| DATA_SET_DESCRIPTION |
Data Set Overview
=================
The Composite Infrared Spectrometer is a dual-interferometer carried on the
Cassini spacecraft Remote Sensing Palette. Cassini was launched on 15th
October 1997 and due to arrive at Saturn on July 1st 2004. En route to
Saturn, Cassini made a gravity-assist maneuver at Jupiter in December
2000, which allowed for a six-month Jupiter observing campaign. CIRS was
operated in full science mode for much of this six month period,
either as the 'prime' instrument controlling pointing, or as a
'ride-along' when another team was 'prime'. The CIRS Jovian dataset
forms the first part of the CIRS archive.
The Cassini Prime Mission after SOI (Saturn Orbit Insertion) is the
first four years of tour, from July 2004 to July 2008. After that
time, if approved and technically possible, an extended mission period
will begin. During the first four years of Prime Mission, Cassini will
undergo 76 orbits (revs) of Saturn, known as 0, A, B, C, 3 ... 74. The
original orbits 1 and 2 were replaced with A, B and C when the mission
was redesigned in order to accomodate radio link issues with the
Huygens probe, discovered after launch. The probe delivery to Titan is
scheduled for rev C. There are 44 targeted (i.e. close) fly-bys of
Titan planned during the first 76 revs.
See MISSION.CAT file for more information on the Cassini mission.
Science Objectives and Observation Strategy
-------------------------------------------
The Cassini/Huygens mission to
the Saturnian system is designed to investigate the following targets:
1. Saturn
2. Rings
3. Titan
4. Other ('icy') satellites.
The overall mission may also be broken in physical target types:
1. Atmospheres (Saturn and Titan)
2. Surfaces (Titan, and other satellites).
3. Magnetosphere: fields, particles and solar wind interaction (Sun-Saturn)
4. Orbital mechanics (rings, satellites esp Hyperion).
CIRS contributes to the investigation of many of these areas:
SATURN
Sensing of tropospheric and stratospheric temperatures and composition.
This includes abundances of the major and minor species, the hunt for new
gaseous species, isotope ratios for major species, and
dynamics. Allocating time for Saturn observations was the
responsibility of the Saturn TWT (Target Working Team), which was in
charge of much of the periapse parts of each orbit (rev). There was
also input from the Atmospheres Working Group (AWG) which dealt with
high-level science reccomendations.
CIRS Saturn atmospheric science goals are met with observations at a
variety of inclinations, range from Saturn and, in some cases, in
conjunction with other teams' observations. The standard CIRS requests
include:
Name Range Details
COMPSIT >60Rs Composition sit and stare at 0.5 cm-1 spectral resolution
using FP1. Uses all 3 focal planes. Search for oxygen
compounds (CO2 and H2O) and new molecules.
MIRMAP 25-40Rs Mid-IR maps, centered at a latitude, with the long axis of
our arrays (Z) pointed toward the pole of Saturn to allow
the planet's rotation to map out a latitude band. 3 cm-1
resolution.
FIRMAP 15-25Rs Far-IR maps, constructed by slewing equatorward over a
hemisphere. Slews overlap and repeat to map out the entire
hemisphere. 15 cm-1 spectral resolution.
FTRACK <10Rs Campaign with VIMS and ISS to track features across the disk
from limb to limb, including limb sounding. FP1 nadir
sounding at 3 cm-1, FP3/4 limb sounding at 15 cm-1.
The goal is to study vertical structure and poleward
heat transport.
LIMBMAP <6Rs Vertical sounding at multiple latitudes on the limb at 15
cm-1.
LIMBINT <6Rs Long integrations for vertical composition at one location on
the limb. Resolution = 0.5 cm-1 or 1 cm-1 (TBD).
NADIROCC<10Rs Helium abundance measurements taken by following the Radio
Science Team's occultation points across the disk, 3 cm-1 res.
OCCLIMB <6Rs Independent verification of vertical temperature profile at
Radio Science's occultation latitudes.
REGMAP <10 Rs Regional mapping and/or composition feature tracks, to fill
COMPFT/ in coverage not obtained by the other observations (0.5 or 3
cm-1)
TEMPSIT Early temperature map. Res=3 cm-1. Sit on CML, FP3/4 N-S, let
planet rotate to cover all longitudes. Arrays cover Southern
Hemisphere (north blocked by rings).
