PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = "P. Christensen, 1999-02-28" RECORD_TYPE = STREAM OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = MGS INSTRUMENT_ID = TES OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "THERMAL EMISSION SPECTROMETER" INSTRUMENT_TYPE = "THERMAL INFRARED SPECTROMETER" INSTRUMENT_DESC = " Instrument Overview =================== The TES instrument consists of three sub-sections, the primary one being a Michelson interferometer that produces spectra from 1700 to 200 cm-1 (~6 to 50 microns), at a spectral sampling of either ~5 or ~10 cm-1 [CHRISTENSENETAL1992]. The instrument cycle time, including collection of the interferogram, mirror flyback, and electronic reset, is 2 sec for 10 cm-1 (single scan) operation, and 4 sec for 5 cm-1 (double scan) operation. The interferometer includes a visible interferometer that is used to generate fringes which are used to control the linear drive servo and to determine position in the interferogram. This system uses two redundant neon lamps that produce an emission line at 703.2 nm for fringe generation and a continuum that is used for a quasi-white-light source for determination of zero path difference. The finite size and off-axis position of the six detectors results in self-apodization and a spectral shift that is a function of both distance from the axis and optical frequency. The resulting full-width half-maximum (FWHM) value is ~12.5 cm-1 for 10 cm-1 sampling at 200 cm-1 and 15.4 cm-1 at 1650 cm-1. For the corner detectors and at the highest frequency (shortest wavelength) there is a significant departure from the ideal, with a worst-case degradation to a FWHM of ~24 cm-1. Because all of the response functions have the same area there is no loss in signal when viewing a smooth continuum scene like Mars. However, there will be a slight loss in contrast of narrow spectral features due to broadening of the spectral width. Because the self-apodization is considerable, the data will generally be used without further apodization. Separate fast fourier transform (FFT) algorithms are used for the center and edge detectors in order to partially correct for the different spectral shifts introduced into these detectors. As a result, the data generated by the two FFTs will have approximately the same frequency sampling. Table 1 TES Performance Characteristics Parameter Expected Performance -------------------------------------------------------------------- NEDe in spectrometer channels 0.002 @ 270 K and 10 micron NEDT in spectrometer channels 0.04K @ 270 K and 10 micron NEDr in solar reflectance channel 0.1% of solar flux NEDT in thermal bolometric channel 0.1K @ 270K Spectral resolution 5, 10 cm-1 Spectral range 200 to ~1600 cm-1 Spatial resolution 3 km (NEDT is the noise-equivalent delta temperature) Table 2 Design Parameters for the Thermal Emission Spectrometer Spectral Range -------------------------------------------------------------------- interferometer 200 to ~1600 cm-1 (6.0 -50 micron) radiometer 5.5 to 100 micron and 0.3-2.7 micron Spectral resolution of 10 cm-1 and 5 cm-1 interferometer Field of view (FOV) 16.6 mrad downtrack, 24.9 mrad crosstrack Instantaneous Field of View 8.3 mrad square (IFOV) Telescope Aperture interferometer 15.2 cm diameter Cassegrain radiometer 1.5 cm diameter off-axis reflecting Pointing mirror range 90 forward, 90 aft step size ~ 0.25 mrad Detectors uncooled deuterated triglycine sulfate (DTGS) pyroelectric spectrometer channel: 6-element array; each 1.75 mm diam NEP = 3.01x10-11 W-Hz-1/2 responsivity = 1000 V/W bolometer channels: 6-element array; each 1mm x 1mm NEP = 2 x10-11 W-Hz1/2 responsivity = 1000 V/W Michelson mirror travel +- 0.25 mm and +- 0.50 mm Mirror velocity 0.0295 cm/sec Neon fringe reference 703.2 nm wavelength Sample rate 839 samples/sec/detector Cycle time per measurement 2 sec and 4 sec Number of samples per 1344 interferogram Number of bits per sample 16 Number of spectral samples 143, 286 Number of bits per spectral 12 sample Data bit rates 668, 1664, 4992 bits/sec Size 21.08 x 34.52 x 39.85 cm Mass 14.47 kg Power 10.6 Watts (ave.) Scientific Objectives ===================== The scientific objectives of the Thermal Emission Spectrometer (TES) investigation include the following: (1) Determine the composition and distribution of surface materials. (2) Determine the composition, particle size, and spatial and temporal distribution of suspended dust. (3) Determine the location, temperature, height, and water abundance of H2O clouds. (4) Determine the composition, seasonal behavior, total energy balance, and physical properties of the polar caps. (5) Determine the particle size distribution of rocks and fines on the surface. Calibration =========== The TES instrument was radiometrically, spectrally, and spatially calibrated prior to delivery. Three categories of calibration requirements were considered: absolute accuracy of all three bands, relative accuracy of spectral measurements within the spectrometer, and calibration stability over the lifetime of the instrument. The spectrometer and thermal bolometric channels were calibrated in a thermal/vacuum chamber using blackbody reference sources operated over the expected Martian temperature range of 130 to >310 K. The calibration sequence was repeated for instrument temperatures over the operating temperature range. The solar reflectance channels