**LEISA/Atmospheric Corrector (LAC) Validation Report**

March 18, 2002

Dennis Reuter

*NASA/GSFC*

*Code 693*

*Greenbelt, Maryland 20771*

George McCabe

*NASA/GSFC and Catholic University*

*Code 693*

*Greenbelt, Maryland 20771*

NASA/GSFC

**TABLE OF CONTENTS**

[1. INTRODUCTION 1](#introduction)

[2. TECHNOLOGY DESCRIPTION 1](#technology-description)

[3. TECHNOLOGY VALIDATION 5](#technology-validation)

[3.1 Ground Test Verification 5](#ground-test-verification)

[3.2 On-Orbit Test Validation 7](#on-orbit-test-validation)

[3.2.1 Dark Current Stability 8](#dark-current-stability)

[3.2.2 Illumination Noise Characteristics
8](#illumination-noise-characteristics)

[3.2.3 Image Quality 10](#image-quality)

[3.2.4 Out-of-band Transmittance 11](#out-of-band-transmittance)

[3.2.5 Absolute Radiometric Calibration and Spectra
12](#absolute-radiometric-calibration-and-spectra)

[3.2.6 Co-location With Landsat 7 13](#co-location-with-landsat-7)

[3.3 On-Orbit Usage Experience 13](#on-orbit-usage-experience)

[4. LESSONS LEARNED 16](#__RefHeading___Toc7511897)

[5. CONTACT INFORMATION 16](#contact-information)

[6. SUMMARY 17](#summary)

[6.1 Water Vapor Retrieval Example 17](#water-vapor-retrieval-example)

[7. REFERENCES 18](#references)

[ACRYONYMS and ABBREVIATIONS 19](#acryonyms-and-abbreviations)

**LIST OF ILLUSTRATIONS AND TABLES**

[Figure 1. LAC electronics module (left) and optics module. The three
lenses are facing outward. The electronic module is 25 cm x 23 cm x 18
cm. The optics module is 19 cm x 18 cm x 14 cm. The cable is separated
into two groups of three, one group carrying the analog signals and
biases, the other carrying the clock signals.
2](#__RefHeading___Toc12674447)

[Figure 2. Optics module with lenses facing upward and solar calibration
tubes facing forward. The red object to the right is a cover for the
optical alignment cube used to align the LAC with the ALI.\
The fitting in front is an N2 purge coupling used before launch.
3](#__RefHeading___Toc12674448)

[Figure 3. Optics module details. The array/LVE assembly is to the left
while light from the scene enters\
to the right through an anti-reflective (AR) triplet lens. The
spectralon target for the solar calibrations is at a 45° angle. The
target is sized to approximate a 30% reflectance surface at 60° solar
angle. The tilted element before the lens is a visible/UV reflector.
4](#__RefHeading___Toc12674449)

[Figure 4. Examples of spectral line-shapes measured with the
monochrometer system. A Gaussian line-shape was fitted.
6](#__RefHeading___Toc12674451)

[Figure 5. Summary of fittings of measured normalized pixel responses to
a single Gaussian function (width in wavenumbers) for the short-(left
panel) and long-wavelength filter segments. Pixels covering the entire
spectral range are included in the fittings. The constant width in
cm^-1^ is consistent with a linear dependence of wavenumber with
position on the filter. 7](#__RefHeading___Toc12674452)

[Figure 6. Histogram of difference between a dark DCE on July 17, 2001
and dark DCEs February 27, 2001 (leftmost panel), April 30, 2001
(center) and a separate dark DCE on July 17, 2001. Note\
the tail of the distribution indicating a slight increase in dark
current as a function of time. The\
rightmost panel shows the difference for two dark DCEs on July 17, 2001.
8](#__RefHeading___Toc12674453)

[Figure 7. SNR as a function of wavelength determined from a sole
calibration DCE in the method described in the text. The rapid drop-off
at wavelengths shorter than 0.97 μm is caused by loss of quantum
efficiency in the InGaAs arrays. 9](#__RefHeading___Toc12674454)

[Figure 8. Plot of row 180 for all three arrays showing the "fixed
pattern" noise. As the pointing of the solar calibration tubes is rolled
about the solar position, the array illumination varies from a maximum
in frame 385. 10](#__RefHeading___Toc12674455)

[Figure 9. Lunar image (expanded by a factor of 8) showing rise and fall
distances (5% to 95%) of 8 to\
10 pixels (on expanded scale), indicating single pixel imaging
capability. Without expansion the moon fills about 20% of an array (\~25
pixels). 11](#__RefHeading___Toc12674456)

[Figure 10. Relative shortwave response for LAC solar calibration. The
lack of signal at wavelengths\
less than 0.93 μm indicates that there is little out-of-band filter
transmittance. 12](#__RefHeading___Toc12674457)

[Figure 11. Two spectra obtained in different regions near the Suez
canal. Note the much deeper absorption at 1.13 μm in the right panel,
indicative of greater atmospheric water vapor.
12](#__RefHeading___Toc12674458)

[Figure 12. Overlay of Landsat 7 image on LAC image taken within 1
minute. Railroad Valley, Nevada (left) and Cuprite, California (right)
are used as calibration areas for these instruments. The\
features in the images are well correlated.
13](#__RefHeading___Toc12674459)

[Figure 13. Schematic of EO-1 LAC data flow. The ACCS (Atmospheric
Corrector Calibration System). 13](#__RefHeading___Toc12674460)

[Figure 14. Efficacy of "fixed pattern" noise correction. The bottom
panel shows the variability in a\
single wavelength without correction. The second panel shows how the
variance is reduced after scanning solar cal while the top panel shows
the final result. Note how variance is reduced by\
both steps. 15](#__RefHeading___Toc12674461)

[Figure 15. Comparison of retrieved water vapor (left panel) with
monochromatic image in the 1.0463-μm relative window region. Regions of
reduced atmospheric water vapor show up as brighter areas on the left.
Note the bright spots in the top center of both images. This is a region
of clouds, which show up as a reduced water vapor because the column
path is reduced. Also note the expected\
lower water vapor amounts above regions of high topography.
18](#__RefHeading___Toc12674462)

Table 1. Instrument Summary 5

#  {#section .unnumbered}

# 

# 1. INTRODUCTION

The LEISA (Linear Etalon Imaging Spectral Array) Atmospheric Corrector
(LAC or AC) on Earth Observing-1 (EO-1) is a hyperspectral imager
providing 256-channel continuous spectra in the wavelength range from
0.89 to 1.58 microns. It has a single pixel spatial resolution of 250
meters, a 185-km swath-width and a spectral resolving power (λ/∆λ) of
150 or greater (∆λ \< 10 nm) throughout its spectral range. The imager
employs a wedged filter, a sophisticated 2-dimensional filter whose
transmission wavelength varies along one of the dimensions to provide
its spectral resolution. The use of the wedged filter greatly simplifies
the optical/mechanical design, so that the instrument is adaptable to a
wide variety of platforms (i.e., it is a \"bolt on\" imager).

