SSUSI: Horizon-to-horizon and limb-viewing spectrographic imager for remote sensing of environmental parameters
Larry J.Paxton, Ching-I. Meng, Glen H. Fountain, Bernard S. Ogorzalek,
E.H. Darlington, S.A. Gary, J.O. Goldsten, D.Y. Kusnierkiewicz,
S.C. Lee, L.A. Linstrom, J.J. Maynard, K. Peacock,
D.F. Persons, and B.E. Smith
Johns Hopkins University, Applied Physics Laboratory, Laurel, MD, 20723-6099
Douglas J. Strickland and Robert E. Daniell
Computational Physics, Inc., Fairfax, VA
ABSTRACT
We review some of the features of the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) and describe the
environmental parameters that will be produced on an operational basis from this instrument's data. The associated algorithms are
summarized. SSUSI consists of a scanning imaging spectrograph (SIS) whose field-of-view is scanned from horizon to horizon
and a nadir-looking photometer system (NPS). The SIS produces simultaneous monochromatic images at five "colors" in the
spectral range 115nm to 180nm. The NPS consists of three photometers with filters designed to monitor the airglow at 427.8nm
and 630nm and the terrestrial albedo near 630nm. SSUSI will fly on the DMSP Block 5D3 satellites S-16 through S-19. In a
companion paper we provide more details on the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) 1.
1. OBJECTIVES
APL is building four state-of-the-art sensors for DMSP. These sensors, the SSUSI, are intended to provide a
quantitative description of the state of the upper atmosphere and the aurora on a global basis. In order for the data to be of use to
the user community, rapid, efficient, and accurate operational algorithms must be developed to convert the radiance observations
into environmental parameters. The development of the SSUSI data handling system and the operational uses of SSUSI data are
described in greater detail in an APL Technical Report2. An extensive description of the instrument is in another SPIE paper 1.
The SSUSI design reflects the operational need for the monitoring of global space weather. Memorandum of the Joint
Chief of Staff MJCS 154-86 ranked environmental parameters in order of priority. Table 1 summarizes those parameters with
priority ranking that SSUSI will measure.
TABLE 1. MJCS 154-86 Rank of SSUSI Environmental Parameters
Rank
5
12
16
18
24
26
31
Environmental Parameter
Electron Density Profile
Neutral Number Density Profile
Solar Radiation (EUV integrated flux)
Auroral Emissions and Airglow
Precipitating Electrons and Ions
Electric Fields
Ionospheric Scintillation
The FUV is ideally suited to determining thermospheric and ionospheric environmental parameters. It possesses optical
signatures of all the major thermospheric species: O, N2, and O2 (O2 is seen in absorption on the limb) and the dominant F-region
ion, O+ (on the nightside). Figure 1 shows that absorption by O2 effectively determines the lowest altitude that can be observed
in the FUV. Most of the excitation processes (e.g. solar photons, photoelectrons, or precipitating particles) deposit their energy
above 100km. Figure 2 illustrates a typical FUV spectrum. In this case it is a theoretical model of the spectrum in the night
aurora for two incident electron energies: 2.0 and 10.0keV. The more energetic electrons are deposited more deeply in the
250
N
O 2
Altitude (km)
200
O
O
2
3
absorption
atmosphere and so produce radiation that is more likely to be absorbed by thermospheric O 2. Figure 3 shows that the cross
section is strongly peaked. This fortunate happenstance allows us to use the ratio of two wavelength regions or "colors" to
determine the characteristic energy or "hardness" of the incident precipitating particles. Thus, since we understand the processes
which produce this radiation, we know that we need not telemeter down the entire FUV spectrum but can identify a few
wavelength bands or "colors" that provide all the information required for an unambiguous determination of these environmental
parameters1-11. These wavelength intervals are essentially monochromatic in that they are the signature of one process. SSUSI
will send down just five colors: HI 121.6 OI 130.4, OI 135.6, N 2 Lyman-Birge-Hopfield (LBH) bands from 140.0 to 150.0nm and
N2 LBH bands from 165.0 to 180.0nm (Note: these color definitions can be changed on orbit and in going from disk to limb). H,
O, and N2 are seen in emission and O2 in absorption. in Figure 2. Figure 2 shows how the shape of the spectrum reflects changes
in the characteristic energy of the precipitating particles. Higher energy electrons are precipitated more deeply into the atmosphere
and O2 absorption becomes more important. Figure 3 shows how the ratio of the intensity of the atomic oxygen emission feature at
135.6nm to two LBH "colors" varies as a function of the characteristic energy of the input energetic electrons. Figure 3 shows the
dependence of the O2 absorption cross section on wavelength. Comparison of Figures 3 and 4 demonstrates our point: by
sampling the long and short wavelength regions of the auroral FUV spectrum we can deduce the characteristic energy of the
precipitating electrons.
