Download Modification of the ocean PHILLS hyperspectral imager for the

Modification of the ocean PHILLS hyperspectral imager for the
International Space Station and the HyGEIA program
Michael R. Corson*a, Jeffrey H. Bowlesa, Wei Chena, Curtiss O. Davisa, Clinton E. Dorrisb,
Kiera H. Gallellia, Daniel R. Korwana, Lisa A. Policastric
a
Naval Research Laboratory, 4555 Overlook Ave. SW, Washington, DC 20375
b
Boeing NASA Systems, 13100 Space Center Blvd. MC HB3-30, Houston, TX 77059
c
Analytical Graphics, Inc., 40 General Warren Blvd., Malvern, PA 19355
ABSTRACT
The Naval Research Laboratory and the Boeing Company have teamed to fly the NRL ocean Portable Hyperspectral
Imager for Low Light Spectroscopy (ocean PHILLS) on board the International Space Station (ISS). This joint program
is named the Hyperspectral Sensor for Global Environmental Imaging and Analysis (HyGEIA). Hyperspectral images
spanning the wavelength range 400 to 1000 nm will be collected at a ground sample distance of 25 m, with 10 nm
spectral binning, and 200 to 1 signal to noise over the visible wavelengths for a 5% albedo scene. These images will be
used to characterize the coastal ocean and littoral zone, crops, and forest areas. The PHILLS will also image over the
same wavelength range at 130 m GSD to produce similar environmental products over a larger ground area. This paper
will describe the modification of PHILLS required for use on the ISS, the modeled on orbit performance, and the
planned on orbit configuration.
Keywords: remote sensing, visible spectroscopy, hyperspectral, International Space Station
1. INTRODUCTION
The ocean Portable Hyperspectral Imager for Low Light Spectroscopy (ocean PHILLS) has been under development at
the Naval Research Laboratory for approximately eight years1,2. This pushbroom instrument is optimized to image the
coastal ocean, with high signal to noise over its 400 to 1000 nm wavelength range, 1.2 nm minimum band spacing, and
very low smile and keystone distortion. Several of these systems are in use on aircraft, providing data for bathymetry,
water and bottom properties, classification of on-shore vegetation and terrain, and providing the basis for algorithm
development. In 2002, the Naval Research Laboratory and the Boeing Company entered into a partnership to adapt
PHILLS to fly on the International Space Station (ISS). This joint program is named the Hyperspectral Sensor for
Global Environmental Imaging and Analysis (HyGEIA). Under HyGEIA, PHILLS will collect hyperspectral images of
the Earth through the optical quality nadir window in the Window Observational Research Facility (WORF). This paper
describes the modifications to PHILLS required for flight on the ISS, and discusses the anticipated operations and
performance.
2. THE PHILLS HYPERSPECTRAL IMAGER
The PHILLS is a high performance pushbroom-scanning hyperspectral imager developed and built at the Naval Research
Laboratory. PHILLS is designed to produce hyperspectral images of the littoral zone and provide data to develop
algorithms to map water clarity and optical properties, bathymetry and bottom type, characterization of terrain and
vegetation, and other products of value to Naval forces. The current PHILLS, shown in Figure 1, makes maximum use
of commercial off the shelf (COTS) components to minimize cost and development cycle time. The current PHILLS
uses a commercial C-mount video camera lens from Schneider Optic, Inc., optimized for the wavelength range 400 to
1000 nanometers, to image the scene onto the entrance slit of a grating spectrometer. The slit is the system field stop,
and allows only light originating from a line in the scene, parallel to the slit, to enter the spectrometer. In pushbroom
operation, the slit is perpendicular to the direction of motion, so that the line in the scene is in the cross-track direction,
and this line is swept forward to collect an image. The light passing through the slit and entering the spectrometer is
dispersed in the direction perpendicular to the length of the slit, and falls on a two-dimensional charge coupled device
* [email protected], phone 1 202 404-2475, fax 1 202 404-5869
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(CCD). The CCD is aligned relative to the dispersed image so that the spatial direction (the direction parallel to the slit)
is parallel to one dimension of the array, and the spectral direction is parallel to the second dimension. The complete
image is constructed by reading out the CCD continuously as the imager moves forward over the scene and building the
spectral image line by line.
