# PART 3. TECHNOLOGY VALIDATION

# 1. INSTRUMENT TECHNOLOGY OVERVIEW

## 1.1 Introduction

The Earth Observing-1 (EO-1) mission is flying three advanced technology
verification land imaging instruments. They are the first
Earth-observing instruments to be flown under NASA's New Millennium
Program. The three instruments are the Advanced Land Imager (ALI), the
Hyperion hyperspectral imager, and the Linear Etalon Imaging
Spectrometer Array (LEISA) Atmospheric Corrector (LAC). These
instruments incorporate revolutionary land imaging technologies that
will enable future Landsat and Earth observing missions to more
accurately classify and map land utilization globally.

## 1.2 Advanced Land Imager (ALI)

![](media/image1.jpeg){width="1.5541666666666667in"
height="1.98125in"}The ALI is designed to produce images directly
comparable to those of the Enhanced Thematic Mapper Plus (ETM+) of
Landsat 7. It employs novel wide-angle optics and a highly integrated
multispectral and panchromatic spectrometer. The focal plane is
partially populated with four sensor chip assemblies (SCA). Operating in
a push-broom fashion at an orbit of 705 km, the ALI provides Landsat
type panchromatic and multispectral bands. The multispectral bands have
been designed to mimic six Landsat bands with three additional bands
covering 0.433-0.453, 0.845-0.890, and 1.20-1.30 µm. The
multispectral/panchromatic (MS/Pan) array therefore has 10 spectral
bands in the visible and near infrared (VNIR) and short wave infrared
(SWIR). The Pan covers the visible portion of the VNIR spectrum
(0.480-0.690 µm) and has a 10 m spatial resolution. The MS detectors
have a 30 m spatial resolution. With a partially populated focal plane,
the ALI wide-angle optics produces a ground swath image width of 37 km.

The following key technologies are incorporated in the ALI instrument to
achieve its dramatic cost, weight and performance advantages.

1.  Silicon Carbide Optics

2.  Wide Field of View Optics

3.  Multispectral Imaging Capability

### ![](media/image2.jpeg){width="2.34375in" height="1.9534722222222223in"}1.2.1 Silicon Carbide Optics

The telescope design incorporates Silicon carbide mirrors and an Invar
truss structure with appropriate mounting and attachment fittings.
Silicon carbide (SiC) offers the advantage of very high stiffness to
density ratio and very high conductivity to heat capacity ratio. These
characteristics are superior to currently used materials for reflective
optical systems. The high stiffness to density ratio of SiC allows
mirrors of very low weight to be designed and still maintain the
necessary surface figure to provide the performance required for
high-resolution optical imaging. Lightweight optics lead to lightweight
optical metering structures required to support them. This in turn leads
to lighter instruments and therefore lighter payloads. The high thermal
conductivity, with relatively low thermal heat capacity, property allows
minimum thermal gradients for a given heat load. This is an advantage
for an optical system in a low Earth orbit that experiences a change in
thermal boundary conditions on a regular basis.

### 1.2.2 Wide Field of View Optics

![](media/image3.wmf){width="2.4902777777777776in"
height="1.9902777777777778in"}The telescope is a f/7.5 reflective
triplet design with a 12.5-cm unobscured aperture diameter and a FOV of
15 degrees cross-track by 1.256 degrees in-track. It uses reflecting
optics throughout to cover the fullest possible spectral range. The
design uses four mirrors: the primary is an off-axis asphere, the
secondary is an ellipsoid, the tertiary is a concave sphere; and the
fourth mirror is a flat folding mirror. The optical design features a
flat focal plane and telecentric performance, which greatly simplifies
the placement of the filter and detector array assemblies. When the
focal plane is fully populated, the detector arrays will cover an
entire-185-km swath on the ground, equivalent to Landsat 7, in a
\"push-broom\" mode.

### 1.2.3 Multispectral Imaging Capability

![](media/image4.jpeg){width="2.4895833333333335in" height="1.875in"}The
Multispectral Imaging Capability (MIC) consists of the multispectral and
panchromatic components of the ALI's Focal Plane System (FPS) and the
ALI's calibration capability. Although the ALI optical system supports a
15º wide FOV, only a 3º FOV segment within the focal plane is populated
with detectors, giving a cross-track coverage of 37 km. The intent was
to provide adequate flight validation of the imaging technologies, but
within the program cost and schedule constraints. The MS/Pan arrays use
silicon-diode VNIR detectors fabricated on the silicon substrate of the
Readout Integrated Circuit (ROIC). The SWIR detectors are
mercury-cadmium-telluride (HgCdTe) photo-diodes that are indium
bump-bonded onto the ROIC that services the VNIR detectors. These SWIR
detectors promise high performance over the 0.9 to 2.5-µm-wavelength
region at temperatures that can be reached by passive or thermoelectric
cooling. The nominal focal plane temperature is 220K and is maintained
by the use of a radiator and heater controls. Application of detectors
of different materials to a single ROIC enables a large number of arrays
covering a broad spectral range to be placed closely together. This
technology is extremely effective when combined with the wide
cross-track FOV optical design being used on the ALI. This is due to the
fact that although the ALI optical design provides a large FOV in the
cross-track dimension, the FOV in the in-track dimension is much
smaller.