MIRCMPSIT Early composition sit and stare. Res = 0.5 cm-1. Sit on
CML, FP3/4 N-S, Arrays cover Southern Hemisphere
(north blocked by rings).
FIRCMPSIT Early far-IR composition. Res = 0.5 cm-1. FP1 centered
on South Pole and on mid-Southern latitudes.
RINGS
Measurement of the infrared spectrum at varying phases, leading to
conclusions about ring particle size, shape, composition, distribution
and dynamics. Allocation of time for rings observing was carried out
by the Rings TWT, in conjunction with the Rings Working Group (RWG).
Ring observations are made as a function of ring opening angle, or
spacecraft elevation:
* Faint Ring Long Integrations. The low optical depths of the
faint D, E, F and G rings will pose particular observing challenges
for CIRS. These rings are best viewed edge-on because this geometry
enhances the instrument fill factor. Low spectral resolution of 15
cm-1 with FP1 provides the best signal-to-noise and should be
sufficient for detecting the variations of emissivity with wavelength,
which is our primary measurement goal. From close range (~ 10 RS) and
small opening angle, the FP1 filling factor will approach 1% when
pointed at the F rings ansa. Integrations of ~ 10 minutes should yield
usable signals. However, because the F ring is so clumpy, it needs to
be sampled at many longitudes before a truly representative spectrum
can be obtained. Observations will consist of alternating between both
ring ansas every ~30 minutes to achieve the most complete rotational
coverage of this ring. The E ring will be observed by pointing FP1
near the orbit of Enceladus, where the long edge-on line of sight
through the ring maximizes the fill factor. However, this fill factor
will still remain quite low, ~ 10-4, so , detecting the E ring will
require many, perhaps 100 or more, hours of integration. On the other
hand, because the ring is so thick vertically, the observing range can
be quite large (3040 RS). More observing time is available then during
these apoapse periods of the tour. The VIMS and UVIS instruments will
also require substantial integration on this ring, so E ring
observations will be cooperative activities between all of Cassinis
optical remote sensing instruments. Unfortunately, the best possible
fill factors for the remaining rings, D and G, are still lower than
for Ring E. It is unlikely that either will be detected with CIRS.
* Composition Integrations. CIRS will determine with unique
accuracy the ring spectrum between 50 and 1000 micron. As intimately
mixed contaminants significantly influence this part of the spectrum,
mixtures derived from the visible and near-infrared spectra will be
tested against this new spectrum. Spectra of the three main rings over
the full CIRS wavelength range will be obtained to determine possible
radial variations in the bulk composition. Two types of observations
will be made: high spectral resolution (0.5 cm-1) FP3 emission
measurements of the A, B and C rings, and high spectral resolution
transmission measurements of the rings with the rings against Saturn.
The former can be obtained from large ranges 2040 Rs because of FP3s
fine spatial resolution; long integrations of 1020 hours will be
obtained on representative locations in each ring. The transmission
measurements will be made from 20 Rs at relatively low ring opening
angles. This will allow a search for absorption features in the A and
C rings, and the Cassini division. The same region of Saturn will be
observed in at a similar spatial resolution when the rings are not
present, to establish the background. The transmission spectra will
be obtained over a series of emission angles.
* Stellar Occultations. A handful of stellar occultations are
observed by CIRS to directly obtain the ring opacity in the infrared.
Only a limited number targets are observable by CIRS, including CW Leo
and Eta Carinae. Eta Carinae occultations are only observable during
the final month of the tour. Occultations are observed in one FP3
pixel (CW Leo) or one FP4 pixel (Eta Carinae) at 15 cm-1 spectral
resolution.
* Radial scans. These scans are typically executed between 5 and
20 Rs over a range of spacecraft inclinations, from low (5 deg) to
highest possible inclination (75 deg), radial mapping (FP1, FP3) of the
rings, on both lit and unlit sides, over a range of spacecraft
elevations, inclinations, local times and phase angles, is performed
to obtain broadband radiometric measurements of the total flux in the
CIRS wavelength range. Sets of observations are obtained in each of
the inclined orbit intervals to map the temperature variation in the
rings with changing solar illumination. Two types of scans are
planned. So called temperature scans will consist of spectra at 15
cm-1 spectral resolution of the lit and unlit sides of the rings at
many incidence and emission angles and provide prime information on
the ring thermal gradient as a function of radial distance to
Saturn. Submillimeter scans will be made of spectra at 1 cm-1 spectral
resolution of the lit and unlit sides of the rings to map the thermal
characteristics and composition of the ring particles out to 1 mm.
