| INSTRUMENT_DESC |
Instrument Overview
===================
Introduction
------------
The Mars Climate Sounder (MCS) represents NASA's third attempt to
observe Martian weather and monitor the planet's climate using
modern infrared remote sensing techniques, including limb sounding.
The campaign began in 1986 with the Pressure Modulator Infrared
Radiometer (PMIRR) [MCCLEESEETAL1986; MCCLEESEETAL1992] on the Mars
Observer mission, which was lost in 1993 during the final
pressurization of the spacecraft propulsion system only a few days
prior to Mars orbit insertion (MOI). NASA re-selected that
scientific investigation, including an enlarged science team and
using the same PMIRR instrument design, for another mission, the
Mars Climate Orbiter (MCO). Launched in 1998, the MCO spacecraft
entered the Martian atmosphere during MOI and was destroyed. NASA
selected the same team once more to build the new MCS instrument
now onboard the Mars Reconnaissance Orbiter (MRO). MCS is not a
copy of the PMIRR instrument launched in 1992, as the 1998 PMIRR II
was, but is a smaller, lighter instrument with the same scientific
objectives as those of the previous two investigations.
The Mars Climate Sounder is the first custom designed infrared
sounder for systematic meteorological measurements at Mars, using
the limb viewing technique to obtain relatively high vertical
resolution of better than one pressure scale height.
The application of modern technology and the use of new high
performance, linear array thermopile detectors with focal plane
signal processing and miniature multilayer interference and mesh
filters has permitted a fivefold reduction in mass, in part by the
elimination of the requirement for cryogenic operation of the focal
plane.
The compact dimensions of the design allow the entire instrument to
be pointed to limb or nadir, for atmospheric and surface mapping
respectively, and special patterns to be implemented, for example
for mapping over the poles where the complex physics of the energy
balance and atmospheric response during the long winter night is to
be addressed.
Mars Climate Sounder Investigation
----------------------------------
MCS is an investigation designed to acquire the high vertical
resolution, horizontally contiguous measurements needed to take the
next major step forward in understanding the Martian atmosphere.
MCS will measure thermal emission from the limb of the atmosphere
in nine spectral bands to obtain profiles with a vertical
resolution of 5 km (approximately one half the atmospheric scale-
height on Mars). The MCS investigation will invert accurately
calibrated measurements of radiance in profiles extending from the
surface to an altitude of 80 km to obtain vertical profiles of
atmospheric temperature, water vapor abundance, and dust and
condensate opacities. MCS will also acquire nadir and off-nadir
observations that provide additional constraints on atmospheric
profiles, and measurements of infrared thermal emission and
broadband solar reflectance of the surface. In the polar regions,
more frequent nadir and off-nadir measurements will be used to
characterize the radiation balance at the top of the atmosphere,
which provides constraints on the rates of condensation and
sublimation of CO2 on the surface and in the atmosphere. Table 1
is a high-level description of the instrument observation modes
and the objectives that they address.
Table 1. MCS Observation Modes and Science Objectives
Observation Mode: In Track Limb Staring
Measurement Objective: Vertical profiles of temperature, pressure,
aerosols, and water vapor
Science Objective: Global monitoring of atmospheric properties
High-Level Science Goal: Atmospheric circulation, interannual climate
variations, annual water, and dust cycles
Observation Mode: Nadir Sounding
Measurement Objective: Surface infrared radiance and broadband solar
reflectance
Science Objective: Characterization of surface and sub-surface
thermal state and thermal properties
High-Level Science Goal: Climate, distribution, and state of surface
and subsurface water
Observation Mode: Polar Buckshot Scanning
Measurement Objective: Net polar radiative balance
Science Objective: Annual carbon dioxide frost budget
High-Level Science Goal: Interannual and secular climate variations,
and global change
Investigation Description
-------------------------
In 1999, following the failure of the MCO spacecraft,
a major redesign of the instrument was undertaken by the
science team. The result is a much-reduced instrument mass,
a smaller footprint on the payload deck, fewer mechanisms,
and lower power requirements. PMIRR was a 44-kg instrument
consuming 40 W. MCS weighs 9 kg and consumes
11 W of continuous power.
The availability of new high performance, linear array thermopile
detectors with focal plane signal processing [FOOTEETAL2003] was
the key that made possible a much-simplified design for MCS without
compromising performance.