were calibrated under ambient conditions using filament lamps traceable to National Institute of Standards and Technology (NIST) standards and a diffuser plate with known bidirectional reflectance distribution function properties. Altering the distance from the lamps to the plate was used to vary the radiance over the expected dynamic range. The absolute accuracy of the calibration was better than 5%. This calibration was confirmed by measurements in the thermal/vacuum chamber over the expected instrument operating temperature range. The inflight radiometric calibration is performed using observations of space (zero level) and an internal blackbody (gain). The instrument has an unobstructed view to space with the line of sight at 85 degrees from nadir in at least one direction, with an unobstructed half angle of 10.75 degrees on either side of this line of sight. These calibration measurements allow the instrument response function and zero levels to be determined and removed from the measured spectra prior to transmission to Earth. This calibration is performed internally to permit coadding of spectra from more than one detector and from more than one measurement. The internal blackbody and lamp calibration sources will be viewed by rotation of the pointing mirror, providing a complete end-to-end system calibration. Operational Considerations ========================== None Detectors ========= Each sensor array consists of uncooled deuterated triglycine sulfate (DTGS) pyroelectric detectors. A narrow bandpass filter is used to isolate the emission line at 703.2 nm for fringe generation and the continuum is used for a quasi-white-light source for determination of zero path difference. A silicon photodiode detector is used for each of these functions. Electronics =========== The outputs from all TES channels are digitized at 16 bits, processed, and formatted before being sent to the spacecraft Payload Data Subsystem (PDS). The outputs of the interferometer receive the following processing within the instrument before transfer to the PDS: 1) selectable apodization; 2) Fast Fourier Transformation (FFT) of data from all six interferometer channels; 3) correction for gain and offsets; 4) data editing and aggregation; 5) data compression; and 6) formatting for the PDS. Filters ======= None Optics ====== The interferometer telescope is a reflecting Cassegrain configuration with a focal ratio of f/4 and an intermediate field stop which limits stray light from being admitted to the interferometer and aft optics sections of the optical system. The afocal output beam of the telescope is 1.524 cm in diameter. After passing through the Michelson interferometer the energy is focused by an off-axis mirror on to a 2 x 3 array of field stops. The focal ratio at the field stops is also f/4. Behind each stop is a field lens operating at approximately f/1 and a pyroelectric detector. A separate 1.5-cm-diameter reflecting telescope, collimated with the main telescope and using the same pointing mirror, is used for the thermal and albedo radiometer channels. The optical system consists of a single off-axis paraboloidal mirror operating at f/8. Location ======== Payload deck of MGS (+Z panel), boresighted with MOC Operational Modes ================= The overall science objectives of the TES experiment will be addressed during the standard mission through a variety of observation types. These include: (1) nadir pointing observations of the surface and atmosphere collected along the spacecraft groundtrack, (2) surface mosaics constructed by observing a particular region forward, nadir, and then aft along the groundtrack, (3) limb observations produced by scanning the pointing mirror to and across the limb, and (4) emission phase functions produced by viewing a particular region at a limited set of emission angles fore and aft. In addition, the TES processor will operate in a wide variety of data collection and processing modes that will allow great flexibility in the types and data volume of observations that will be made. Substantial on-board data processing is necessitated by data rate constraints. A variety of observing modes will be used. These will be based on: (1) combining outputs from selected combinations of detectors (spatial averaging), (2) retention of limited numbers of spectral points (spectral editing) and (3) averaging results over several instrument cycles (temporal averaging). Data modes will be selected, depending on position in the orbit and on scientific requirements, that limit the variable data flow into the internal TES buffer to an orbitally averaged level consistent with the telemetry rate. Internal tables will be used to select between the possible operational modes. For example, the full sampling rate can be utilized over the warmest region of the planet, whereas data can be spectrally and spatially averaged at night and over the poles to decrease the data volume while increasing the signal-to-noise ratio. Control of the instrument parameters and processor activities will be accomplished using an internal command language and internal tables to select between the possible operational modes. These modes include control of the pointing mirror position and motion, spectral selection, spatial and temporal averaging and editing, and data compression. The basic instrument parameters will be set for each two-second observation. Sequences