* Azimuthal scans. These observations are executed between 5 and
20 Rs at spacecraft inclinations greater than 20 deg. They will be used
to study both the surface properties, the vertical dynamics and the
spin of ring particles. Observations of the cooling and heating of the
ring particles entering and emerging from the planetary shadow are
planned to derive particle thermal inertias for all three main
rings. It will make measurements at moderate radial resolution
(typically 1000 km) across the shadow boundaries at low spectral
resolution (15 cm-1) with the FP1 field of view. To constrain the
vertical dynamics of ring particles, the temperatures of the main
rings will be measured by CIRS along the ring azimuth of the main
rings, from the exit of the shadow (morning) to the evening ansa, both
on the unlit and unlit faces. This unique experiment will be realized
with spectra at low spectral resolution (15 cm-1). Spins create both
an azimuthal asymmetry in the ring temperature and a dependence of the
temperature with the emission angle, due to day/night
contrast. Circumferential scans at a variety of phase and emission
angles will be executed to detect azimuthal asymmetries and the
anisotropy in the ring particle emission function which are both
function of particles spin and thermal inertia. Occasionally, when
observing time is highly disputed, long azimuthal scans (8-to-20 hours
long depending on geometry and face) will be replaced by a series of
radial scans at different azimuths.
TITAN
Sensing of tropospheric and stratospheric temperatures and composition.
This includes abundances of the major and minor species, the hunt for new
gaseous species, isotope ratios for major species, and dynamcics.
CIRS may also be able to sense the surface near 600 cm-1. Allocation
of time for Titan observations was primarily done in the TOST group
(Titan Orbiter Science Team), in conjunction with recommendations from
the AWG.
CIRS can achieve different science goals at different distances from
Titan. Typically, CIRS makes the following requests (symmetric about
closest approach):
+ 0 to +10 mins HIRES surface mapping (e.g. slew over south pole).
+10 to +45 mins FIRLMBT - radial limb scans with FP1 to derive
temperatures in the 8--100 mbar region via the N2-N2
collision-induced absorption between 20--100 cm-1.
+45 to +75 mins FIRLMBAER - radial limb scans with FP1 to measure/
characterize particulate and condensate distributions,
abundances and properties.
+75 to +135 mins FIRLMBINT - integrate at two altitudes on the limb
with FP1 to search for signals of CO, H2O and new
species.
+2:25 hrs to +5 hrs FIRNADMAP - slow scan north-south or east-west on
the disk to sound tropospheric temperatures at 40--200
mbar, via the N2-N2 absorption,
OR,
slow scans at constant emission angle on the disk,
to retrieve surface temperatures in the presence of
aerosols around 520 cm-1.
+5 to +9 hrs MIRLMBMAP - map 1/4 limb using the FP3 and FP4 arrays,
to infer stratospheric temeratures via the 1304 cm-1
band of CH4. The arrays are placed perpendicular to
the limb at two altitudes, chosen to provide
overlapping coverage of the altitude range 150 to 420
km. The arrays are used in blink (ODD-EVEN)
mode. After mapping both altitudes, the arrays are
stepped 5 degrees in latitudes for the next step.
OR,
MIRLMBINT - as in MIRLMBMAP, except that only a
single latitude is covered, at two over-lapping
altitudes for 2 hrs in each position. To search for
and measure new species in the mid-IR: methyl, benzene
etc
+9 to +13 hrs FIRNADCMP - integrate on the disk at emission angle
approximately 60 degrees with the FP1 detector, in
order to measure spatial abundance distribution of
weak species and search for new species in the far-IR.
+13 to +22 hrs MIDIRTMAP - scan the entire visible disk with the
FP3/FP4 arrays perpendicular to the scan direction
('push-broom'), to measure stratospheric temperatures
via the CH4 v4 band. Used for later dynamical
analysis, for winds, waves etc.
