PART 3. TECHNOLOGY VALIDATION
1. INSTRUMENT TECHNOLOGY OVERVIEW
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)
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
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
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
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
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.
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)
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 -1resolution of approximately 35 to 55 cm, with a linear dependence of wavenumber on position. Reflective ?-wave stacked layers placed on both sides of one, or more, ?-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
2 application, the single pixel spatial resolution is 360 x 360 mradian, 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.