Measurement Approach
--------------------
MCS employs a nearly continuous limb viewing strategy in order to
achieve greatly increased sensitivity to minor and trace
constituents, the atmospheric limb path length being approximately
50 times that of a nadir view on Mars. The vertical resolution is
also improved compared with the vertical resolution achievable by
nadir viewing instruments. Martian targets of interest to the MCS
investigation are typically low radiance, such as the cold
low emittance atmosphere and low temperature surfaces (<150 K in
the polar night). This is another factor that led to the
selection of a radiometer design that favors large energy grasp
over spectral resolution. MCS measurements are made in nine
spectral intervals with 20 cm-1 and broader spectral passbands in
the range 0.3 - 45 um; see Table 2.
Table 2. MCS Spectral Channel Band Passes and Measurement
Functions
Telescope/ Band Pass Band Center Measurement
Channel # (cm^-1) (um) Function
------------------------------------------------------------------------
A1 595-615 16.5 temperature 20 to 40 km
A2 615-645 15.9 temperature 40 to 80 km and pressure
A3 635-665 15.4 temperature 40 to 80 km and pressure
A4 820-870 11.8 dust and condensate (D&C) extinction
0 to 80 km
A5 400-500 22.2 temperature 0 to 20 km, D&C extinction
0 to 80 km
A6 3300-33000 1.65 polar radiative balance
B1 290-340 31.7 temperature 0 to 20 km and D&C extinction
0 to 80 km
B2 220-260 41.7 water vapor 0 to 40 km and D&C extinction
0 to 80 km
B3 230-245 42.1 water vapor 0 to 40 km and D&C extinction
0 to 80 km
Instrument Specifications
=========================
Overview
--------
The instrument consists of an optical bench assembly (OBA),
containing the optics bench, telescopes, and analog and digital
electronics boards, suspended from a yoke via the elevation
actuator. The yoke supports the instrument, accommodates instrument
calibration targets and power conditioning and drive electronics,
and provides the mechanical and electrical interface with the
spacecraft via a twist-capsule mounted on the azimuth actuator. A
similar twist capsule, mounted on the elevation actuator assembly,
conveys power and signals between the yoke and OBA electronics.
Table 3 summarizes the primary physical and operational parameters
of the instrument.
Table 3. MCS Instrument Specifications
Instrument type: Filter radiometer
Spectral range and channels: 0.3 to 45.0 um in nine spectral channels
Telescopes: two identical, 4 cm aperture,
f/1.7 telescopes
Detectors: nine arrays near 290 K
Fields of view: Detector IFOV:
3.3 x 6.2 mrad
4.4 x 8.2 km (at limb)
Instrument IFOV:
75 x 75 mrad
105 x 105 km (at limb)
Instrument articulation: two-axis azimuth/elevation
range/resolution:
Azimuth: 270/0.1 degrees
Elevation: 270/0.1 degrees
Operation modes: Single Operating Mode,
2.048s signal integration period
Observation strategy: Limb Staring; limb, nadir and off-nadir
The entire MCS optical bench is articulated in two axes in order
that the limb as well as the nadir can be observed. The range of
articulation is 270 degrees in both azimuth and elevation axes
providing views of the full hemisphere beneath the nadir-pointed
payload platform.
Each telescope consists of mirrors and a focal plane assembly
supported by a metering structure, which is suspended within the
instrument optical bench. The two identical MCS telescopes are
differentiated primarily by their focal planes. Telescope A has a
focal plane with six spectral channels, labeled A1 through A6 in
Table 2. These visible and mid-IR channels are defined spectrally
by optical interference filters mounted over six, 21-element linear
thermopile detector arrays. The telescope B focal plane has three
far-infrared spectral channels, defined spectrally by conductive
mesh filters mounted over three detector arrays.
Each MCS focal plane assembly consists of a detector chip, focal
plane signal processing chips, and a bandpass filter assembly, all
mounted on a focal plane block.
Instrument Optical Design / Telescopes
--------------------------------------
The telescopes are of identical design. Both are off-axis, all-
reflective, telecentric with 4 cm apertures, FOVs of 4.3 degrees,
and comprising three mirrors. The telescopes are mounted with
parallel optical axes; rotated 180 degrees relative to each other
about their prime axes. The instrument aperture is
defined by the perimeter of the secondary mirror and the tertiary
mirror focuses an f/1.7 beam through band-pass filters and baffles
on to the focal plane detectors. The baffles, mounted close to the
back surfaces of the filters, act as light pipes for the detectors
and define the FOV response of the individual detector elements.