150
100
50
0
0
50
100 150 200
250 300 350
Wavelength (nm)
Figure 1. The altitude at which the pure absorber optical depth reaches unity as a function of wavelength. The approximate
region of importance of the various atmospheric constituents for absorption is shown by the dashed lines.
1000.0
Q = 1 erg cm-2 s-1
Gaussians
Eo = 2.0 kev
130.4 nm
Column Emission Rate (R/Å)
Eo = 10.0 kev
100.0
135.6 nm
10.0
1.0
0.1
1300
1400
1500
1600
Wavelength (Å)
Figure 2. The modeled response of the FUV auroral spectrum to varying the incident energy of the precipitating energetic
electrons.
4.0
2
Q = 1 er g /cm /s
In ten s ity R a tio
3.0
O I 135.6n m /N LBH 138.3-150.0n m
2
O I 135.6n m /N2 LBH 165.0-180.0n m
2.0
1.0
0.0
0
1
2
3
4
5
E 0(keV)
Figure 3. The ratio of the OI 135.6nm/N2 LBH band intensity as a function of characteristic energy. Monitoring the intensity of
selected lines in the FUV provides quantitative information about the characteristic energy of the incoming energetic electrons. In
this figure the input electron energy distribution is assumed to be a Maxwellian with an energy flux of 1 erg cm-2 s -1.
10
-16
Cross section (cm 2 )
10
10
10
-17
-18
-19
10 -20
100
120
140
160
180
200
Wavelength (nm)
Figure 4. O2 absorption cross section as a function of wavelength. The difference in absorption cross section between the
spectral region near 140nm and that near 160nm is used to determine the O 2 number density in limb observations of the dayglow
and the characteristic energy of precipitating electrons in disk images of the auroral region.
2. SCANNING IMAGING SPECTROGRAPH (SIS)
The imaging spectrograph builds multispectral images by scanning spatially across the satellite track (see Figure 5). One
dimension of the detector array contains 16 spatial pixels (along the spacecraft track), and the other dimension consists of 160
spectral bins over the range of 115 to 180 nm. The scan mirror sweeps the 16 spatial pixel footprint from horizon to horizon
perpendicular to the spacecraft motion, producing one frame of 170 cross-track lines in 22 seconds. The imaging mode scan cycle
consists of a limb viewing section followed by an Earth viewing section. Limb viewing pixels are collected from -72.8° from nadir
(the start of scan) to -63.2° from nadir. The limb viewing section has a cross track resolution of 0.4° per pixel, and consists of 24
cross track pixels by 8 along track pixels at five wavelengths. The 8 along track pixels are formed by co-adding adjacent pixels in
the 16 spatial pixel footprint. At -72.8° from nadir and a spacecraft altitude of 830 km, the spectrograph will view approximately
520 km above the horizon. Simultaneous image frames are generated over the entire wavelength range in the imaging mode, but
the data rate allocation limits the downlinked image data to five different wavelength intervals or "colors".
The 11.8° field-of-view was chosen so that there was contiguous coverage at nadir for a 22sec scan period. Figure 6
shows the amount of overlap between two consecutive scans. This overlap ensures that uninterupted coverage of large-scale
geophysical phenomena, such as the auroral oval, can be obtained. There is an additional advantage when the field-of-view is
directed toward the limb; each element on the limb is sampled on three successive scans (see Figures 5 and 6). This improves our
effective responsivity on the limb by a factor of three. This is particularly important for nightglow observations where the signal is
can be just a few Rayleighs.