This paper presents an overview of the camera that highlights its unique
features from both a design and data standpoint. It discusses the
pre-launch instrument characterization process and on-orbit performance.
This includes the use of solar and lunar calibration techniques to
obtain absolute radiometric calibration. The application of the data to
the problem of atmospheric correction of high-spatial resolution
multi-spectral satellite images (e.g., Landsat-type) is also discussed.

A problem common to all measurements obtained by high-spatial resolution
multispectral (MS) imagers aboard satellites is the systematic errors in
the apparent surface reflectances caused by atmospheric effects.
Temporal and spatial variability in atmospheric scattering and
absorption due to aerosols, clouds and molecular species, primarily
water vapor, must be accounted for in the retrieval of accurate surface
properties. The research team has developed the high spectral/moderate
spatial resolution wedged filter hyperspectral (HS) imager, the LAC, to
correct high-spatial, low-spectral resolution MS imagery for atmospheric
effects. The spectra it provides may be used to correct images from the
Advanced Land Imager (ALI) on EO-1. Because EO-1 is flying in formation
one minute behind Landsat 7, the data may be used to correct Landsat 7
also. The LAC provides scientific return both in terms of improved
imagery and moderate spatial resolution hyperspectral sensing
capabilities. It advances three technologies that are relevant to a
number of remote sensing applications. These are listed as the first
three items in the introduction to Section 3. TECHNOLOGY VALIDATION.

# 2. TECHNOLOGY DESCRIPTION

The LAC employs a state-of-the-art wedged infrared (IR) filter (a linear
variable etalon or LVE) placed very close to a two-dimensional IR
detector array to produce a 2-D spatial image that varies in wavelength
along one of the array dimensions. (The LVE is a wedged dielectric film
etalon whose transmission wavelength varies along one dimension.) The
filter has a nearly linear dependence of wavenumber on position. It has
a 0.45-cm section which covers the 1.2- μm to-1.6-μm spectral region at
a resolution of \~35 cm^-1^, and a 0.55-cm section covering the
0.9-to-1.2-μm spectral range at a resolution of \~55 cm^-1^. The
sections are bonded together to form a single filter assembly. This
filter represents an advance in dielectric thin film technology.
Reflective 1/4-wave stacked layers placed on both sides of a 1/2 -wave
etalon cavity provide the spectral resolution. Out-of-band suppression
of the etalon is accomplished with lower resolution filter layers. The
wavelength transmitted by all the layers varies as a function of
position, thus providing the spectral information.

The two-dimensional spatial image is formed on the array by a small,
wide field of view (WFOV) lens. The filter is mounted within 200 μm of
the array, so the image is formed simultaneously on the array and the
filter. The spectrum of each point of the area imaged is obtained as the
orbital motion of the spacecraft scans the image of that area along the
focal plane in the variable wavelength dimension. This creates a
three-dimensional spectral cube. The spatial resolution is determined by
the angular resolution of the imaging optic, the image scan speed and
the readout rate of the array. The LAC has a single pixel spatial
resolution of 360 μradian x 360 μradian corresponding to a single pixel
FOV of 250 m x 250 m for a 705 km orbit. The LAC uses three identical
subassemblies in a single module to match the Landsat 7 swath width of
185 km (\~15°). Each subassembly consists of a focal plane formed of a
wedged filter mounted to a 256 x 256 pixel indium gallium arsenide
(InGaAs) detector array placed behind a lens covering a 5° x 5° FOV. The
InGaAs detector material was fabricated by Sensors Unlimited of
Princeton New Jersey, and was hybridized to a TCM 2620 multiplexer by
the Boeing (Rockwell) Science Center in Thousand Oaks, California. The
middle lens is used on-axis while the other two lenses are used off-axis
to cover the full 15°. So each off-axis lens must have a usable 15° x 5°
FOV. Thus a single frame consists of an effective focal plane with 768
pixels in the cross-track direction and 256 pixels in the along-track
direction. A frame readout rate of approximately 28 Hz matches the image
motion caused by the orbital velocity to the interpixel angular
separation. That is, the orbital motion of the spacecraft moves the
image 1 row in \~0.0356 second. The wedged filter design was chosen for
its optical and mechanical simplicity compared to conventional grating,
prism, or Fourier transform spectrometers tunable filter systems. As
discussed below, this method of obtaining spectra does place some
stability constraints on the spacecraft and the scene. The LEISA concept
was originally developed at NASA Goddard Space Flight Center (GSFC)
under the Advanced Technology Insertion Program for the Pluto Fast Flyby
Mission \[1\]. The filters were designed and fabricated by the Optical
Coating Laboratories Inc. of Santa Rosa, California, with the aid of
test results obtained at NASA (GSFC) \[2,3\]. The wedged filter concept
has also been developed independently at Hughes Santa Barbara Research
Center \[4\] and their Wedged Imaging Spectrometer (WIS) operating in
the visible and near-IR (VNIR) regions has been demonstrated in aircraft
flights \[5\].

The LAC is composed of two modules, the optics module and the
electronics module. Figure 1 shows both modules being tested in the
laboratory. The optics module contains the lenses, focal planes, and
electronics necessary to operate the arrays and to transfer the
digitized pixel data to the electronics module. It is mounted to the
nadir deck of the spacecraft and bore sighted with the ALI. The
electronics module contains the command and data interface to the
spacecraft, the array timing and bias circuitry, the thermal electric
cooler (TEC) control circuitry, and the instrument power supply. It is
mounted in a bay below the nadir deck. Because the long-wavelength
cut-off of the arrays is 1.6 μm, they may be operated at near room
temperature. However, to decrease the dark current and increase the
signal-to-noise ratio, TECs are used to stabilize the array temperatures
at [\<]{.underline}285K.

![](media/image1.wmf){width="3.5027777777777778in"
height="2.286111111111111in"}

[]{#__RefHeading___Toc12674447 .anchor}Figure 1. LAC electronics module
(left) and optics module. The three lenses are facing outward. The
electronic module is 25 cm x 23 cm x 18 cm. The optics module is 19 cm x
18 cm x 14 cm. The cable is separated into two groups of three, one
group carrying the analog signals and biases, the other carrying the
clock signals.