Limb-sounding atmospheric measurements with the resolution required
by MCS place fairly modest requirements on image quality but are
strongly influenced by the far wings of the FOV response in the
vertical direction. Focal plane baffles reduce FOV response wings
produced by scattering within the filters and by thermal radiation
re-emitted from the detectors. Also, the secondary mirror is
apodized at the edges, using deposited gold black, to reduce FOV
wings caused by diffraction at the longer wavelengths.
The telescope mirrors are diamond turned from nickel-plated
aluminum and post-polished to reduce scattering at visible
wavelengths. The reflective surface is a silicon dioxide-protected
aluminum coating, chosen for its reflectivity at both visible and
infrared wavelengths and its robustness. The aluminum metering
structure supports the mirrors and its internal surfaces form
baffles that shield the space between the tertiary mirror and the
focal plane. An external baffle, mounted on the optical bench,
prevents stray light reaching the focal plane after only one
reflection. All the exposed internal surfaces of the metering
structure and external baffle are treated with Martin Black to
reduce scattering. Alignment tolerances for the MCS telescope
permit a 'snap together' design, although provisions are made for
shimming the primary mirror and focal plane. In practice, only the
focal plane is adjusted to achieve best focus.
Because MCS is an unchopped radiometer, considerable care is taken
to minimize thermal drifts in the telescope and focal plane
assembly. The primary source of variable radiant energy that will
tend to destabilize the instrument temperature is exposure, once
per orbit, to the large day-night contrast in solar illumination
and Martian surface temperature. High thermal stability is achieved
in MCS by maximizing the thermal mass of the telescope, maximizing
the thermal conductivity between its component parts, and
conductively isolating the telescopes from the surrounding optical
bench, which sees most of the variable environmental heat load from
the instrument thermal blankets and external baffles. The MCS
telescopes are each mounted at three points to the optical bench
via struts with flextures. Conductive isolation between the struts
and optical bench is provided by fiberglass (G10) washers.
Temperature control is applied to the optical bench, which acts as
a stable radiative 'oven' for the telescopes and focal planes.
The stable thermal environment, all aluminum mirrors and metering
structure, and flextured interfaces with the optical bench and
focal plane assembly also minimize thermal distortion in telescope
alignment.
Focal Plane Filters
-------------------
The spectral passbands of MCS are defined using individual spectral
filters mounted in metal frames in front of each of the 9 detector
arrays. For telescope A, the five mid-infrared filters in the focal
plane cover the 11.8- to 22.2-um wavelength range (channels A1
through A5). These are multilayer interference filters manufactured
by the University of Reading, UK. Germanium was the substrate
material of choice for the five mid-infrared filters (A1 through
A5) because of its robust mechanical properties and high optical
transmission over the full spectral range. The germanium absorption
band near 20 um was not a major concern because the substrates are
very thin. The multilayer filters are of conventional design but
they are unusually small and five filters must fit directly over
the MCS focal plane. Small filters are cut from a larger substrate.
Good yield was achieved with this approach with little chipping or
other damage to the substrates or coatings. The broadband 0.3 to 3
um filter, A6, is made of UV22 glass, chosen for its blocking from
3 to 5 um.
The three far infrared filters in focal plane B consist of stacks
of mesh filters developed by Cardiff University. The same group
successfully produced air spaced mesh filters for the PMIRR
instruments. Cardiff produced a 'hot pressed' version of their
filters consisting of copper meshes on polypropylene substrates,
which can be cut to fit the smaller MCS filter frames.
Thermal stability of all the spectral filters is critical. The MCS
thermopile arrays respond to radiation at all wavelengths where
their surfaces absorb, i.e., where gold black has low reflectivity.
Small excursions in the temperature of the filters will produce
measurable signals, because the filters themselves are sources of
blackbody radiation at all wavelengths away from their narrow
spectral windows. Temperature changes in the filters due to the
changing scene, e.g., Mars and the limb of the atmosphere, are
potentially large enough to cause a measurable, but spurious,
effect. Note that the filters fill nearly the whole 2pi FOV of the
detector. A change in the temperature of 5 mK produces a spurious
signal equivalent to a 1% true signal change in channel A3.
