Instrument Overview
  ===================
    The THEMIS flight instrument is a combined infrared and visible
    multi-spectral pushbroom imager [CHRISTENSENETAL2002].  It has a
    12-cm effective aperature telescope and co-aligned infrared and
    visible area arrays.  The imaging system is comprised of a
    three-mirror anastigmat telescope in a rugged enclosure, a
    visible/infrared beamsplitter, a silicon focal plane for visible
    detection, and a microbolometer for infrared detection.  A major
    feature of this instrument is the use of an uncooled IR
    microbolometer array operated at ambient temperature, eliminating
    the need for complex passive or active cryogenic coolers.  A small
    thermal electric cooler is used to stabilize the detector
    temperature to 0.001K.  A calibration flag, the only moving part in
    the instrument, provides thermal calibration and a DC restore
    capability, and will also be used to protect the detectors from
    unintentional direct illumination from the Sun when the instrument
    is not in use.  The electronics provide digital data collection and
    processing as well as the instrument control and data interface to
    the spacecraft.  Infrared data will be collected in 9 wavelengths
    centered from 6.6 to 15.0 microns at 100 meter per pixel resolution;
    the 6.6 micron band is collected twice to result in a 10 band image.
    Visible data will be collected in 5 spectral bands at a resolution
    of 18 meters per pixel.  The instrument weighs 11.2 kg, is 29 cm by
    37 cm by 55 cm in size, and consumes an orbital average power of
    14W.
 
 
    Optical Design
    --------------
      In order to integrate the visible and IR bands into a single
      telescope, a fast, wide field-of-view reflective telescope has
      been used.  The 3.5 degree (down-track) by 4.6 degree
      (cross-track) field of view is achieved with a 3 mirror f/1.6
      anastigmat telescope with an effective aperature of 12 cm and a
      20-cm effective focal length.  The design allows for excellent
      baffling to minimize scattered light.  It is based on a
      diamond-turned bolt-together approach to telescope design,
      fabrication, alightment and testing.  The manufacture utilized
      high-precision machining capabilities that allowed the entire
      optical stage to be machined and assembled without manual optical
      component adjustments, and achieved diffraction-limited
      performance in both the visible and infrared.  The optical
      surfaces were machined with extremely tight tolerances (0.0002in).
      The optical surfaces were machined directly from high order
      aspheric equations.  The telescope was manufactured with aluminum
      to reduce cost and to be significantly light-weight.  Nickel
      plating and automated post polishing were used to keep the surface
      scatter to levels unobtainable with conventional diamond turning
      techniques.  The system was optimized to match the high signal
      performance required for the IR imager and the spatial resolution
      needed for the visible camera.  The 9 micron pitch of the visible
      array maps to a ground sample distance (GSD) of 18 meters with an
      MTF of approximately 0.1 at Nyquist.  Similarly, the 50 micron
      pitch of the IR focal plane array maps to a GSD of 100 meters.
 
 
    Focal Plane Assemblies
    ----------------------
      The THEMIS infrared design is based on a Raytheon hand-held
      imager developed for rugged military use.  The microbolometer
      array contains 320 pixels cross track by 240 pixels along track,
      with a square 50 micron pixel pitch.  The microbolometer arrays
      were grown directly on the surface of Readout Integrated Circuits
      (ROIC) which are designed by Raytheon Santa Barbara Research
      Center (SBRC) and utilize custom Digital Signal Processing
      electronics.  Spectral discrimination in the infrared is achieved
      with ten narrowband stripe filters.  Each filter covers 16 lines
      in the along track directiona with an 8-line 'dead-space' between
      filters.  The stripe filters were fabricated as separate stripe
      filters butted together on the focal plane.  The along-track
      detectors under a common spectral filter are combined by the use
      of time-delay and integration (TDI) to improve the instrument's
      signal-to-noise performatnce.  The calculated dwell time for a
      single pixel, at a martian orbit of 400km and a 100-meter
      footprint is 29.9 msec, which closely matches the 30Hz frame rate
      for the standard microbolometer.  The ten stripe filter produces
      nine ~1 micron wide wavelength bands from 6.6 to 15 microns.  Two
      filters (bands 1 and 2) cover the same spectral region centered at
      6.6 microns.  The nine IR wavelengths include eight surface-
      sensing wavelengths (bands 1 - 9) and one atmospheric wavelength
      (band 10).
 