Figure 1. The PHILLS hyperspectral imager. The imager is approximately 25 cm long, and has a mass of approximately 6 kg.
The HyperSpec™ VM-15 spectrometer used in the PHILLS, shown in cross section in Figure 2, was developed in a
collaboration between NRL and American Holographic, Inc., now Agilent Technologies. The spectrometer is an Offner
design that has inherently low smile (change in dispersion with field position) and keystone distortion (change in
magnification with spectral position), both modeled to be less than 0.1%. The spectrometer design incorporates a
convex reflective grating corrected for astigmatism, and was further optimized by selecting mirror tilts and the grating
holographic construction points to balance third- and fifth-order astigmatism. The dispersed image illuminates 1024 x
512 pixels of a 1024 x 1024 thinned, backside-illuminated CCD in a camera from PixelVision, Inc. The wavelength
range from 400 to 1000 nm is dispersed over approximately 500 pixels, yielding spectral binning as fine as 1.2 nm. The
backside illumination provides high quantum efficiency in the blue wavelengths, and the approximately 30 electron read
noise and 14 bit digitization enable the high signal to noise required to derive quality data products from the low albedo
water scenes. The focal plane assembly incorporates a zero order beam dump and an order-sorting filter to block the
second order spectrum. The PHILLS camera is controlled by a Windows-based personal computer through a PCI
interface card. The camera is usually commanded to bin internally in the spectral direction by 4 or 8, producing nominal
5 or 10 nm spectral bins, and when binning by 8 can achieve frame rates in excess of 70 frames per second. The image
data are written to the computer hard drive for later processing, analysis, and archiving.
Figure 2: Cross section of the HyperSpec™ spectrometer used in the PHILLS hyperspectral imager. The numbered components are:
1. lens mount and entrance slit, 2. fold mirror, 3. concave mirrors, 4. convex grating, 5. location of CCD.
An important adjunct to the PHILLS imager is the Optical Real-time Adaptive Spectral Identification System3
(ORASIS). ORASIS is a data processing algorithm that demixes the data in the hyperspectral image to find the
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underlying physically meaningful subspectra, or endmembers, in the scene. The endmembers can then be used to
express the hyperspectral image by specifying the quantity of each endmember in each pixel, resulting in a significant
reduction in the data volume with little degradation in data quality. This algorithm has been implemented on a personal
computer, and the data compression factor that it achieves is typically greater than ten to one.
3. THE INTERNATIONAL SPACE STATION WINDOW OBSERVATIONAL RESEARCH
FACILITY
The U.S. Destiny Laboratory Module of the International Space Station, currently on orbit, contains a 20 inch diameter,
optical quality, nadir-facing window allowing Earth observation. The Window Observational Research Facility,
manifested for launch in 2004, will accommodate experiments using the window and provide mechanical mounting,
electrical power, cooling, and data communications. The window facility is inside the Destiny Module at Station
atmospheric pressure, approximately equal to air pressure at sea level, and can be accessed by Station crewmembers for
experiment setup and maintenance. The WORF area will be able to be enclosed to provide a dark environment to
minimize reflected glare off the window.
The ISS orbits at an inclination of approximately 51 degrees, at altitudes in the range 280 and 450 km. The current
operations plan anticipates that the altitude will average approximately 380 km in 2004 when the Station PHILLS will
be in operation, with a few tens of kilometers variation around this average due to the reboost cycle. The Station
normally orbits in a Local Vertical/Local Horizontal (LV/LH) attitude, with the velocity vector nominally parallel to the
long axis of the forward pressurized module group. The observation window faces nadir, however the Station attitude
can vary over a several degree band around LV/LH. The speed of the sub Station point at the location of the Earth’s
surface is approximately 7,250 m/s.
4. PHILLS OPERATING PARAMETERS ON BOARD THE SPACE STATION
The PHILLS pushbroom mode of imaging is compatible with use on the Space Station for the HyGEIA mission. The
orbital motion of the Station corresponds to the forward motion of the aircraft environment in which the PHILLS is
designed to operate, and successive frames (readouts of the CCD) will build the hyperspectral image of the swath of the
Earth’s surface beneath the Station. The speed of the Earth’s surface due to its rotation is relatively small compared to
the speed of the sub-Station point, and will be corrected for during image processing.