Consequently, it is essential that the filters be mounted in good
thermal contact with a large thermal mass.
The filter assemblies throughout the instrument incorporate
aluminum alloy baffles located between the detectors and the
filters to reduce the transmission of offaxis rays produced from
reflections within the assemblies and by reflection and emission
from the detector surfaces. These baffles, produced using EDM
(Electrical Discharge Machining), have the same external dimensions
as the detector arrays with individual cell walls surrounding
each of the individual detectors. The baffles have an anodized
coating with low large angle reflectivity that preferentially
reduces off-axis reflections.
Detectors and Signal Processing
-------------------------------
The MCS focal plane assembly A (FPA-A) comprises six arrays with
two front-end signal read-out chips and FPA-B three arrays with one
chip. The detector arrays are micro-machined thermopile arrays
using Bi-Sb-Te and Bi-Te thermoelectric materials. Each pixel
consists of a silicon-nitride membrane with twelve Bi-Sb-Te/Bi-Te
thermocouples connected in series. These thermocouples measure the
temperature difference between the thermally isolated absorber and
the substrate. The detectors are coated with gold black, providing
high absorptivity and nearly flat spectral response from 0.3 to 45
um. During deposition, the gold black forms bridges across the
membrane slits, thermally shorting detectors to each other and to
the substrate. These unwanted gold black bridges are eliminated by
laser ablation from the detector backside with 248 nm excimer laser
radiation. The detector membrane acts a mask during this ablation
process to confine the laser energy to the gold black bridges. It
is necessary to remove the few remaining gold black bridges using a
focused ion beam.
The MCS observation cycle consists of two-second signal integration
intervals, interspersed every 34 seconds with instrument views of
space. This corresponds to a 0.03 to 0.25 Hz frequency range. Thus
low 1/f noise in both the detectors and the readout circuitry is
essential. Thermopile detectors intrinsically have low 1/f noise
because when read out with high-input-impedance voltage amplifiers
they exhibit negligible current flow. Each group of 64 detector
pixels is read out with a custom CMOS readout integrated circuit. A
readout chip is connected to each thermopile array with two wire
bonds per detector pixel. The roughly DC signal from each pixel is
modulated at 64 kHz by an electronic chopping circuit. The
resulting AC signal is amplified, demodulated, and integrated.
Because amplification occurs at 64 kHz rather than near DC, the 1/f
noise in the CMOS amplifier is dramatically reduced. Integrated
signals from 64 channels are multiplexed into a single analog
output stream.
Thermopile detector noise is dominated by Johnson noise, which, for
the 100kOhm MCS detectors, is 40 nV/(Hz^1/2). With a 100 kOhm
source, the readout chip has an input-referred noise of 70
nV/(Hz^1/2) for 0.03 to 0.25 Hz. Thus the readout chip is the
dominant focal plane noise source. Even with this readout noise,
the MCS focal planes demonstrate D* values of 8 x 10**8
(cmHz^1/2)/W for 0.03 to 0.25 Hz.
Mechanisms
----------
The instrument FOV can be directed over the full operating range of
270 degrees in azimuth and elevation by stepper motor actuators.
Twist capsule assemblies for each axis contain the stepper motors,
gearing, position sensing diodes, and flexible leads.
Both motors are brushless stepper motors with a step size of 30
degrees, geared down by a factor of 297:1 by planetary gears and
harmonic drives in series. The resulting step size granularity for
instrument rotation is 0.1 degrees, or half the vertical instrument
FOV at the limb of Mars, which corresponds to roughly one quarter
of the atmospheric scale height (the scale-height is ~10 km).
Encoders are not used. Instead, diodes coupled with drive phase
information define two reference positions and software checks for
inconsistencies between commanded and reference positions. If
inconsistencies are detected, motor positions are re-initialized.
Flexible leads within the twist capsule assemblies supply data,
signal, and power connections between the OBA, the yoke, and the
spacecraft across the moving interfaces.
The motors are driven open loop by electronics in the yoke
controlled by software. In order to minimize mean torque and
jitter, and the possibility of induced disturbance of the
spacecraft, instrument slews are divided into constant
acceleration, constant velocity, and constant deceleration phases.