      The visible camera was supplied by Malin Space Science Systems
      (MSSS) and is a derivative of the MS'98 MARDI camera.  It consists
      of a small (5.5 x 8.5 x 6.5 cm, <500 gm) unit incorporating a
      focal plane assembly with five color filters superimposed on the
      CCD detector, a timing board, a data acquisition subsystem and a
      power supply.  The visible sensor utilizes a Kodak KAI-1001 CCD.
      This detector has 1024 by 1024 9-micrometer pixels (1018 x 1008
      photoactive).  The visible imager used a filter plate mounted
      directly over the area-array detector on the focal plane.  On the
      plate are multiple narrowband filter strips, each covering the
      entire cross-track width of the detector, but only a fraction of
      the along-track portion of the detector.  The five filter bands
      are centered near 425, 550, 650, 750, and 860 nanometers.  Band
      selection is accomplished by selectively reading out only part of
      the resulting frame for transmission to the spacecraft computer.
      The imager uses 5 stripes each 192 pixels in along-track extent.
      The entire detector is read out every 1.3 seconds.
 
 
    Electronics Design
    ------------------
      Both the visible and infrared cameras utilized commercial,
      off-the-shelf electonics with modifications to accommodate space
      environmental requirements.  Dedicated, miniaturized electronics
      provide ultra-stable, low-noise clock and bias signals to the
      focal planes, stabilize IR focal plane temperature with 0.001
      degree C, and perform analog and digital processing of the output
      signals.
 
      The microbolometer readout electronics includes an initial 8-bit
      analog DC offset correction which occurs on the focal plane, an
      Analog-to-Digital Converter (ADC) coverts the signals to 12-bit
      words, which are then corrected for gain and offsets.  The
      correction is provided by the electronics of the IR camera and
      consists of a 12-bit fine offset and 8-bit gain and responsivity
      adjustment, performed in real time on a pixel-by-pixel basis.
      This process eliminates all the significant noise elements with
      the exception of the fundamental random noise term.  This noise is
      reduced by applying TDI to the corrected digital data.  Internal
      THEMIS data processing in firnmware includes a 16:1 TDI processing
      and lossless data compression for the IR bands using a hardware
      Rice data compression algorithm chip.
 
      The visible sensor requires 7 clock signals: a two-phase
      vertical clock (V1/V2), a two-phase horizontal clock (H1/H2), a
      sub-state clear clock (S), a reset clock (R), and a fast-dump
      clock (F).  In addition, the ADC requires a convert clock.  The
      amplified CCD signal is digitized by an Analog Devices AD1672
      12-bit ADC running at its maximum rate of 3 MSPS.  For each pixel,
      both reset and video levels are digitized and then subtracted in
      the digital domain to perform correlated double sampling (CDS).
      The digital electronics are responsible for clock generation,
      sampling of the CCD signal, conversion of the 12-bit samples to
      8-bit encoded pixels, storage of the pixels, and finally readout
      of the pixels to the spacecraft.  The zero reference ('reset')
      level for each pixel is digitized and stored in a register.  The
      sum of the video plus zero reference ('video') level is then
      digitized, and an arithmetic subtraction is performed to produce
      the final result.  The CCD output only requires scaling to the
      ADC range; no analog sampling, delay or differencing is required.
      The digital signal processor within the visible sensor generates
      the CCD clocks, reads the reset and video levels from the ADC,
      performs the correlated double sampling subtraction, reduces the
      pixel from 12 to 8 bits, applies lossless (2:1) first-difference
      Huffman compression, and transmits it digitally with handshaking
      over the serial communications interface to the spacecraft CPU.
 
      The spacecraft interface electronics supply final processing of
      the focal plane data, a data and command interface to the
      spacecraft, and overall instrument power conditioning.  The bulk
      of the interface electronics is performed using Actel Field
      Programmable Gate Arrays (FPGAs), that are packaged using a
      mixture of conventional, and Sealed Chip-On-Board, High-Density
      Multiple Interconnect technology and chip stack memory.  The
      visible and IR subsystems have independent power supplies, the IR
      power supply uses off-the-shelf modules and requires only a few
      discrete components for input filtering to assure electromagnetic
      compatibility with the rest of the spacecraft.  The spacecraft
      processor performas final data stream formatting for both the IR
      and the visible data.
 