Figure 2 shows a view of the optics module as it is oriented on the
nadir deck. In normal operation the lenses would be facing earthward
while the solar calibration tubes would be facing along the direction of
the spacecraft motion. During a solar calibration the solar calibration
tubes are pointed to within 1° of the sun, and light entering the tubes
is reflected off a small spectralon target, providing a reproducible
illumination pattern across the focal planes. Because the sun is not
imaged through the lenses, it provides only an approximate absolute
radiometric calibration. In flight radiometric calibration is provided
by lunar scans and by comparisons with the other instruments on EO-1
(ALI and Hyperion). Several well-instrumented ground sites are also used
for radiometric calibration. Figure 3 shows the interior detail of the
optics module. In normal operation, light comes from the right through
the triplet lens and is imaged onto the focal planes at the left. The
cylinder directly above the focal plane is a stray light baffle. The
baffles in the solar calibrators are evident, as is the spectralon
reflector. Note that there are three electronics boards, one for each
focal plane. The far-left side is mounted to the nadir deck and the TEC
heat sinks are mounted to this side. The use of a two-module design for
the LAC allows a greater degree of flexibility than using a single
module. Optical modifications (e.g., wavelength range, resolution, etc.)
require changes only to the optics module while spacecraft interface
changes (e.g., command and data handling (C&DH), power levels, etc.)
require changes only to the electronics module.

![](media/image2.wmf){width="4.000694444444444in"
height="3.4784722222222224in"}

[]{#__RefHeading___Toc12674448 .anchor}Figure 2. Optics module with
lenses facing upward and solar calibration tubes facing forward. The red
object to the right is a cover for the optical alignment cube used to
align the LAC with the ALI. The fitting in front is an N2 purge coupling
used before launch.

![](media/image3.wmf){width="5.502083333333333in"
height="4.507638888888889in"}

[]{#__RefHeading___Toc12674449 .anchor}Figure 3. Optics module details.
The array/LVE assembly is to the left while light from the scene enters
to the right through an anti-reflective (AR) triplet lens. The
spectralon target for the solar calibrations is at a 45° angle. The
target is sized to approximate a 30% reflectance surface at 60° solar
angle. The tilted element before the lens is a visible/UV reflector.

The LAC has a total mass of 10.5 kg: 4.4 kg for the electronics module,
3.9 kg for the optics module, and 2.2 kg for the cable connecting them.
It uses a maximum power of 45 W on start-up, which decreases to about 35
W for a TEC temperature setting of 275 K, once the temperature is
stabilized. At the nominal fame rate of 28 Hz, which slightly
oversamples the spatial dimension in the along-track dimension, the data
rate is 95 Mbits/s (12-bit analog-to-digital (A/D) converters are used).
A frame rate of 56 Hz allows double sampling in the along-track spatial
dimension at the expense of reduced single-frame signal-to-noise ratios
(SNRs). The LAC frame rate could have been adjusted to provide exact
single sampling as was done for ALI, however, for simplicity this was
not done.

A significant difference between push broom filter imagers such as LAC
and ALI and slit imagers such as Hyperion is in the way spectra are
formed. In an LVE camera an image is obtained, however, several images
are required to complete the spectrum of a given point in the scene.
Thus, excess drift in the image motion can cause a point to cross
columns during the acquisition of a full spectrum (see \[6\] for a
discussion of re-sampling in wedged filter spectrometers). Control and
knowledge of the EO-1 spacecraft, and the alignment of the LAC
along-track axis to the ALI along-track axis to within 2 arc-minutes,
limits this uncertainty to a fraction of a pixel. In the worst case,
across-track drift and along-track timing errors lower the effective
spatial resolution to 2 pixels, or the Nyquist sampled spatial
resolution. In comparison, slit-type imagers obtain all the spectral
elements of a given point on the surface simultaneously. In practice,
however, off-axis image errors, as well as the effect of oblique
incidence for gratings and non-linearity effects for prisms, cause the
spectrum to be tilted and/or curved in the focal plane, and thus not to
track along a column. These geometric effects are the analog of
imperfect sampling in the wedged filter camera, and also cause a loss of
effective spatial resolution. A wedged filter camera is inefficient for
small sources (e.g., to obtain the spectrum of a point source), but it
is essentially as efficient as a slit-type instrument for extended
sources (e.g., images of the earth\'s surface from a near earth
orbiter). The instrument characteristics of the LAC are summarized in
Table 1.

Table 1. Instrument Summary

  ----------------- ---------------- ---------------------------------- ------------
  **Type of                                                             
  Spectrometer**:                                                       
  Wedged Filter                                                         

  **Spectral                                                            
  Coverage**: 0.89                                                      
  to 1.58 μm                                                            
  continuous                                                            
  (SNR\>10, 0.94 to                                                     
  1.58 μm)                                                              

  **Spectral                         \~35 cm^-1^ (Δλ: 5 nm @ 1.2 μm, 9  
  Resolving                          nm @ 1.6 μm)                       
  Power**:                                                              

                                     \~55 cm^-1^ (Δλ: 4 nm @ 0.9 μm, 8  
                                     nm @ 1.2 μm)                       

  **Wavelength                                                          
  Dependence**:                                                         
  Linear in                                                             
  Wavenumber                                                            
  (Δλ^-1^)                                                              

  **Imaging                                                             
  Optics**: Three                                                       
  Multi-Element                                                         
  Refractive, 15°                                                       
  FOV Lenses (f/8,                                                      
  11.4 cm focal                                                         
  length)                                                               

  **Dynamic                                                             
  Range**: 2048                                                         

  **Frame Rate**:                                                       
  Nominal 27.8 Hz,                                                      
  Maximum 55.6 Hz                                                       

  **Single Pixel                                                        
  FOV**: 0.36 x                                                         
  0.36 mrad2 (250 x                                                     
  250 m2 for 705 km                                                     
  orbit)                                                                

  **Three Array                                                         
  FOV**: 264 x 92                                                       
  mrad2 (185 x 64                                                       
  km2 for 705 km                                                        
  orbit)                                                                

  **Memory Required                                                     
  for One 185 x 64                                                      
  km2 Spectral                                                          
  Map**: 810 Mbits                                                      

  **Data Rate**:                                                        
  Nominal, 95                                                           
  Mbits/sec;                                                            
  Maximum, 190                                                          
  Mbits/sec                                                             

  **Arrays**: 256 x                                                     
  256 PV InGaAs, 40                                                     
  μm pixels (TCM                                                        
  2620 Multiplexer)                                                     