+22 to +48 hrs COMPMAP or TEMPMAP - map a meridian across the planet
either E-W or N-S, using the Fp3/FP4 arrays in two
positions longwise. To search for new species, and/or
monitor temperatures.
ICY SATELLITES
Surface mapping in the IR, providing information on the surface composition,
morphology, and age. Close passes of icy satellites resulted in time
allocated to the SOST (Satellites Orbiter Science Team), which divided
the time between teams. There was also input from the Surfaces Working
Group (SWG).
During the Cassini tour, there are eight flybys of the classical icy
satellites targeted at 1000 km or less, as well as a number of
'Voyager-class' (less than 300,000 km) encounters. There are also
flybys of several much smaller satellites, such as Janus and
Epimetheus, at various distances. The dimensions of these objects
range from less than 100 km to as much as 1530 km (for Rhea).
Consequently, it is most useful to discuss CIRS observations in terms
of the angular diameter of the object, rather than its distance from
the spacecraft.
Normally the spacecraft orientation is controlled by momentum wheels,
which provide pointing accuracy and precision of ~2 mrad and ~0.04
mrad, respectively. Consequently, the full spatial resolution of the
FP3 and FP4 pixels (0.3 mrad) cannot be utilized with reasonable
confidence until the target exceeds 1 mrad in diameter. At that
point, the separation of focal planes 3 and 4 (0.9 mrad), and the
large size of FP1 (nominally 3.9 mrad) offer reasonable assurance of
obtaining useful data in either FP1 or at least one of FP3 and FP4.
As with the planet and the rings, the order of magnitude difference in
FOV scale between FP1 and the other focal planes plays heavily in the
design of the icy satellite observations. Approaching from a
distance, CIRS observations might proceed roughly in the following
order.
FP1INT, FP34INT: Compositional integrations (typically
performed at ranges where the target AD is 1-3 mrad). Because
spectral features of solid materials are generally broader
than those of molecular lines, these observations are made at
1-3 cm-1 resolution. Conducted in staring mode, using FP1 and
center detectors of FP3 and FP4, these are concentrated in
geometries with low phase angles, so as to view the icy
satellite surfaces at their warmest and maximize the
signal-to-noise ratio. The steep rolloff of intensity with
increasing wavenumber requires increasing integration time to
extend the spectrum. For a 100K surface and a spectral
resolution of 3 cm-1, a SNR of 10 at 600 cm-1 can be obtained
with 1 minute of integration, but extending the spectrum to
800 cm-1 with the same SNR requires integration of 100 minutes
Therefore, extending the spectrum to the highest wavenumbers
will involve coadding spectra over the entire tour.
SECLN, SECLX: Satellite eclipse entry (N) or exit (X)
observations (AD>1 mrad, phase angle phi <100 degrees,
spectral resolution delv = 15.5 cm-1; a
typical eclipse duration is ~2 hours). The high sensitivity
of FP1 permits observations of objects even when the FOV is
incompletely filled, or when the phase angle is moderately
high. At low phase angles, even FP3 can be used to follow the
initial portion of the cooling curve for relatively dark
objects; this allows observations with body-centric
spatial resolution better than 10o to be made when the
apparent target size exceeds ~3 mrad. Eclipses provide the
thermal inertia of the upper mm or so of the surface, as well
as estimates of surface coverage by relatively large fragments
of consolidated material.
FP1FAZ: Phase/longitude (diurnal cycle) coverage (AD>1 mrad,
delv=15.5 cm-1, 10-20 minutes). Focal plane 1 will be utilized to
determine the disk-averaged temperature of the satellites.
From the resulting diurnal behavior, mean thermal inertias in
the upper cm or so of the surface will be derived.
FP34FAZ, FP34MAP, FP34REG: Global thermal inertia mapping
and/or hot spot monitoring (AD>3 mrad, delv=15.5 cm-1). Maps
are made by slewing and rastering FP3 and FP4 across the disk
at rates not to exceed that for Nyquist sampling (16 microrad/sec
in blink mode); observation durations will typically be 10-30
minutes. These maps will be successful for varying portions
of a satellite, depending on its albedo and thermal inertia.