 
    Mechanical Design
    -----------------
      The THEMIS main frame is composed of aluminum and provides the
      mounting interface to the spacecraft as well as the telescope
      assembly, thermal blankets, and thermal control surface.  The
      focal plane assemblies are mounted in the main frame using
      brackets that provide for the necessary degrees of freedom for
      alignment to the telescope.  The calibration shutter flag is
      stored against a side wall that will maintain a known termperature
      of the flag for calibration purposes.  Aluminum covers are
      installed over the electronics circuit cards to provide EMI, RFI,
      and radiation shielding as required.  There is no reliance on the
      spacecraft for thermal control of THEMIS, other than the
      application of replacement heater power when the instrument is
      off.  The themal control plan includes the use of multi-layer
      insulation blankets and appropriate thermal control surfaces to
      provide a stable thermal environment and a heatsink for the
      electronics and the TE termperature controller on the focal plane
      arrays.
 
 
    Performance Characteristics
    ---------------------------
      The predicted performance for the infrared bands produced noise
      equivalent delta emissivity values ranging from 0.007 to 0.038
      when viewing Mars at surface temperatures of 245K to 270K.  The
      measured SNR values for each band at a reference surface
      temperature of 245K are as follows:
 
                      band 1,2 = 45
                      band  3  = 107
                      band  4  = 169
                      band  5  = 193
                      band  6  = 187
                      band  7  = 194
                      band  8  = 167
                      band  9  = 128
                      band 10  = 120
 
      SNR ratios for the visible imager were computed for a low albedo
      (0.25), flat-lying surface viewed at an incidence angle of 67.5
      degrees under aphelion conditions.  The SNR values for this case
      vary from 200 to 400.
 
 
    Software
    --------
      The flight software for the IR imager resides on the spacecraft
      computer and performs the formatting and data packetization.
      Instrument commanding will be done using discrete spacecraft
      commands to the THEMIS instrument over an RS-232 command line.
      These commands will consist of:
 
         1) IR camera on/off/standby;
         2) visible camera on/off/stand-by;
         3) calibration flag shutter control and electronics
            synchronization;
         4) instrument parameter settings (gain, offset, integration
            time, etc.).
 
      The visible imager software runs on two processors: the main
      spacecraft CPU and the internal DSP.  The CPU will be responsible
      for instrument operational commands and image post-processing and
      compression.  The DSP is responsible for generating the CCD
      clocks, emulating the required analog processing and transmitting
      the data output to the CPU.  Lossless predictive compression is
      implemented as part of the DSP firmware.  The algorithm employed
      compresses each image line independently by encoding first
      differences with a single, fixed Huffman table.  Selective readout
      and pixel summing can also be performed by the DSP software.  The
      result of an imaging command is a stream of raw or compressed
      8-bit pixels.
 
      Experience has shown that the volume of data likely to be
      returned from a spacecraft often evolves during a mission.
      Implementing data compression in software on the spacecraft
      computer provides the maximum flexibility for the science and
      spacecraft team to trade-off data return and buffer space usage.
      The compression nodes developed are:
 
         1) lossless predictive (capable of applications by the SDP in
            real-time);
         2) a relatively fast discrete cosine transform compression
            (applied by the spacecraft CPU in 'near realtime' a few
             tens of seconds);
         3) high-quality lossy wavelet compression (applied by the CPU
            on a longer timescale).
 
      Each compression node has optional pixel summing.
 
 
  Scientific Objectives
  =====================
    The objectives of the THEMIS experiment are:
 
        1) to determine the mineralogy and petrology of localized
           deposits associated with hydrothermal or sub-aqueous
           environments, and to identify future landing sites likely to
           represent these environments.
        2) to search for pre-dawn thermal anomalies associated with
           active sub-surface hydrothermal systems.
        3) to study small-scale geologic processes and landing site
           characteristics using morphologic and thermophysical
           properties.
        4) to investigate polar cap processes at all seasons using
           infrared observations at high spatial resolution.
        5) to provide a direct link to the global hyperspectral
           mineral mappingfrom the Mars Global Surveyor TES
           investigation by utilizing the same infrared spectral region
           at high (100m) spatial resolution.
 
 
  Calibration
  ===========
    The THEMIS instrument was radiometrically, spectrally, and
    spatially calibrated prior to delivery.  Three categories of
    calibration were performed: 1) spatial calibration; 2) spectral
    calibration; and 3) radiometric calibration.  The radiometric
    calibration included the absolute rediometry, the relative precision
    (SNR), and the calibration stability during an image aquisition.
    The data returned from the instrument in-flight will be converted
    to scene radiance by:
 
        1) adjusting for the gain and offset that were applied in the
           instrument to optimize the dynamic range and signal
           resolution for each scene;
        2) correcting for drift or offest that occur between
           observations of the calibrations flag;
        3) converting signal to radiance using the instrument response
           function determined prior to launch.
 