Scan Mirror
Detector
an
Sc
b
m
Lim
5 k ls
44 ixe
8p
9
ca
kS
ac
r
s T els
ros 4 pix
C
2
.6°
160 spectral elements
n
16 spatial elements
+Y
11.8° FOV
Along Track Motion
148 km/22 sec
–Z
153 Km
10 Km x 10 Km resolution
16 pixels
124.8° Cross Track Scan
156 pixels
(horizon to horizon)
+Z
Figure 5. The SIS produces horizon-to-horizon images at 160 wavelengths simultaneously. The scan mirror sweeps the
field-of-view across the disk and onto the limb. The two-dimensional detector records spatial imformation in the along slit
direction and spectral information in the other. In the 22 seconds required for one complete scan cycle the spacecraft moves
148km. The 11.8° field-of-view maps 153km at the emitting layer. From scan to scan, SSUSI images overlap by at least 5km. The
limb scans are handled differently than the disk scans. On the limb, adjacent pixels are combined to reduce the required data rate.
The step size is also reduced. The projected field-of-view on the limb is about 445km. Since the spacecraft only moves about
153km along track from scan to scan, each pixel on the limb is sampled three times.
Along Track Distance (km)
500
400
300
200
Scan 1
100
Scan 2
0
-100
-200
-300
-60
-40
-20
0
20
40
60
Scan Angle (deg)
Figure 6. The overlap from scan to scan of the SSUSI imaging spectrograph (SIS) when operated in imaging mode. The scan rate
is defined so as to provide contiguous coverage. This means that an element on the limb is scanned three times before leaving the
field-of-regard. This enhances our effective responsivity.
SSUSI has another mode of operation: the spectrograph mode. In the spectrograph mode, the scan mirror is held at a
fixed viewing angle and the entire spectrum is downlinked. In order for the spectrum to fit into the available telemetry rate of
3816kbps, the integration period is increased to 2.99 seconds. The along track dimension of the detector array is binned into 6
spatial pixels. The spectrograph mode would be used predominantly during stellar calibration operations and for "ground truth"
campaigns in which we will stare at the radiating volume above a ground site.
The SIS (see Table 2) consists of a cross-track scanning mirror at the input to a 75mm focal length off-axis parabola
system with a 25mmx50mm clear aperture and a Rowland circle spectrograph. The SIS is an f/3 system with a toroidal grating.
The optical path incorporates baffles to prevent stray light from reaching the focal plane at the slit and the detector. The scan
mirror and the grating are coated with ARC Coating #1200. The telescope mirror is coated with ARC Coating #1600. This
combination of coatings was chosen to reduce the SIS responsivity to OI 130.4 and HI 121.6 nm radiation since the total input
count rate can approach 200kHz for very bright scenes. Figure 7 shows the SIS in schematic form. The scan mirror feeds the
off-axis parabola and the spherical toroidal grating. Two detectors lie at the focal plane. The secondary detector is accessed via
a folding mirror.
The imaging spectrograph contains three entrance slits of different widths corresponding to fields-of-view of 0.74°,
0.30°, and 0.18°. The intermediate width slit is intended for use during normal imaging mode operations. The widest slit will be
used in imaging mode to increase the sensitivity should the optical efficiency of the system decrease over time or to minimize the
statistical error for low count rate scenes such as when the FUV nightglow is to be observed. The narrowest slit improves the
spectral resolution. Any slit can be used in any mode of operation.
slit mechanism
nadir
telescope mirror
cross track scan range
180nm
140nm
115nm
scan mirror
toroidal grating
secondary detector
primary detector
Figure 7. The SSUSI Imaging Spectrograph (SIS). The principle features are indicated: the scan mirror which has a 140°
field-of-regard, the toroidal grating, and the two detectors. The long axis of the slit is into the page: this is the spatial dimension of
the image. The direction of spectral dispersion and the wavelength range corresponding to the active area of the detector is
indicated.