  **Selectable                                                          
  Focal Plane                                                           
  Temperatures**:                                                       
  \~ 285 K, 280 K,                                                      
  275 K, 265 K                                                          

  **Power**: 48 W                                                       
  peak; 35 W                                                            
  nominal, \<10 W                                                       
  orbit average                                                         
  (maximum 20% duty                                                     
  cycle)                                                                

  **Mass**: 10.5 kg                                                     

                    *Electronics                                        4.4 kg
                    Module* (1773                                       
                    Interface,                                          
                    Power, RS 422                                       
                    Data Interface,                                     
                    Array Control):                                     

                    *Optics Module*                                     3.9 kg
                    (Optics, Arrays,                                    
                    Solar                                               
                    Calibrators,                                        
                    TECs,                                               
                    Amplifiers,                                         
                    A/D):                                               

                    *Inter-module*                                      2.2 kg
                    *Cable*                                             
  ----------------- ---------------- ---------------------------------- ------------

# 

# 3. TECHNOLOGY VALIDATION

The primary purpose of the LAC on EO-1 from an orbiting technology
validation standpoint is fourfold: 1) to validate the use of the wedged
filter method for obtaining hyperspectral images, 2) to validate the use
of a multi-array, multi-telescope system to synthesize a wide-field
imager, 3) to validate the use of non-cryogenic InGaAs IR arrays for
moderate resolution spectroscopy and, 4) to validate the use of lunar
and solar measurements (in conjunction with ground-based measurement
campaigns) to provide calibration. The science validation is to provide
a demonstration of the ability of moderate-spatial resolution
hyperspectral measurements to provide real-time atmospheric correction
information to high-spatial resolution multispectral sounders. This
report will not deal with the scientific validation issues.

## 3.1 Ground Test Verification 

Prior to launch, the operational characteristics of the LAC, including
dark current, read noise, radiometric sensitivity, central wavelength,
bandwidth and angular position, were measured for each pixel. The
low-illumination noise limit (read noise + dark current) were about 1 --
2 counts. While near saturation, in the photon noise limited regime,
SNRs of several hundred were obtained. The overall photon to
photoelectron conversion efficiency ranged from 0.10 to 0.20. For the
most part the characteristics measured prior to launch have not changed
with two exceptions: the number of non-responsive pixels has increased
since launch, and a systematic noise source not apparent in pre-launch
tests, and therefore presumably occurring after launch, has modified the
radiometric calibration. These effects will be discussed in the next
section.

The pre-launch spectral characteristics of each pixel were measured
using a grating monochrometer whose resolution was much higher than that
of the wedged filters. A white light source was imaged onto the entrance
slit of the spectrometer and a grating blazed for 12 μm was used to
disperse the light. Thus at each grating setting several orders would be
present simultaneously, for example, at a 12-μm setting light in orders
13 thru 8 would fall into the 0.92 to 1.6-μm range. The output slit of
the monochrometer was placed near the focal point of each lens of the
LAC, filling each array with collimated light in 5 to 7 orders. Thus, at
each grating setting, light would be transmitted at several rows of the
array. The central wavelength and bandwidth of each pixel were
determined by stepping the grating in small (1 to 5 nm) increments and
fitting the resultant intensity vs. wavelength data to a Gaussian
function. Examples of these fittings are shown in Figure 4. As may be
seen from this figure, the resolving power of the filter is quite high
(e.g., \> 200 at 1 μm).

![](media/image4.wmf){width="5.401388888888889in" height="3.05625in"}

[]{#__RefHeading___Toc12674451 .anchor}Figure 4. Examples of spectral
line-shapes measured with the monochrometer system. A Gaussian
line-shape was fitted.

In fact, the spectral line-shape at each pixel is closely described by a
Gaussian function whose full-width at half maximum is 53.34 cm^-1^ for
the short-wave filter segment (0.89 to 1.23 μm) and 36.84 cm^-1^ for the
long-wave segment (1.21 to 1.58 μm). Figure 5 summarizes the results of
fitting Gaussian functions with a single width to normalized intensity
vs. wavenumber measurements for several pixels in each filter segment.
The wavenumber of each pixel depends nearly linearly on row number.

![](media/image5.wmf){width="5.998611111111111in"
height="2.402083333333333in"}

[]{#__RefHeading___Toc12674452 .anchor}Figure 5. Summary of fittings of
measured normalized pixel responses to a single Gaussian function (width
in wavenumbers) for the short-(left panel) and long-wavelength filter
segments. Pixels covering the entire spectral range are included in the
fittings. The constant width in cm^-1^ is consistent with a linear
dependence of wavenumber with position on the filter.

The angular positions of each pixel in each array were determined
relative to an alignment cube attached to the surface of the optics
module prior to launch. The measurement was performed using two
theodolites. One theodolite was aligned with the surfaces of the cube
and used as a transfer standard to determine the angular displacements
of the second theodolite. The second theodolite was equipped with a
cross-hair target at the focal point of a lens simulating a source at
infinity. When re-imaged by the lenses of the LAC, an image of the
crosshair was formed on each of the focal planes. The image of the
crosshair was \~20 pixels across, allowing the center of the crosshair
to be located to \~0.1 pixel. The image of the crosshair was translated
along the nominal along-track direction of each focal plane by rotating
the theodolite source. Position measurements were made at several
locations for several rows of each array. In this way, the roll and yaw
angles of each array were determined to approximately 40-microradian
accuracy (0.1 pixel). This information was used to map the angular
position of each pixel for use in geolocating the on-orbit image data.
These data were also used to mount the LAC to the nadir deck of the
spacecraft in such a way that the yaw angle of the LAC array that
overlaps the field-of-view of the ALI was parallel to the ALI yaw angle
to within 1 arc-minute. This ensured that there was minimal across-track
drift (\< 0.25 pixel) during a scan.

## 3.2 On-Orbit Test Validation

On-orbit validation includes: 1) determining the stability of the arrays
under conditions of no illumination, 2) determining the instrumental
noise characteristics under conditions of illumination, 3) determining
the image quality, 4) determining the absolute radiometric calibration
accuracy, 5) assessing the ability to obtain spectra and 6) co-locating
LAC images with Landsat 7 images. A typical on-orbit data collection
event (DCE) lasts about 40 seconds during which \~1100 frames are
obtained at the 28-Hz frame rate (2200 at 56 Hz) and produces a complete
spectral image over an area of \~210 km x 185 km. The operation of the
instrument is extremely simple. Power is applied to the instrument about
9 minutes before image data is taken to insure the array temperatures
and electronics have stabilized. The frame rate, array temperature
set-point and array sensitivity are set by issuing three commands. The
data clock is enabled about 8 seconds before the instrument is pointed
at the desired surface location to insure that all spectral bands are
obtained for the entire target area.