    The response functions necessary to perform this calibration were
    acquired prior to instrument delivery using a thermal vacuum chamber
    at the SBRS facility.  See calibration report for details on IR and
    visible image calibration methodologies [CHRISTENSEN2002]
 
    THEMIS images will be calibrated using periodic views of the
    internal calibration flag.  This flag will be closed, imaged, and
    reopened at selectable intervals throughout each orbit.  Calibration
    data are expected to be acquired every 3-5 minutes.  However, the
    optimum spacing of these observations that meets the calibration
    accuracy requirements, while minimizing the loss of surface
    observations, will be determined in Mars orbit.
 
 
  Operation of THEMIS
  ===================
    The THEMIS instrument is operated from ASU, building on the
    facility and staff developed and in place for the MGS TES
    investigation.  No special spacecraft operation or orientation is
    required to obtain THEMIS data.  The instrument alternates between
    data collection (<3.5 hours per day) and idle modes based on
    available DSN downlink rates.  These modes will fall within
    allocated resources (e.g.  power), and will not require power
    cycling of the instrument.  All instrument commands are generated
    at ASU, delivered electronically to the Odyssey Project, and
    transmitted to the spacecraft.
 
 
    Image Collection
    ----------------
      IR images can be acquired at selectable image lengths, in
      multiples of 256 lines (25.6 km).  The image width is 320 pixels
      (32 km from the nominal mapping orbit) and the length is variable,
      as given by ((#frames)*256 lines) - 240.
      The allowable number of frames varies from 2 to 256, giving
      minimum and maximum image lengths of 272 and 65,296 lines
      respectively (27.2 km and 6,530 km from the nominal mapping
      orbit).  The bands returned to the ground are selectable.  THEMIS
      visible images can be acquired simultaneously with IR images, but
      the spacecraft can only transfer data from one of the two THEMIS
      imagers at a time.  The IR image transfers data as it is being
      collected, while the visible images are stroed within an internal
      THEMIS buffer for later transfer to the spacecraft CPU.
 
      Visible images can be acquired in framelets that are 1024 samples
      crosstrack by 192 lines downtrack in size.  The images can be
      composed of any selectable combination of bands and image length
      that can be stored within the 3.734 Mbytes THEMIS internal buffer.
      The size of an image is given by:
 
          (1024 * 192) * #framelets * #bands < 3.734 Mbytes.
      Thus, for example, either a single-band, 19 framelet (65.6 km)
      image or a 5-band 3-framelet (10.3 km) image can be collected.
      This buffer must be emptied to the spacecraft before a subsequent
      image can be acquired.
 
 
    Data Allocation
    ---------------
      THEMIS data collection will be distributed between the
      mineralogic, temperature, and morphologic science objectives in
      both targeted and global mapping modes.  A baseline observing plan
      has been developed to prioritize the total data volume returned by
      THEMIS between the different objectives.  This plan currently
      devotes 62% of the total data to the IR imager and 38% to the
      visible, averaged over the course of the Primary Mission.  In the
      baselne plane the IR data will be further sub-divided into
      9-wavelength daytime mineralogic observations (47% of total data
      return) and 2-wavelength nighttime and polar temperature mapping
      (15% of total data).  The visible data will be sub-divided into
      monochromatic images (36% of total data) and 5-band multi-spectral
      images (2%).
 
      We have assumed a lossless data compression factor of 1.7 for
      the IR imager and a combination of lossless (40%), and lossy with
      compression factors of 4 (30%) and 6(30%) for the visible imager.
      With these allocations, the Science and Relay phases of the
      mission will fully map Mars in daytime IR and will map the planet
      1.5 times in nighttime IR. The visible imaging will cover 60% of
      the planet at 18 meter resolution in one band (80,000 18x50Km
      frames) and <1% in 5-band color.  Tradeoffs between monochromatic
      and multi-spectral imaging, as well as variations in the degree of
      lossy compression , will be made to maximize the science return
      from the visible imager.
 
      THEMIS data volume return varies significantly will mission
      phase due to variations n the Earth-Mars distance.  In addition,
      the equator crossing local time will vary between ~15.5 H (24 H
      equals one martian day) and 18 H over the course of the mission.
      The first ~180-day phase of the mission provides some of the best
      viewing conditions for the IR imager and will allow ~40% of the
      planet to be observed.  IR images acquried during this period will
      be carefully selected using the TES global mineral maps to focus
      on the sites of highest mineralogical or morphological interest.
      During a second IR mapping phase, when the data rates are again
      high and the local time close to 16.5 H, the remaining ~60% of the
      planet will be mapped.
 
 
  Principal Investigator
  ======================
    Philip R. Christensen
    Arizona State University