Figure 8 illustrates the flow of events through the SIS detector electronics from the arrival of a photon at the window of
the detector to its final disposition by the Electronics Contol Unit (ECU).
After a photon has created a charge cloud at the anode, the charge is collected using a low-noise charge-sensitive
preamplifier which generates a voltage step proportional to the amount of collected charge. A filter-and-amplifier network shapes
the small voltage steps into unipolar Gaussian pulses. This reduces the noise while providing a pulse-height distribution which is
easier to work with. A baseline restorer circuit monitors the output of the shaping network and eliminates any DC offsets. The
peaking time of the shaping network is independent of the signal amplitude. The three networks are matched so that their
corresponding analog-to-digital converters can be triggered from a single timing signal.
A single fast amplifier detects arrivals by amplifying the unshaped signal directly from the back of the microchannel plate
stack. The "Fast Amp" signal indicates the start of an event which drives the A/D, it guarantees the proper settling time between
events, rejects processing of "piled-up" or near-coincident events, and provides the best true input rate. The input rate is used to
"calibrate" the output signal.
Figure 9 shows the detector. The white area of the picture is the area which is binned and read out to form spectrographic
images. The resolution in the spectral dimension is adequate for the narrowest slit (0.18°) as it provides nearly triple oversampling.
The electronics require a fixed amount of time to process an event. There are two components to this: the time it takes to
do the analog-to-digital conversion and the time required to compute the digital position. Due to the statistical nature of the arrival
of events, some fraction of events will not be recorded during the deadtime incurred by these two processes. This percentage is
reflected in Figure 10 in the difference between the input rate and the output rate.
TABLE 2. SIS Characteristics Summary
Entrance Aperture
Clear aperture
Distance to mirror
Telescope Mirror
Type
Clear aperture
Off-axis distance
Distance to slit
Entrance Slit
Size (wide)
Angular Resolution
0.74° x 11.8°
Size (standard)
Angular Resolution
0.30° x 11.8°
Size (narrow)
Angular Resolution
0.18° x 11.8°
Distance to grating
Grating
Radius of curvature
Clear aperture
Type
Ruling
Focal Plane
Spatial
(Y)
Spectral (X)
20 x 25 mm rectangular
105.2 mm
Off-axis parabola
25 mm by 50 mm
22.5 mm
75 mm (along parabola axis)
0.97 mm x 15.7 mm
0.39 mm x 15.7 mm
0.236 mm x 15.7 mm
194.6 mm (along ray)
200 mm (spectral), 193.7 mm
(spatial)
65 mm (groove length) x 54 mm
(ruled width)
Toroidal
1200 grooves/mm
16.5 mm
Figure 8. Diagram for the
SIS
detector
electronics.
15.6 mm
Photons enter through a
window in the sealed tube. The
System Parameters
photon
produces
a
photoelectron
which
is
Focal length
75 mm
amplified as it cascades
through the Z stack of
F/number
3.0
microchannel plates. The
number of electron clouds
Beam diameter
25 mm
produced in the tube (hence
the true input count rate) is
monitored with the "fast
amp" circuit. The position of
the electron cloud is recorded by a position sensitive anode. We have chosen a wedge-and-strip readout. A sophisticated proven
focal plane electronics (FPE) package contains the front-end analog circuits, corrupted event rejection logic, and analog-to-digital
converters. The FPE transfers the raw digital data to a detector processing unit (DPU) which computes the 2D position for each
event and bins the event into image memory. The DPU is under ECU control which can halt the DPU and read out the
accumulated image at the end of each integration period. The Electronics Control Unit (ECU) determines which locations in the
accumulated image to downlink.
16 element
resolution
(16.5 mm)
1030 
98

160 element
resolution
(15.6 mm)
Figure 9. Active area of the detector. The position sensitive wedge-and-strip anode is read out with a spatial resolution of 16
elements and a spectral resolution of 160 elements. The data processing unit (DPU) then bins the data into 8 bins for limb
observations or selects 6 bins for spectrograph mode.