In the pushbroom mode of operation, the pixel size and the focal length of the collecting lens determines the cross-track
instantaneous field of view (IFOV). The speed of the nadir point and the frame rate of the CCD determine the along
track IFOV. The IFOV and the altitude will then determine the ground sample distance (GSD). After selecting these
quantities, the camera performance for the desired GSD must be modeled to make sure that the signal to noise is
sufficient, the frame rate is within the capabilities of the camera and computer, and that other parameters are within
acceptable bounds. As discussed below, these considerations lead to the conclusion that ground motion compensation
(GMC) is required for the PHILLS on board the Station; GMC has not been required to date for aircraft operation.
The NRL and Boeing team are planning to image at two ground sample distances, 25 m and 130 m. The images at 25 m
GSD are of interest for characterizing the coastal environment, mapping water clarity, phytoplankton chlorophyll,
colored dissolved organic matter (CDOM), suspended sediments, bathymetry, on shore vegetation and terrain, and have
applications for crop and forest management. The 25 m PHILLS GSD is similar to the 20 m GSD of the NASA
Airborne Visible/Infrared Imaging Spectrometer4 (AVIRIS) hyperspectral imager, which flies in a NASA ER-2 aircraft
above most of the atmosphere. AVIRIS has produced hyperspectral images used by many research groups, and the 25
m GSD PHILLS data offers a link to compare space hypserspectral imagery to the well established AVIRIS imagery.
The images at 130 m GSD will provide similar environmental characterization at coarser resolution, with a 130 km wide
ground swath that is sufficient to capture the entire coastal zone or a substantial land area. The GSD will be changed by
manually changing lenses, which will be done by a Station crewmember.
The cross-track dimension of the ground sample is determined by the effective pixel size at the spectrometer entrance
slit, the altitude, and the focal length of the collecting lens. The pixel dimension of the PixelVision CCD in the PHILLS
is 12 microns square, and the magnification ratio of the HyperSpec™ VS-15 Offner spectrometer is unity so that the
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effective pixel size at the entrance slit is also 12 microns. The anticipated Station altitude during operation is 380 km, so
that imaging at 25 m GSD requires a 180 mm focal length lens. The along track dimension of the ground sample equals
the speed of the sub Station point divided by the camera frame rate. Ignoring the Earth’s rotation, the ground speed of
the sub Station point is approximately 7,250 m/s, so that square 25 m x 25 m ground samples would require a frame rate
of 290 frames per second for a nadir-pointing line of sight. This frame rate is beyond the capability of the PixelVision
camera, and modeling shows that even if the frame rate were possible the signal to noise would be unacceptable with no
ground motion compensation because of the short integration time. Therefore, it is necessary to implement ground
motion compensation for PHILLS imaging from the Station at this GSD.
With GMC the imager line of sight is pointed at a forward looking slant angle at the beginning of the imaging sequence,
and the line of sight rotates smoothly from forward to aft during imaging. The motion of the line of sight is selected so
that the apparent speed of the ground across the field of view is reduced by the factor necessary to achieve the desired
frame rate and signal to noise ratio. For hyperspectral imaging, the practical limits on the forward and aft angles are
approximately 30 to 45 degrees relative to nadir, to limit the effects of the additional atmosphere resulting from the slant
angles. Of course, GMC limits the length of a continuous ground swath.
The total cross track field of view for 25 m GSD is 3.8 degrees, which is comparable to the variation in the Station
attitude relative to LV/LH, indicating that many intended ground scenes will be only partially within the ground swath
without cross track pointing to compensate for attitude variation. Finally, the relatively narrow 25 km wide ground
swath leads to poor revisit frequencies. Based on these considerations, the Station PHILLS will implement cross track
pointing in addition to along track ground motion compensation.
