's rings.
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 Saturn's interior based on the results
5) Gravity Field and Occultation of Untargeted Satellites:
a) To determine the masses of Mimas, Tethys, Dione, Hyperion, and
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
1) Saturn System Scans:
a) EUV and FUV low resolution spectra of magnetosphere neutral and ion
b) System scans at every apoapsis.
a) Latitude, longitude and phase coverage coordinated through SSWG.
b) Distant stellar occultations to determine satellite orbits and
Saturn reference frame.
a) Vertical profiles of H, H2, hydrocarbons, temp in exo,
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
- 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
g) Measure dust abundance in diffraction aureole.
h) Simultaneously search for flashes from 0.1 m - 1.0 m meteoroid
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
1) Ring Observation:
a) Determine ring composition and its spatial variations.
b) Determine light scattering behavior of rings as a function of I, e,
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
- 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
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
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.
For more information see [ASMAR&RENZETTI1993]"
|REFERENCE_DESCRIPTION|| "Asmar, S.W., and N.A.Renzetti, The Deep Space Network as an Instrument forRadio Science Research, Jet Propulsion Laboratory Publication80-93, Rev.1, 15 April 1993."
"Cassini Mission Plan,Revision N (PD 699-100), JPL Document D-5564,Jet Propulsion Laboratory, Pasadena, CA, 2002."