### 3.2.1 Dark Current Stability

The LAC dark current is measured within 1 orbit of a DCE by imaging a
region on the unlit hemisphere of the earth's surface for 1 second (a
dark DCE). The average of these dark data is subtracted from each frame
of the DCE to remove offsets and dark current at each pixel. The
subtraction is done as part of the ground processing of the data. The
dark current is very uniform during a dark DCE, with the frame-to-frame
standard deviation of each pixel typically being less than 1 count. The
dark current is also very stable from day to day, with only a slight
drift apparent. This is illustrated in Figure 6, which shows histograms
of the differences between a dark DCE on day 198 of 2001 (July 17, 2001)
and two previous dark DCEs (February 27, 2001 and April 30, 2001). From
this figure one can see that there is a general broadening of the dark
current difference distribution over long periods of time (i.e. weeks)
and there appears to be a tendency for the dark current to increase
slowly as a function of time. The final panel in the figure shows the
reproducibility of the offset on a single day, which is indicative of
the error in the offset subtraction methodology. This error of somewhat
less than 1 count is the limiting error in the low illumination case.
The bias of 0.5 count in the last panel is representative of the array
temperature control stability.

![](media/image6.wmf){width="6.002777777777778in"
height="1.7638888888888888in"}

[]{#__RefHeading___Toc12674453 .anchor}Figure 6. Histogram of difference
between a dark DCE on July 17, 2001 and dark DCEs February 27, 2001
(leftmost panel), April 30, 2001 (center) and a separate dark DCE on
July 17, 2001. Note the tail of the distribution indicating a slight
increase in dark current as a function of time. The rightmost panel
shows the difference for two dark DCEs on July 17, 2001.

The slow broadening of the dark current distribution is probably related
to an observed slight increase in non-responsive pixels since launch. At
launch there were about 400 non-responsive pixels (\~0.2% of the total)
and in the 8 months since then another 1100 have been added (for a total
of \~0.8%). Typically this number grows at a rate of one per day,
however on two occasions several hundred pixels have "died" within a
single DCE. (Note: Since the instrument is turned off between DCEs it is
possible the pixel mortality could have occurred at any time since the
turn-off after the previous DCE and the beginning of data at the current
DCE). There were no other operational (e.g. radiation, solar cell
overvoltage, etc.) or instrumental (e.g., overvoltage, excess current,
etc.) anomalies at these times and the cause of the increased
non-responsive pixels is still being investigated. It has had no major
operational effect since the relative number of pixels affected is still
quite small (\<1%) and they are scattered throughout the arrays. Prior
to launch the arrays were damaged by contact with the wedged filter
holders during a vibration test. The contact was caused by an incorrect
assembly procedure that was subsequently corrected and no further damage
was observed in additional vibration testing. However, gradual long-term
effects on the multiplexer bump bonds from this incident remains a
strong candidate for the cause of the pixel mortality.

### 3.2.2 Illumination Noise Characteristics

The limit on the SNR under most illumination conditions is determined by
the Poisson distributed statistical variance of the photon flux from the
source. The solar calibration DCEs are used to evaluate this limit for
the LAC, since the illumination is constant. The standard deviation of
each pixel is measured over the 200 frames obtained during the
calibration and the ratio of this to the average signal is used to
determine the SNR. Figure 7 shows a wavelength dependent summary of the
SNR obtained in this fashion. The discontinuity at \~1.2 μm marks the
boundary between the two filter segments. Note that the SNR is greater
than 100 everywhere, consistent with pre-launch calculations. In fact,
since the solar calibration level is only about 25% of the maximum
before saturation is reached, the maximum SNR is expected to be about a
factor of 2 higher than shown here. However, as discussed below, a
systematic noise source that apparently occurred after launch limits the
actual SNR to values lower than shown in Figure 7. Since digital
saturation occurs well before the actual analog detector saturation
(digital saturation occurs at about 20% of the 2.5 million electron
detector element full well), the signals do not show large
non-linearities near saturation.

![](media/image7.wmf){width="6.002083333333333in"
height="4.095138888888889in"}

[]{#__RefHeading___Toc12674454 .anchor}Figure 7. SNR as a function of
wavelength determined from a sole calibration DCE in the method
described in the text. The rapid drop-off at wavelengths shorter than
0.97 μm is caused by loss of quantum efficiency in the InGaAs arrays.

As mentioned above, the actual LAC noise is dominated by a systematic
source that appeared after launch. Initially, it was very difficult to
obtain images not showing excessive pixel-to-pixel variation (streaking)
at nearly the 10% level. The source of this streaking was found only
after a scanning solar calibration showed that for most illumination
levels there was a relatively constant "fixed pattern" noise that did
not appear in the dark frames. In a scanning solar calibration, the
solar calibration tubes are nodded ± 6° about the solar direction, thus
varying the amount of light scattered off the spectralon target in a
non-calibrated but uniform way. This maneuver was initially performed to
assess the stability of illumination during a solar calibration, and not
as an additional calibration method. Figure 8 shows a plot of a typical
row of all three arrays for several positions of the solar calibration
tubes.

![](media/image8.wmf){width="6.0in" height="3.598611111111111in"}

[]{#__RefHeading___Toc12674455 .anchor}Figure 8. Plot of row 180 for all
three arrays showing the "fixed pattern" noise. As the pointing of the
solar calibration tubes is rolled about the solar position, the array
illumination varies from a maximum in frame 385.

The offset has been subtracted in this plot. Two observations
immediately present themselves: 1) for most illumination levels there is
a large, relatively illumination independent pixel-to-pixel variation
which 2) does not decrease the number of counts below the dark level
(i.e., there are no large negative numbers in the differences). Data
from the scanning solar calibrations are now used in the calibration
method to account for this "fixed pattern" noise. That it is very
reproducible for a given illumination level is evidenced by the nearly
photon-noise-limited variance of the signal from any given pixel during
a non-scanning solar DCE (e.g., Figure 7). Unfortunately, one
irreversible effect of this noise is that once the resultant of the
illumination and the noise combines to give the same output count as the
offset level, further reductions in illumination do not lower the output
level. Thus, there are uncorrectable non-linearities at low illumination
levels. Prior to launch, the only time all spacecraft systems could be
operated was during thermal vacuum testing and the "fixed pattern" noise
was not seen in LAC data from the last illuminated test carried out at
that time. Since subsequent LAC tests were carried out in
non-illuminated conditions, all that may be said is that this noise
appeared sometime between the last thermal vacuum test and the first
on-orbit DCE. The most likely causes of this systematic noise component
are: 1) although the thermal-vacuum tests were designed to closely
simulate orbital conditions it is possible that there is a system
interference that was not present in these simulations, 2) launch
vibration could have altered the LAC and, 3) power supply filter
components that were stressed by an overvoltage condition during the
conducted susceptibility test may have failed. The causes are still
being investigated.