A model has been developed at NRL to predict the signal to noise ratio of the PHILLS imager in the WORF, viewing
the Earth through the Destiny Module window. The spectral radiance at Station altitude is determined using Modtran
4.0, where for the results presented here the model assumes a 45 degree solar elevation, 1976 Standard Atmosphere, and
Rural 5 km aerosols. Water is a dark scene, and for this work the ocean is modeled as having a wavelength independent
5% albedo. It is important to note that for a surface albedo of 5%, almost 90% of the spectral radiance above the
atmosphere at blue wavelengths is due to scattering off the atmosphere. The effect of this atmospheric scattering must
be removed in processing to retrieve data products from the underlying signal from the water, and this consideration
leads to the requirement of a high signal to noise ratio. NRL has considerable experience deriving environmental
products from coastal ocean images collected by the NASA AVIRIS hyperspectral imager. AVIRIS flies above most of
the atmosphere so that AVIRIS images exhibit atmospheric effects similar to those expected from the Station. Based on
this experience, NRL has set a signal to noise goal for the HyGEIA mission of 200 to 1 for wavelengths in the range 400
to 700 nm, which penetrate the water. For the signal to noise computation, the total spectral radiance above the
atmosphere is considered signal, including both light reflected from the Earth’s surface and scattered by the atmosphere.
The signal to noise model also incorporates: the anticipated use of the collection lens at f/4; the transmission of the
collection lens; the reflectivity of the mirrors; the grating efficiency; the quantum efficiency of the CCD; the shot noise,
read noise, and quantization noise in the data; the level of ground motion compensation, and uses the 10 nm spectral
binning that will be used for Station imaging at 25 m. The Station PHILLS model includes the measured transmission
of a stack of witness samples simulating the multi-pane Destiny Module window, and this transmission is shown as a
function of angle of incidence in Figure 3. The reader is referred to Reference 5 for a discussion of the transmission of
the Destiny Module window. The modeled signal to noise per 10 nm spectral bin is shown in Figure 4 for the Station
For 130 m GSD images, the Station PHILLS will use a commercially available video camera lens from Schneider
Optics, Inc. that has been designed to provide good image quality over the extended wavelength range from 400 nm into
the near infrared at 1000 nm. This lens has a focal length of 35 mm. Square 130 m x 130 m ground samples require a
frame rate of 56 frames per second. This frame rate is within the capability of the PixelVision camera when the spectral
pixels are binned by eight, yielding 9.6 micron spectral bins. However, spectral smear is significant at this frame rate
because row shifting and clocking out the pixels for readout occupies a significant fraction of the frame time.
The modeled signal to noise is shown in Figure 4 for the Station PHILLS with: a 35 mm focal length collection lens
operated at f/4; no ground motion compensation; a frame rate of 56 frames per second; and 5% surface albedo. This
graph shows that without ground motion compensation, the modeled Station PHILLS signal to noise ratio falls short of
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Figure 3: Measured transmission of the viewing window in the Space Station for several wavelengths. The wavelengths given in the
figure are in nm.
Figure 4: Modeled signal to noise ratio for 25 m GSD and ground motion compensation factor of 10.
Figure 5: Modeled signal to noise ratio for 130 m GSD and ground motion compensation factors of 1 and 3
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the goal of 200 to 1 over the 400 to 700 nm range, and as mentioned spectral smear is significant at the 56 frames per
second rate. Therefore, the current plan is to take advantage of the ground motion compensation capability implemented
for the 25 m GSD images. The signal to noise ratio for 130 m GSD imaging at a ground motion compensation factor is
3 is also shown in Figure 5, and this signal to noise ratio meets the goal.
5. CONFIGURATION OF THE PHILLS SYSTEM ON THE SPACE STATION
The hardware and software modifications to PHILLS required for the HyGEIA mission are under way. The components
of the flight system are: the PHILLS imager; the two axis gimbaled mount providing ground motion compensation and
off-track pointing; a computer running under Windows 2000 that receives imaging scripts generated on the ground, time
and attitude data from the Station, operates the PHILLS, operates the mechanical shutter to take dark frames, controls
the gimbaled mount, processes the data, stores the data on ruggedized hard drives, and sends health data and some image
data to the ground; a power supply for the PHILLS, gimbal motors, and computer; and ancillary brackets and cables. A
solid model of PHILLS on the gimbal mount is shown in Figure 6. The PHILLS imager is at the center of the figure,
and this model gives some indication of the physical constraints involved in achieving motion within the WORF while
staying out of the keep-out zones. The PHILLS imager, computer, gimbal mount, and mechanical shutter are all
powered by 28 vdc from the WORF.