The drive waveforms are micro-stepped to smooth the low frequency
jitter produced by individual steps. Drive voltage and phase
information that can be used to generate slews are stored in
uploaded azimuth and elevation slew tables in MCS memory. The slew
tables specify accelerations and decelerations of 25 degrees/s/s in
azimuth and 42 degrees/s/s in elevation with a top speed of 26.5
degrees/s in both axes.
A complete flight spare actuator assembly was used to perform
accelerated life testing of the MCS actuators over 3.5 x 10**6
measurement cycles, or 5.2 x 10**8 motor revolutions. For the
purposes of the test, a cycle consists of two opposite 90 degree
slews. This represents a factor of two margin over the total
anticipated elevation actuator motions during the nominal MRO
mission of one Mars year. The life test was made more conservative
by combining the higher number of elevation actuator cycles with
the larger azimuth actuator mechanical load at cold (-5 degrees C),
warm (20 degrees C), and hot (50 degrees C) operating temperature
plateaus. The life test finished successfully in March 2006,
Electronics and Software
------------------------
The main elements of the MCS electronics are the analog, control,
and power boards. The analog board accommodates analog signal
processing electronics and the control board contains digital
controller electronics. These boards are mounted on opposite sides
of the OBA to minimize mutual interference. The power boards are
mounted on the yoke, to isolate them from the other boards, and
accommodate power conditioning, actuator driver, and spacecraft
interface electronics.
The core of the MCS electronics is the digital controller,
comprising a processor coupled with 32 kBytes of RAM, 32 kBytes of
ROM and a FPGA. Its primary functions are as follows:
1. Control focal plane readout integrated circuits and digitize,
collect, and process signals.
2. Provide set-points and variable heater power for the optical
bench and blackbody targets.
3. Control housekeeping data sampling, collection, and process
housekeeping data.
4. Control the actuators.
5. Receive and process spacecraft commands.
6. Generate telemetry packets and transmit them to the spacecraft.
7. Provide DC/DC converter synchronization and the 31.25 Hz real-
time clock.
The focal plane interface electronics digitizes the multiplexed
integrated analog signals from the three 64-channel readout
integrated circuit chips on the two MCS focal planes. All 192
signals are sampled at 1 kHz, and 2048 samples are digitally
integrated by the FPGA resulting in an effective signal integration
period of 2.048 seconds.
Instrument Nomenclature
=======================
Instrument Id : MCS
Instrument Host Id : MRO
Pi Pds User Id : UNK
Instrument Name : MARS CLIMATE SOUNDER
Instrument Type : ATMOSPHERIC PROFILER
Build Date : UNK
Instrument Mass : UNK
Instrument Weight : 9 kg
Instrument Length : UNK
Instrument Width : UNK
Instrument Height : UNK
Instrument Power Requirements : 11 W of continuous power
Instrument Manufacturer Name : UNK
Science Objectives
==================
The scientific goals of the MCS investigation are focused on defining
and understanding the current and past climate of Mars. They include:
(1) Global monitoring of atmospheric properties,
(2) Characterization of the surface and subsurface thermal state
and thermal properties,
(3) Monitoring the annual carbon dioxide frost budget
The Mars Climate Sounder observes the temperature, humidity, and
dust content of the martian atmosphere, making measurements that
are needed to understand Mars' current weather and climate, as
well as potential variations that may occur.
Scientists will use these measurements to understand how the
martian atmosphere circulates and varies over time. The
measurements will also help explain how and why the martian polar
caps vary in response to the atmosphere and the energy input from
the Sun.
Calibration
===========
In-Flight Calibration
---------------------
MCS performs a two-point in-flight radiometric calibration of its
signals, to correct for the effects of thermal drifts, using views
of space and of calibration targets mounted on the instrument yoke.
Views of space, accomplished by using the elevation actuator to
slew the instrument above the limb, provide a zero reference for
all MCS spectral channels. Space views are obtained at 34-second
intervals during standard limb viewing observations.
