| DESCRIPTION |
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
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