Instrument Information
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| IDENTIFIER |
urn:nasa:pds:context:instrument:mcs.mro::1.1
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| NAME |
MARS CLIMATE SOUNDER
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| TYPE |
SPECTROMETER
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| DESCRIPTION |
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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D. McCleese, J. Schofield, F. Taylor, S. Calcutt, M. Foote, D. Kass, C. Leovy, D. Paige, P. Read, and R. Zurek, 'Mars Climate Sounder: An Investigation of Thermal and Water Vapor Structure, Dust and Condensate Distribution in the Atmosphere, and Energy Balance of the Polar Regions', Journal of Geophysical Research, TBD
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