Figure 6. Solid model of the PHILLS and gimbal mount in the WORF. The round WORF window and a cone of incident light are in
the upper left. The computer is on the right. The gimbal mechanism is mounted to the WORF wall, shown at the bottom of the
figure, and the PHILLS imager is in the center. The figure includes some construction lines defining surfaces.
The images will be acquired according to an imaging script developed on the ground and sent to the Station PHILLS
through Station telemetry. As currently envisioned, the imaging script will contain a sequence of time-tagged tasks.
The PHILLS computer will receive the task list through the WORF data interface, receive time and attitude update
information from the Station, reset the computer clock and compute any pointing offsets required because of Station
attitude variation, turn on the camera and gimbal electronics, initialize the position of the gimbal mount, close and open
the mechanical shutter and acquire dark frames as needed, acquire the hyperspectral image, store the image data to a
hard disk inside the computer, process the image using the NRL ORASIS algorithm for hyperspectral data compression,
send health and status information to the Station telemetry system, and shut down the system if required.
Health and status telemetry will be sent by the PHILLS computer to the Station data system for relay to the ground.
Current plans are for some image data to be sent via telemetry, especially during the checkout phase of the mission,
however the image data volume during operation far exceeds allowable telemetry. Therefore, the data will be stored on
removable hard disks in the PHILLS computer, which will be removed by Station crewmembers and returned to the
ground during resupply missions. The crewmembers will also change PHILLS lenses on a periodic basis to achieve the
two image resolutions.
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6. MODELED REVISIT TIMES
A simulation of three months of HyGEIA operation, from June 1, 2002 through August 31, 2002, was performed using
STK from Analytical Graphics, Inc., and the numbers of accesses to selected ground scenes during the total three
months were recorded for two GSDs. The simulation was performed during early planning stages of the PHILLS
project, and assumed slightly different GSDs than are now being implemented: in particular the study assumed 100 m
nadir GSD with no cross-track pointing; 20 m GSD with no cross-track pointing; and 20 m GSD with cross-track
pointing up to +/- 30 degrees. For this simulation, a valid access of the ground scene is defined as taking place when
any portion of the sensor field of view overlaps any portion of the selected scene, with the condition that the local solar
elevation is greater than 40 degrees to insure adequate lighting. The selected ground sites, some of which are extended,
and the number of accesses over three months are shown in Table 1. Table 1 shows that number of accesses for nonextended ground sites, such as Key West, is significantly increased by the cross-track pointing capability.
Bahamas
Bermuda
Bottom Coast South America
Camp Pendleton, CA
Chesapeake Bay, MD
English Channel
Great Barrier Reef, Australia
Gulf of Maine
Hawaii
Hobart, Tasmania, Australia
Key West FL
Lake Okeechobee, FL
Melbourne Harbor, Australia
Mississippi River Delta
Mobile Bay, AL
Monterrey Bay, CA
New Jersey Shelf
Puget Sound, WA
Santa Barbara, CA
Straits of Gibraltar, Spain
Straits of Hormuz
Tampa Bay, FL
Top Coast South America
Virginia Coast, VA
100 m GSD
no tilt
67
26
20
27
33
166
32
35
60
24
11
10
20
19
17
21
33
65
26
19
45
19
34
33
20 m GSD
no tilt
57
21
9
20
27
143
29
21
56
20
7
4
13
13
10
14
19
47
17
9
41
16
30
24
20 m GSD
with tilt
119
84
66
86
106
257
72
131
102
88
67
62
65
74
75
90
120
309
87
85
101
64
79
100
Table 1. Selected ground sites and the simulated number of valid accesses from the Station PHILLS over a three month period. The
assumed cross track pointing is +/- 30 degrees.
8. ACKNOWLEDGEMENTS
This work is supported by the Office of Naval Research, the Space Test Program, and Navy TENCAP.
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