The exception is Enceladus, which is so cold that even the
subsolar regions will be barely detectable in an individual
measurement. In this case, mapping will serve to monitor the
satellite for ongoing endogenic activity; for example, active
sources at or above the NH3.H2O eutectic temperature will
be easily observed if they fill more than a few percent of an
FP3 or FP4 pixel.
FPGREEN: Search for a solid state greenhouse (AD>10 mrad,
phi <40 degrees, delv =15.5 cm-1, 15 minutes).
The decreasing absorption coefficient of water
ice with decreasing wavenumber in the
far-IR permits detection of radiation from increasingly far
below the surface, reaching as deep as 1 cm at 10 cm-1. Slow
east-west slews using FP1 allow following the daily
penetration of the thermal wave into the regolith.
FP1POLE: Polar night (annual cycle) coverage (AD>10 mrad,
delv=15.5 cm-1, 15 minutes). FP1 observations of the dark winter
polar region from high latitude permit determination of the
seasonal cooling curve. This enables an estimate of thermal
inertia in the upper tens of cm of the polar regolith.
FP1DAYMAP, FP1DRKMAP: Hemispheric FP1 mapping near closest
approach (AD>10 mrad, delv=15.5 cm-1). The high sensitivity of
FP1 permits rapid mapping of satellite disks near closest
approach, where time is at a premium. Nyquist-sampled maps at
low spectral resolution (slew rate 400 microrad/sec) will
typically take 10-30 minutes. These permit identification of
minor thermal anomalies, even on the night hemisphere.
Other TWT and WGs
The cross-discipline TWT (XD TWT) allocated time near apoapse on some
revs. The magnetospheres TWT allocated critical time for magnetospheric
observations, such as time near critical transition boundaries.
Instrument Overview
-------------------
CIRS consists of two interferometers; a far-IR polarizing interferometer
sensitive from 10-600 cm-1, and a mid-IR interferometer sensitive from 600
-1400 cm-1. The far-IR radiation is sensed by a large (~4 mrad diameter)
thermopile detector (actually two, one for reflected and one for
transmitted beam at the polarizer/analyzer), known as FP1. The mid-IR
radiation falls on two 1X10 arrays, known as focal planes 3 & 4. FP3
is an array of photoconductive detectors sensitive from 600-1100 cm-1,
and FP4 is an array of photovoltaic detectors seneitive from 1100-1400 cm-1.
FP2 was dropped from the original design.
Each of the 1X10 arrays has a 5-channel amplifier/signal processor, meaning
that only 5 of the 10 detectors in each array can be used at a time. The
maximum number of simultaneous interferograms from CIRS is therefore 11
(1 from FP1, 5 from FP4, 5 from FP4). However, there are observing modes
in which some of the focal planes are not used, leading to a lesser number
of active channels.
Almost the entire instrument is thermostated to 170 K, including the
primary and secondary mirrors, internal optics, scan mechanisms, and the FP1
focal plane assembly. The only exception is the FP3 and FP4 arrays (the
mid -IR FPA), which are cooled by a passive radiator to between 74 and 85K.
See the INST.CAT file for full details.
Calibration Overview
--------------------
Wavelength calibration is achieved by use of a reference diode laser
interferometer which uses the same scan mechanism as the IR
interferometer. Careful attention must be paid to the laser mode and
voltage which may drift/mode jump.
Radiometric calibration in the far-IR (FP1) is simplified due to the
fact that there is only one temperature inside the
instrument. Therefore, use of a deep space (2.7 K) reference target is
sufficient to calibrate radiance.
The fact that the mid-IR has two temperatures to contend with means
that a second reference target is required. Therefor, in the mid-IR
optical path, a shutter may be lowered for calibration purposes, which
gives a 170K reference body.
Calibration must be done using complex number algebra and fourier
transforms, to correctly account for phase changes which occur in the
beamsplitter. See the DATASIS.TXT for details.
Parameters
----------
The major parameters when observing with CIRS are as follows:
1) mid-IR shutter (open or closed). This is used for radiometric
calibration in the mid-IR, which requires two thermal reference
bodies (space and shutter).
2) mid-IR focal plane temperature set point (74-85 K). This was
usually set to the lowest level in the range at which the radiator
could cope with heating from the sun and/or Saturn/rings, and
therefore, achieve stability of FPA temperature.