### 3.2.3 Image Quality

Prior to launch, the image quality was checked by imaging a simulated
source at infinity onto a single pixel. The on-orbit terrestrial surface
images indicate that the focus has remained good after launch. As a
further test, images of the Moon obtained during the lunar calibration
scans have been used to determine the rise distance of a sharp boundary.
The results, as illustrated in Figure 9, show a rise from 5%
illumination to 95% illumination in approximately 1 pixel, again
indicating excellent focus and image quality.

![](media/image9.png){width="5.998611111111111in" height="3.0in"}

[]{#__RefHeading___Toc12674456 .anchor}Figure 9. Lunar image (expanded
by a factor of 8) showing rise and fall distances (5% to 95%) of 8 to 10
pixels (on expanded scale), indicating single pixel imaging capability.
Without expansion the moon fills about 20% of an array (\~25 pixels).

### 3.2.4 Out-of-band Transmittance

A concern frequently raised about wedged filter spectral imagers is that
there may be excessive out-of-band spectral transmittance. If this were
significant, then flux measurements at any wavelength would be
contaminated by the flux at all other wavelengths in a complex manner.
LAC solar calibration measurements show that the integrated effect of
out-of-band transmittance is less than 1%. This is illustrated in figure
10, which shows a plot of the relative response of the LAC, averaged
over all three arrays, as a function of λ for wavelengths near the
InGaAs response cut-off. The sharp cut-off at wavelengths less than 1 μm
follows the curve expected for these back-illuminated arrays. For
wavelengths where the in-band transmission maximum is less than
\~0.93μm, the maximum signal is always less than 1% of the available
in-band flux because the detector elements are not sensitive to these
short wavelengths. If there were significant out-of-band transmittance,
wavelengths longer than the array cut-off would impinge on the
detectors, which would give rise to a signal.

![](media/image10.wmf){width="4.5in" height="3.373611111111111in"}

[]{#__RefHeading___Toc12674457 .anchor}Figure 10. Relative shortwave
response for LAC solar calibration. The lack of signal at wavelengths
less than 0.93 μm indicates that there is little out-of-band filter
transmittance.

### 

### 3.2.5 Absolute Radiometric Calibration and Spectra

Because of the relatively recent identification of the "fixed pattern"
noise in the images, the absolute radiometric calibration is still
proceeding. The ability to create spectra is illustrated in Figure 11,
which shows two spectra obtained in regions near the Suez Canal. The
differing water vapor absorption is clearly visible here.

![](media/image11.wmf){width="6.002777777777778in"
height="2.4034722222222222in"}

[]{#__RefHeading___Toc12674458 .anchor}Figure 11. Two spectra obtained
in different regions near the Suez Canal. Note the much deeper
absorption at 1.13 μm in the right panel, indicative of greater
atmospheric water vapor.

### 3.2.6 Co-location With Landsat 7

As part of the data processing, single wavelength LAC spectral images
are placed on a latitude-longitude grid for comparison with Landsat 7
multispectral data. This involves two steps: 1) using the complete pixel
by pixel central wavelength map described in Section 3.1, pixels within
½ resolution element of a desired wavelength are selected and also the
angular position of these pixels is taken from the position map
described in Section 3.1; and 2) the angular positions are projected
onto a surface latitude-longitude map using the spacecraft attitude
information provided as part of the ancillary data on each data tape. As
shown in Figure 12, this process produces accurately positioned maps,
further verifying the pre-launch angular measurements. The potential
utility of the LAC for correcting Landsat 7 data is illustrated here,
since the real-time atmospheric features are the same in both data sets.

![](media/image12.wmf){width="5.502083333333333in"
height="2.472916666666667in"}

## 

[]{#__RefHeading___Toc12674459 .anchor}Figure 12. Overlay of Landsat 7
image on LAC image taken within 1 minute. Railroad Valley, Nevada (left)
and Cuprite, California (right) are used as calibration areas for these
instruments. The features in the images are well correlated.

## 3.3 On-Orbit Usage Experience

As of this writing, the LAC has obtained approximately 1600 DCEs without
any instrument failures. As shown diagrammatically in Figure 13, Level 0
image data is received from the GSFC Data Processing Facility (DPF) on
tape media and processed to Level 1R (radiometrically calibrated) data
at the LAC DPF.

![](media/image13.wmf){width="5.8125in" height="2.2819444444444446in"}

[]{#__RefHeading___Toc12674460 .anchor}Figure 13. Schematic of EO-1 LAC
data flow. ACCS (Atmospheric Corrector Calibration System).

More highly processed data sets, such as the geo-located data sets, are
done on an as needed basis and require more computation time. These
processes are currently being automated as well. Data is provided to
researchers as requested through the GSFC Science Validation Facility
(SVF). Currently, research requests through the EO-1 science validation
NASA Research Announcement (NRA) are supported.

To correct for the "fixed pattern" noise an extended calibration
procedure was created for the ACCS. The standard offset and scale
calculation for converting raw values to radiometrically calibrated ones
is exchanged for a discreet look-up-table (LUT) approach. The LUT is an
indexed array that contains a scaled value for every raw value within
the dynamic range of the system. Solar scanning and staring DCEs are
used in generating the calibration dataset. Direct solar irradiance, not
modified by the earth's atmosphere, may be detected through the
calibration inputs of the LAC, designed specifically for this purpose.
Light passing through a calibration port is uniformly scattered by a
Spectralon surface onto the image plane. In scanning solar DCEs the
average transmission of sunlight through the calibration inputs varies
with illumination angle, and all levels from 0 to 100% are recorded
during rotation of the spacecraft around an axis perpendicular to the
line of site. The relative response of each detector pixel to uniform
illumination may be determined in this way. Staring solar scans may be
used to refine the estimates of pixel response at the corresponding
input level. The individual response "functions" of the LUT represent
corrections of each pixel relative to one or a group of "master" pixels.
Absolute radiometric correction of detector response is achieved by
rescaling LUT values according to the ratio of response of master
pixel(s) values in lunar calibration scans to the U.S. Geologic Survey
(USGS) model of moon intensity \[Keifer, private communication, 2002\].
Intercomparisons of LAC data for Earth scenes with ground truth
measurements, and data from independently calibrated sensors (Hyperion)
may be used to verify/refine the first stage correction.