will be constructed to form a self-contained set of observations; for example, calibration observations followed by three minutes of nadir viewing. Orbit Schedules will be constructed from a list of Sequences, each timed to begin at a specified time following the nighttime equator crossing. Two types of Schedules will run in parallel: 1) a basic observing plan designed to be used repetitively; and 2) a targeting Schedule to be used for specific, targeted observations that vary from orbit to orbit. Finally, a Mission Plan will be constructed and stored within the instrument. It will contain the Schedules for the next 3 to 18 days of operation. Using this scheme the TES instrument can be controlled completely internally using minimum number of uplink commands, yet utilizing the full, inherent flexibility of a microprocessor-controlled instrument. Mapping Operations ------------------ Because of the limited (9 km) cross-track FOV, the TES instrument will build up a global image using multiple orbits, with approximately 200 days required to obtain full coverage at the equator. During the mission, the TES could observe each point on the equator three times and each point on the planet an average of 4.7 times. Given the likelihood of dust obscuration during a substantial portion of the mission, this coverage may be significantly reduced. It will therefore be necessary to acquire observations in a well defined, systematic manner. Seasons of highest surface temperature will be chosen for surface compositional mapping, and opportunities provided by increased spacecraft data rate will be incorporated into the observing plan. Observations of temporal phenomena, such as dust storms, polar cap growth and retreat, seasonal pressure variations, and atmospheric phenomena, will be incorporated into the basic plan and collected whenever possible. Nadir Observations ------------------ The nominal TES operating mode will provide a nadir oriented view of the planet, utilizing all three of the cross-track IFOVs. These observations will be assembled as part of the standard data reduction procedure into global maps of the surface observations. Emission Phase Angle Observations --------------------------------- Multiple emission angle observations will provide information on the scattering properties of the surface and atmosphere over regional areas. Because of planetary rotation (0.24 km/sec at the equator) it will not be possible to view exactly the same surface point at multiple emission angles on a single spacecraft revolution. However, regional characteristics can be determined in one revolution and observations from different revolutions may be combined to refine surface photometric estimates. Individual emission angle sequences will consist of 2-5 off-nadir views spaced at fixed angles. Surface Mosaics --------------- The TES instrument has the capability to construct mosaics up to 50 km wide by 110 km long from a single revolution with little loss of spatial resolution by utilizing the planetary rotation. These observations will permit direct comparison with Mars Observing camera and Viking images, and will permit the study of regional features, such as dune fields, wind streaks, and polar lanes on a single orbit. Atmospheric Observations ------------------------ A wide range of atmospheric observations will be accomplished using the TES instrument. These utilize both limb scans and variable emission angle observations of the surface and atmosphere. The observing strategy uses a combination of nadir sounding, fore and aft limb scans, and variable emission angle (nominally+-60 deg ) observations to allow retrieval of vertical temperature profiles, atmospheric aerosol characterization, determination of condensates in the north polar hood, measurement of water ice and vapor and possibly O3, pressure retrievals under high surface temperature conditions, and characterization of localized dust storms. Observing Strategy ------------------ The TES flight software has been programmed with four default operating modes to allow data collection immediately upon instrument turn-on and in the event of an interruption in instrument commanding. This illustrates the level of complexity and flexibility that can be programmed into the TES observing strategy. A default mode has been designed to provide 28, uniformly spaced atmospheric limb observations, distributed in both fore and aft viewing directions. In addition, it optimizes the data collection, with full spectral and spatial data obtained during the day, 6 x 9 km data at half spatial resolution collected over the poles, and 12 x 9 km data at full spectral resolution collected at night. Emission phase observations and limb occultation observations are also collected to permit characterization of polar ices, clouds, and the atmosphere. Actual, mapping orbit observations will vary from this default case, in order to optimize seasonal viewing opportunities, but will probably maintain the basic structure outlined above. Subsystems ========== None Measured Parameters =================== Spectral radiance (spectrometer) - W cm-2 str-1/cm-1 Integrated radiance (bolometer channels) - W cm-2 str-1 Principal Investigator ====================== Philip R. Christensen Arizona State University " END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "CHRISTENSENETAL1992" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END