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.