## 1.3 Hyperion

The focus of the Hyperion instrument is to provide high quality
calibrated data that can support evaluation of hyperspectral technology
for Earth observing missions. The Hyperion is a "push broom" instrument.
Each image frame taken in this \"push broom\" configuration captures the
spectrum of a line 30 m long by 7.5 km wide (perpendicular to the
satellite motion). It has a single telescope and two spectrometers, one
visible/near infrared (VNIR) spectrometer and one short-wave infrared
(SWIR)) spectrometer. The instrument consists of 3 physical units: (1)
the Hyperion Sensor Assembly (HSA); (2) the Hyperion Electronics
Assembly (HEA); and (3) the Cryocooler Electronics Assembly (CEA). The
HSA includes the telescope, the two grating spectrometers and the
supporting focal plane electronics and cooling system. The HEA contains
the interface and control electronics for the instrument and the CEA
controls the cyrocooler operation.

![](media/image5.wmf){width="1.8in" height="2.4in"}

The Hyperion telescope (fore-optics) is a three-mirror astigmate design.
The telescope images the Earth onto a slit that defines the
instantaneous field-of-view which is 0.624° wide (i.e., 7.5 km swath
width from a 705 km altitude) by 42.55 m radians (30 meters) in the
satellite velocity direction. This slit image of the Earth is relayed at
a magnification of 1.38:1 to two focal planes in the two grating imaging
spectrometers. A dichroic filter in the system reflects the band from
400 to 1,000 nm to one spectrometer (VNIR) and transmits the band from
900 to 2,500 nm to the other spectrometer (SWIR). The SWIR overlap with
the VNIR from 900 to 1000 nm will allow cross calibration between the
two spectrometers. Both spectrometers use a JPL convex grating design in
a 3 reflector Offner configuration and provide a spectral resolution of
10 nm. The HgCdTe detectors in the SWIR spectrometer are cooled by an
advanced TRW cryocooler and are maintained at 110 K during data
collections. Therefore, the Hyperion provides earth imagery at 30 m
spatial resolution and with a 7.5 km swath width in 220 contiguous
spectral bands at 10 nm spectral resolution.

## 1.4 LEISA Atmospheric Corrector (LAC)

![](media/image6.wmf){width="3.0097222222222224in" height="2.0in"}The
third EO-1 instrument is the LEISA Atmospheric Corrector (LAC). The LAC
uses three 256 x 256 pixel InGaAs IR detector focal plane subassemblies
in a single module. Each array is placed behind a lens covering a 5° x
5° field of view to obtain a swath width of 185 km (15 degrees). A
state-of-the-art wedged dielectric film etalon filter (a linear variable
etalon) is placed in very close proximity to a two-dimensional IR
detector array. This produces a 2-D spatial image that varies in
wavelength along one dimension. The filter is 1.024 cm x 1.024 cm and
covers the 890 to 1580 nm spectral region at a resolution of
approximately 35 to 55 cm^-1^, with a linear dependence of wavenumber on
position. Reflective ¼-wave stacked layers placed on both sides of one,
or more, ½-wave etalon cavity(s) provide the spectral resolution.
Order-sorting of the etalon is accomplished with lower resolution filter
layers. In operation, the two-dimensional spatial image is formed by a
small, wide field of view lens. Unlike the grating spectrometer that
captures the spectra at a point \"instantaneously\", the spectrum for
the LAC is obtained as the orbital motion of the spacecraft scans the
image across the focal plane in wavelength, thereby creating a
three-dimensional spectral map. The spatial resolution is determined by
the spatial resolution of the imaging optic, the image scan speed, and
the readout rate of the array. For the EO-1 application, the single
pixel spatial resolution is 360 x 360 mradian^2^, corresponding to a
single pixel field of view of 250 m x 250 m (at nadir) from a 705 km
orbit and a readout rate of approximately 28 Hz. Because the spatial
resolution is relatively coarse (250 meters) and the wedge uses light
efficiently, the optical system is compact. This design simplicity is
offset by the need to build up the spectral image over a series of
frames, increasing the satellite attitude control system requirements.
For LAC, the large pixel size minimizes this impact. Therefore, the LAC
is a high spectral (256 bands)/moderate spatial (250 m) resolution
wedged filter imager.

The primary purpose of the LAC, from a 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 high 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 objective is to provide a
demonstration of the ability of moderate spatial resolution
hyperspectral measurements to provide real-time atmospheric water vapor
correction information to high-spatial resolution multispectral
sounders. The imaging data will be cross-referenced to the Hyperion data
where the footprints overlap.