A second calibration point is supplied for the infrared channels by
rotating the instrument so that the blackbody target in the yoke
fills the aperture and FOV of both telescopes. The telescopes are
directed toward the target approximately 10 times per orbit using
the elevation actuator. In order to meet the MCS absolute
radiometric calibration requirement of better than +/- 0.5% at 300 K,
the target must be thermally uniform, and its temperature and
emissivity known with an accuracy of better than +/- 0.25 K and
+/- 0.0035, respectively. The required temperature and emissivity
accuracies are met using a grooved aluminum plate, blackened by the
Martin Black anodize process. Target temperature is monitored by
two platinum resistance thermometers embedded at the center of each
viewed area of the plate. One sensor is used to control the target
temperature via a distributed heating element on the rear surface
of the target. MCS is designed so that the blackbody target is
mounted very close to the telescope apertures. Controlling the
target temperature within a few degrees of the telescope
temperature compensates, in part, for non-zero reflectivity of the
target surface.
For the visible channel, a second calibration point is provided by
the solar target that fills the aperture of telescope A. Telescope
A is rotated to view the solar target twice per orbit as the sun
rises or sets over the limb of Mars. The sun is at an angle of
roughly 15 degrees below the local horizontal for these
measurements. The solar target diffusively scatters solar radiation
into the telescope. In order to meet an absolute radiometric
calibration requirement of better than +/- 3%, the target
reflectivity must be nearly Lambertian. The target was
characterized before launch over the expected range of viewing and
illumination angles. It is also critical that the target surface
reflectivity not change significantly over the lifetime of the
mission. A textured aluminum target plate is suspended within and
thermally isolated from a supporting base structure. A means of
self calibration of the target is included in its design. In the
configuration used here, the target and base form a calorimeter.
Monitoring the target and base temperatures when the target is
heated by the sun provides a direct indication of any changes in
reflectivity during the mission. Corrections can then be applied to
the instrument calibration measurements.
During spacecraft system thermal vacuum testing, the MCS solar
target was exposed to potential contamination by organics. This
raised the question of whether the target might change, most
probably darken, when exposed to solar UV radiation in space. The
MRO spacecraft was configured during cruise to enable the MCS team
to conduct two previously unscheduled tests of the solar target.
These tests permitted a two-point intercomparison of target
reflectivity after a total of 40-50 hours of exposure to the sun.
Analysis of the test data demonstrated that any change in target
reflectance is less than 0.1%.
Post-Launch Calibration
-----------------------
Limited testing of MCS has been performed in Mars orbit. On 25
March 2006, near apoapsis of the elliptical capture orbit,
tables were up loaded and the instrument scanned Mars. The
primary purpose of collecting scans of the planet was to
demonstrate the capability of the MCS to support aerobraking
planning, should it be needed. Over a period of half an hour,
the MCS detector arrays were scanned across the planet,
producing a raster scan of the planet. These data are also being
used by the team to validate the ground calibration of FOVs.
Pre-Launch Calibration
-----------------------
MCS was operated in a stand-alone configuration under laboratory
thermal-vacuum conditions for approximately 500 hours. During
this time, extensive pre-flight calibration was
performed. Radiometric calibration was accomplished using
external high-emissivity blackbody targets controlled and
monitored at temperatures extending over the full range of
Martian conditions, as well as simulations of deep
space. Detector linearity was measured for all detectors. The
internal instrument blackbody target was calibrated. Channel
electronic gain and offsets were established. The results of
analyzing the calibration observations are then used, with the
flight calibration observations, to convert the detector counts
(EDR values) into the radiances (RDR values).
The FOVs of all detectors were measured using a target
projector, and their spectral response was determined using a
monochrometer, both within the thermal vacuum test chamber.
Operational Strategy / Considerations
=====================================
The observation strategy employed by MCS performs uninterrupted
repetitive measurements over the life of the mission. Minimizing gaps
in limb sounding is consistent with our objective to accumulate a
climatology of Mars. MCS performs repetitive observations
autonomously, utilizing an internal table-drive scheme that is
loosely keyed to the spacecraft's latitude. The planet's aspheric
shape and MRO orbital eccentricity are sufficient to require
latitude dependent, although not longitudinal, adjustments to MCS
pointing for limb-staring observations.
The near-polar orbit enables MCS to use the descending (night side)
equator crossing to synchronize its observations with spacecraft
latitude. Ephemeris routines onboard the spacecraft are used to
generate a command containing equator-crossing time that is
transmitted to MCS every orbit. The acceptable error in equator-
crossing time is 10 seconds. If equator-crossing time is not received
by MCS, the instrument extrapolates from earlier equator crossing
times.