The ACCS extended calibration procedure may be briefly summarized as
follows. Reducing solar scan data to numbers for the calibration LUT is
an automatic process, nevertheless it is time consuming. The pertinent
frame subset of the solar scan is selected. Spurious noise is filtered
from each pixel scan profile. The location and value of the peak of the
profile is estimated by a low order fit function. The ratio of response
relative to the master pixel(s) for each input level is saved in a
binary file, the calibration LUT, where each column-row layer of the 3
dimensional array corresponds to a single input level. The relative
scale of LUT response "functions" is refined with staring solar DCE
data. The result of the LUT procedure is a reduction of the systematic
error in the calibrated result to 5%. Converting Level-0 data to
Level-1R is computer-resources intensive and can take four times as long
as the standard calibration procedure. To conserve disk space and
maximize access speed, values are stored as 16 bit integers with a real
type-scaling factor for translating to radiometric units. For the
correction subprocedure, input levels are indexed to relate pixel values
to directly array elements. In other words, The LUT is constructed such
that raw integer counts identify the location of calibrated values in
the result. Typically, the calibration file must be accessed randomly
10^8^ times in converting a Level-0 dataset to Level-1R. A caching
algorithm planned for the next release of the ACCS software will improve
performance significantly.

Figure 14 illustrates the improvement in uniformity gained by using the
LUT obtained in two steps from

the scanning and fixed solar calibrations.

  ----------------------------------------------------- ---------------------------------
  ![](media/image14.png){width="3.3847222222222224in"   []{#__RefHeading___Toc12674461
  height="8.315972222222221in"}                         .anchor}Figure 14. Efficacy of
                                                        "fixed pattern" noise correction.
                                                        The bottom panel shows the
                                                        variability in a single
                                                        wavelength without correction.
                                                        The second panel shows how the
                                                        variance is reduced after
                                                        scanning solar cal while the top
                                                        panel shows the final result.
                                                        Note how variance is reduced by
                                                        both steps.

  ----------------------------------------------------- ---------------------------------

[]{#__RefHeading___Toc7511897 .anchor}

# 4. LESSONS LEARNED

1)  The procurement process must be started as early as possible, using
    planning purchase requests if necessary. By using zero dollar
    planning purchase requests to start the long-lead purchasing process
    before funds were allocated, but after the instrument was approved
    for the mission, the LAC team was able to advance the contract award
    dates for items such as the detector arrays and the linear variable
    (wedged) filters by several months.

2)  All subsystem elements should be subjected to internal, informal,
    independent peer reviews early in the design cycle. The LAC
    initially had particular problems obtaining a reliable, well-modeled
    design for one of the electronic boards (the digital board). An
    independent peer review identified several inherent design flaws,
    which were subsequently corrected. This allowed a robust working
    digital board to be fabricated. Several other lesser problems in
    other subsystems were also caught by good independent reviews.

3)  Appropriate computer modeling tools should be used for all complex
    circuitry. One of the problems with the digital board design process
    was that the initial software used to model the behavior of the
    field programmable gate arrays (FPGAs) and the microcomputer on the
    digital board did not provide a high-fidelity representation of the
    behavior of the actual hardware. Thus, results from the model were
    not borne out by actual hardware tests. This meant that multiple
    FPGAs were irreversibly programmed and incorporated into what was
    supposed to be the flight version of the digital board. Since the
    models did not correctly predict the behavior of the board, this
    wasted time and money.

4)  Instrument delivery schedules should be driven by a realistic
    assessment of the launch schedule, and not by sliding deadlines. The
    LAC was delivered approximately one year before shipment for launch.
    Much of this time could have been spent in better calibrating the
    instrument.

5)  The number of formal, external reviews should be limited to one, or
    two at most. These reviews absorb resources in the form and time and
    money and rarely identify problems not already identified by the
    team. Conversely, informal contacts with knowledgeable outside
    reviewers should be encouraged.

6)  The ability of an agile spacecraft to provide calibration
    opportunities not specifically planned before launch is an extremely
    valuable asset. These possibilities should be anticipated as much as
    possible during mission planning, so that calibration procedures can
    be improved. The LAC was able to use a nodding solar scan, a
    maneuver not planned before launch, to identify and correct a
    systematic noise source that apparently appeared after launch.

# 5. CONTACT INFORMATION

# 

+---------------------------------+------------------------------------+
| Dr. Dennis C. Reuter            | Mr. George H. McCabe               |
|                                 |                                    |
| NASA/GSFC                       | NASA/GSFC and Catholic University  |
|                                 | of America                         |
| Code 693 (Planetary Systems     |                                    |
| Branch)                         | Code 693                           |
|                                 |                                    |
| Greenbelt, MD 20771             | Greenbelt MD 20771                 |
|                                 |                                    |
| Phone: 301-286-2042             | Phone: 301-286-8283                |
|                                 |                                    |
| Fax: 301-286-0212               | Fax: 301-286-0212                  |
|                                 |                                    |
| e-mail:                         | e-mail:                            |
| <dennis.c.reuter@gsfc.nasa.gov> | [george.mccabe@gsfc.nasa.go        |
|                                 | v](../george.mccabe@gsfc.nasa.gov) |
+---------------------------------+------------------------------------+

# 6. SUMMARY

The LAC on EO-1 has successfully demonstrated a wedged filter
hyperspectral imager on a space-based platform. This technology is
highly desirable because of its inherent mechanical, electrical, and
optical simplicity, its low mass and its robust nature because of the
total avoidance of moving parts. The two-module design allows a wide
range of optical design possibilities to be accommodated by changing the
lenses or filters and/or arrays in the optics module while keeping the
interface and power functions unchanged in the electronics module.
Conversely, different spacecraft command and data interface
architectures may be accommodated by modifying the electronics module
and leaving the optics module unchanged. Although, on first glance, the
data produced by imagers of this type are unusual, the data system
developed for EO-1 produces products identical to those produced by more
conventional optical systems. Indeed, the wedged filter system may be
used for any application requiring low to moderate spatial resolution
hyperspectral imaging data, except for those with a rapid temporal
variation (i.e., variability on a time scale shorter than that required
to scan the spectrum). The wedged filter system is especially attractive
for applications covering a wide wavelength range, but where only
segments of the spectrum are needed. In this case, a single array may be
used segmenting the wedge to cover only the spectral ranges desired.