3) FP3 pixel mode (ODD, EVEN, CENTER, PAIRS). Odd mode: detectors
1,3,5,7,9 active. Even mode: detectors 2,4,6,8,10 active. Center mode:
detectors 3,4,5,6,7 active. Pairs mode: 1&2, 3&4, 5&6, 7&8, 9&10
signals combined and amplified.
4) FP4 pixel mode (ODD, EVEN, CENTER, PAIRS). As FP3 modes, except
that detectors 4,5,6,7,8 used in CENTER mode, to spatially match the
detectors of FP3 which are numbered in reverse physical order.
5) co-add mode (COADD or NO-COADD). In co-add mode, two successive
scans are added together in the on-board electronics (buffers), to
reduce the data rate by half.
Data
----
This dataset is composed of CIRS Time Sequential Data Records,
and related calibration software.
READOUT, DOWNLINK AND DECOMPRESSION
CIRS data is normally read out at a rate of 4 kilobits per second
(kbs) and 8kbit packets are transfered via the Bus Interface Unit
(BIU) to the spacecraft Command Data Subsystem (CDS) every 2 seconds.
CIRS has many ways of reducing data rate, such as co-adding IFMs
(reduces rate by factor 2), dropping FP3 or FP4 readout (reduces by
45%), dropping FP3 and FP4 readout (reduces by 91%), or going to
housekeeping only mode.
Data packets are compressed in the instrument electronics, packetised
on the spacecraft, transmitted to the Earth via the Deep Space Network
(DSN) and then decompressed by software on the ground. At this stage,
the data goes into a processing pipeline, which organizes the science
data into binary tables and interpolates housekeeping
records. Pointing and genometry tables are also produced when the NAIF
data becomes available.
VANILLA PROGRAM FOR READING TABLES
The data is in the form of binary tables, also known as the
Vanilla database. Vanilla is a simple database access tool,
also included on each volume as both source and executable.
Note however that the Vanilla software is NOT REQUIRED in order
to read the binary data, but it may simplify the task. Notably,
Vanilla links together all binary files into a huge, `logical'
table, which can be searched based on key fields.
However, the user may read the binary data directly, using any
programming language desired. If this approach is taken it is
extremely important to pay attention to binary field widths:
field widths vary, and typically trying to read an entire record
into a data structure will fail: due to assumed padding of fields.
The user must read the data in field by field, specifying width.
Field widths are described in the *.FMT files, which are placed
in each data directory.
Some tables have both fixed-length and a variable-length fields.
In this case, the variable length part of the record is stored
separately, in a file with the same name but an extension .VAR,
instead of .DAT. The third type is file extension is .LBL, which
is the PDS detached label.
DATA LEVELS
The level-0 tables (base level) are contained in /DATA/UNCALIBR.
These are described in detail in the document DATASIS.PDF
(DATASIS.TXT). These tables types (IFGM, OBS, IHSK, FRV) are essentially
the raw-level information which came from the instrument: the
only process applied was unpacking (uncompressing) the instrument
packets and, in the case of the housekeeping data, interpolating
the 64-sec interval housekeeping data onto the science scan intervals.
Original housekeeping data is stored in /DATA/HSK_DATA, which is read
out at 64-sec intervals when the instrument is either ON or in SLEEP mode.
Pointing and geometry information is stored in /DATA/NAV_DATA.
These tables are described in detail in the DATASIS document, and
also in the FMT format files in the NAV_DATA area. Primarily, these
store the spacecraft attitude and position w.r.t. major bodies
(GEO file type), the position of the 11 detectors (99 q-points,
or fiducial reference marks) on all bodies in the FOV (POI file
type), and the list of bodies seen in each detector (TAR type).
Pointing and geometry information is derived from the spacecraft
re-constructed attitude ('C') and trajectory ('SPK') kernels, using
NAIF (Navigation and Ancillary Information Facility) toolkit routines
supplied by JPL.
Calibrated spectra are stored in two variations: apodised and
unapodised. These are stored in /DATA/APODSPEC and /DATA/UNAPSPEC
respectively. Apodisation is the process of mathematical
filtering or smoothing, which removes `ringing' effects from
finite-width mathematical FFTs, widening the instrument line
shape in the process. Either form is valid for science, depending
on personal preference. Apodization will normally use the Hamming
function window, but other types may be used and the type will be
given in the tables.
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