Articulation of MCS, and thus its pointing direction, is controlled
by three sets of nested tables through which instrument software
loops repetitively. Running at a top level is the Orbit Schedule
Table (OST) that determines which of three Event Schedule Tables
(EST) is to be used from orbit to orbit. The EST controls all
activities over the course of an orbit and contains a list of calls
to Scan Sequence Tables (SST) ordered by time relative to the last
equator crossing. An SST describes a small, frequently used scan
pattern, such as a limb view or a calibration cycle, and consists of
a list of scanning instructions including which axis to scan, where
to go, and how many soundings to acquire at that destination. When an
SST completes, software determines whether to call it again or move
on to the next SST, based on time since the last equator crossing in
the EST. Each SST normally executes to completion so that some orbit-
to-orbit variability in timing is introduced. Two other tables also
modify observations round the orbit. An Elevation and Oblateness
Correction Table (EOCT) contains a list of elevation step
perturbations, again ordered by time relative to the last equator
crossing. This table allows selected SSTs, such as a limb view, to be
perturbed to track the limb by correcting for the MRO orbit and the
oblateness of Mars. Finally, a Radio Occultation Table (ROT) contains
the time and geometrical information needed to perturb a limb view so
that it views the limb where and when the Earth is rising or setting
through it.
Default scan tables retained in MCS memory are utilized by instrument
software if alternates are not uploaded following instrument turn on.
Default SSTs enable limb viewing, with approximations for orbit
eccentricity, to retrieve vertical profiles of temperature, dust,
water, and condensates. These default tables also include SSTs unique
to the polar regions where MCS performs so-called 'buckshot'
observations: varying the angle of observation of the upward going
radiation. Prescribed 'buckshot' patterns provide optimized
coverage of the bi-directional reflectance function (BDRF) of the
polar caps for determining the polar energy balance in the north and
south. MCS also performs limb observations in the polar regions to
provide nearly continuous atmospheric profiling. Default scan tables
can be replaced by uploads from Earth to accommodate orbit changes,
seasonal variations, changes in observational strategies, and
correlative observations with other instruments or the MRO
spacecraft. Table loads have already been used in flight to
facilitate MCS checkouts during the cruise, including performing
solar target calibration tests and characterizing potential
interactions among payload elements, e.g., instrument motion-induced
disturbances.
A nadir-fixed platform is preferred by MCS. However, other elements
of the MRO payload require the spacecraft to roll its nadir pointing
orientation by as much as 30 degrees. Roll angles of less than 9
degrees can be accommodated using the two detector margin at the top
and bottom of the vertical profile. However, when the angle of
spacecraft roll exceeds about 9 degrees, the MCS detector arrays no
longer cover the entire vertical range of atmosphere (0 to 80 km) in
the forward limb view. Consequently, the MRO Project Science Group
(the formal group comprising the payload Principal Investigators) has
undertaken to limit requests for large rolls greater than 9 degrees.
This compromise is possible because, with planning, most surface
targets can be viewed within 30 days using spacecraft roll angles
less than 9 degrees. Were these practices not acceptable to the other
MRO instrument teams, more frequent gores in MCS data would have
seriously compromised the climatology record.
These observations will disrupt the continuity of MCS measurements
over a period of 15 minutes, or 48 degrees in latitude. To mitigate
this problem and reduce the number and extent of incomplete vertical
profiles, MCS can be commanded to react to rolls larger than 9
degrees by changing its nominal observing pattern. In these cases,
MCS alternates its FOV between two positions that together overlap
covering the desired 0- to 80-km altitude range. Data taking will
proceed through all roll events and calibrated radiances are flagged
in the archive (column 10 in the EDR and column 26 in the RDR).
In addition to disruptions due to spacecraft rolls, the HiRISE
instrument needs a quiet platform from which to obtain its
highest-resolution images. Ground testing indicates that MCS-induced
spacecraft jitter exceeds permitted levels when either azimuth or
elevation actuators are operated. Therefore, through a signal sent
from the spacecraft to MCS the instrument will be commanded to
'Freeze' for the duration of high-resolution imaging. In Freeze mode,
MCS moves to view the limb and then remains stationary for 60-90
seconds when it is then commanded to return to routine
activities. These periods are flagged in the archive in columns 8
and 9 of the EDR or columns 24 and 25 of the RDR.
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