The LAC has also demonstrated the utility of space based natural sources
for calibration purposes. In future systems the solar calibration tubes
will have calibrated off-axis performance characteristics, so that
scanning solar calibrations may be used to provide absolute radiometric
calibration as well as relative measures. Future atmospheric correctors
will also employ InGaAs arrays whose lattice matching InP layers have
been thinned to allow shorter wavelength operation. These arrays already
exist and will allow operation throughout the visible spectral region,
increasing the usefulness for aerosol monitoring.

## 6.1 Water Vapor Retrieval Example

Figure 15 illustrates the usefulness of the LAC in determining
integrated vertical atmospheric water vapor abundances. The example
shown here is for an area near Venice, Italy. The left panel shows a
measure of the atmospheric transmittance as defined by the ratio
I(1.0945)/I(1.0425), where I(λ) is the measured reflected intensity at
wavelength λ. Since atmospheric water vapor absorbs radiation at 1.0945
μm, while 1.0425 μm is relatively free of water vapor absorption, this
ratio is larger (approaching a maximum of 1) when the atmosphere is dry,
while it decreases (approaching 0) as the water vapor increases. The
wing of the band (1.0945 μm) is used rather than the band center at
about 1.133 μm, because the band center is frequently saturated,
decreasing sensitivity at this wavelength. The right panel shows an
image of the scene at 1.0463 μm, another wavelength at which there is
little water vapor absorption. This band is used for comparison rather
than the image at 1.0425 μm used in the ratio, to avoid serendipitous
correlations. Note that regions of clouds in the monochromatic image
(bright) show up as regions of low water vapor (also bright) in the
ratio image. This is because the reflecting clouds are at higher
altitude than the surface and therefore there is less integrated water
above the cloud layer than above the surface. Also note that the water
vapor decreases in the areas of raised topography for a similar reason.
This rough estimate will be improved upon with better modeling of the
atmospheric water vapor band, but it already illustrates the usefulness
of the approach.

![](media/image15.wmf){width="5.998611111111111in"
height="2.111111111111111in"}

[]{#__RefHeading___Toc12674462 .anchor}Figure 15. Comparison of
retrieved water vapor (left panel) with monochromatic image in the
1.0463 μm relative window region. Regions of reduced atmospheric water
vapor show up as brighter areas on the left. Note the bright spots in
the top center of both images. This is a region of clouds, which show up
as a reduced water vapor because the column path is reduced. Also note
the expected lower water vapor amounts above regions of high topography.

# 7. REFERENCES

\[1\] \"The Highly Integrated Pluto Payload System (HIPPS): A
Sciencecraft Instrument for the Pluto Mission. \" S. A. Stern , D. C.
Slater, W. Gibson, H. J. Reitsema, Alan Delamere, D. E. Jennings, D. C.
Reuter, J. T. Clarke, C. C. Porco, E. M. Shoemaker and J. R. Spencer,
*SPIE Proceedings, EUV, X-RAY and Gamma-Ray Instrumentation for
Astronomy VI*, **2518**, 39 - 58, San Diego, CA; July 1995.

\[2\] \"Logarithmically Variable Infrared Etalon Filters.\" K. P.
Rosenberg, K. D. Hendrix, D. E. Jennings, D. C. Reuter, M. D. Jhabvala,
and A. T. La, *SPIE Proceedings, Optical Thin Films IV: New
Developments*, **2262**, 25 - 27 July, 1994, San Diego, CA.

\[3\] \"Hyperspectral Sensing Using the Linear Etalon Imaging Spectral
Array.\" D. C. Reuter, D. E. Jennings, G. H. McCabe, J. W. Travis, V. T.
Bly, A. T. La, T. L. Nguyen, M. D. Jhabvala, P. K. Shu and R. D. Endres,
*SPIE Proceedings of the European Symposium on Satellite Remote Sensing
III: Conference on Sensors, Systems, and Next Generation Satellites II,*
**2957**, 154-161, September 23-26, 1996, Taorima, Sicily, Italy.

\[4\] G. T. Elerding, J. G. Thunen and L. M. Woody. \"Wedge Imaging
Spectrometer: application to drug and pollution law enforcement.\" *SPIE
Vol. 1479, Surveillance Technologies*, Orlando, FL; April 1991.

\[5\] L. M. Woody and J. C. Dermo. \"Wedge Imaging Spectrometer (WIS)
hyperspectral data collections demonstrate sensor utility.\"
*International Symposium on Spectral Sensing Research (ISSR) 1994*, San
Diego, CA; July 1994.

\[6\] "Observations Using the Airborne Linear Etalon Imaging Spectral
Array (LEISA): A 1- to 2.5- micron Hyperspectral Imager for Remote
Sensing Applications." G. McCabe, D. C. Reuter, SC. Tsay, P. L.
Coronado, D. E. Jennings, P. K. Shu, P. Mantica, S. Cain, M. Abrams,
Arthur L. Boright and J. L. Ross, *SPIE Proceedings of the European
Symposium on Satellite Remote Sensing VI: Conference on Sensors,
Systems, and Next Generation Satellites V,* September 20-24, 1999,
Florence, It.

\[7\] Keifer, H. Private Communication, 2002.

# ACRYONYMS and ABBREVIATIONS

2-D Two-dimensional

A/D Analog-to-Digital

AC Atmospheric Corrector

ACCS Atmospheric Corrector Calibration System

ALI Advanced Land Imager

C&DH Command and Data Handling

cm centimeter

DCE Data Collection Event

DPF Data Processing Facility

EO Earth Observing

FOV Field of View

GSFC Goddard Space Flight Center

HS Hyperspectral

Hz Hertz

I Measured Reflected Intensity

InGaAs Indium Gallium Arsenide

InP Indium Phosphide

IR Infrared

K Kelvin

kg kilogram

LAC LEISA Atmospheric Corrector

LEISA Linear Etalon Imaging Spectral Array

LUT Look-up Table

LVE Linear Variable Etalon

Mbits/s Megabits per second

mrad milliradian

MS Multispectral

N~2~ Nitrogen

nm nanometer

NRA NASA Research Announcement

PV Photovoltaic

s second

SNR Signal-to-Noise Ratio

SVF Science Validation Facility

TEC Thermal Electric Cooler

USGS U.S. Geologic Survey

VNIR Visible and Near-Infrared

W watts

WFOV Wide Field of View

WIS Wedged Imaging Spectrometer

μm micrometer