120
100
Output Rate (kHz)
80
Analog Rate
Digital Rate
60
40
20
0
0
100
200
300
400
500
600
Input Rate (kHz)
Figure 10. SIS detector throughput. The input rate is expected to be less than 200kHz for typical scenes observed by SSUSI. The
response of the detector electronics (the analog/digital conversion and the microprocessor's position calculation) are indicated.
The lower curve (labeled "digital") reflects the actual throughput measured in our laboratory.
3. NADIR PHOTOMETER SYSTEM (NPS)
The NPS operates only on the nightside. It is intended to provide the height of the F-region ionosphere in conjunction
with the SIS observations of the OI 135.6nm nightglow and to corroborate the characteristic energy and flux of precipitating
electrons in the aurora as determined by the SIS2,6. To do this, three detectors are required for the SSUSI photometer subsystem.
The detectors will be identical except for the optical filter characteristics. For 427.8 nm observations, one detector is required with
a fixed wavelength filter at 427.8 nm with a bandwidth of 5.0 nm. Two detectors are required for the 630 nm observations
because a correction must be made for the Earth albedo and the contribution from backscattered moonlight (see Figure 11) and
starlight. One detector will have a filter with a center wavelength of 630 nm and a bandwidth of 0.3 nm, and the detector
measuring background will use a filter with a center wavelength of 629.4 nm and a bandwidth of 0.3 nm. The characteristics of the
three units are summarized in Table 3.
1000
FULL
60 deg moon angle (FULL)
Intensity (R/Å)
100
3/4
1/2
10
1/4
1
2000
4000
6000
8000
10000
Wavelength (Å)
Figure 11. Dependence of the backscattered lunar radiance on wavelength and lunar zenith angle. These LOWTRAN calculations
were done for a surface albedo of 1 and for four different phases of the moon.
The photometer baffle design received a good deal of attention because the NPS may operate in a near-dusk environment
(see Figure 12). This environment is particularly difficult to model. In order to be able to function at a solar zenith angle of 98°,
a two-dimensional model of the twilight Rayleigh scattering radiation field was developed 10,11. The photometer has a glint zone
of ±25 degrees and is located on the shaded side of the spacecraft GLOB. The NPS has dual, redundant illumination sensors. The
illumination sensor triggers on earth albedo and inhibit the photometer detectors by gating the HVPS. The illumination sensor
field of view is 10 degrees which provides an adequate margin for the near-terminator orbits.
TABLE 3. NPS Characteristics
Unit #1
427.8 nm
5.0 nm
Bi-Alkali
Glass
25 mm
427.8 nm
0.5 Watts
-30°C to -20°C
Unit #2
630 nm
0.3 nm
Tri-Alkali
Glass
25 mm
630 nm
0.5 Watts
-30°C to -20°C
Pixel Field of View full angle, circular
Spatial resolution at nadir
Optic diameter
Clear aperture
Pixel Integration Time
Sensitivity (cnt/sec/Rayleigh)
Maximum count per sec
Dark count (maximum)
2.0 °
25 km
1.0 inch
0.5 inch
1.0 sec
5
500,000
40 cps
2.0 °
25 km
2.0 inch
1.8 inch
1.0 sec
30
100,000
40 cps
Unit #3
629.4 nm
0.3 nm
Tri-Alkali
Glass
25 mm
629.4 nm
0.5 Watts
-30°C to
-20°C
2.0 °
25 km
2.0 inch
1.8 inch
1.0 sec
30
100,000
40 cps
LOCAL ZENITH ANGLE (deg)
Parameter
Center Wavelength
Spectral Bandwidth
Photocathode
Input Window
Cathode diameter
Wavelength
Power
Operating Temperature (in spec)
INTENSITY (R)
Figure 12. The Rayleigh scattered intensity at 630nm observed from a spacecraft at 830km whose subpoint solar zenith angle is
98° as a function of the angle between the local zenith and the look direction. The local zenith angle of 180° corresponds to
looking straight down. There the intensity is about 25R. If the observer were to look at 150° the intensity has increased by about
three orders of magnitude. This curve is an azimuthal average.
4. OVERVIEW OF THE ALGORITHM DEVELOPMENT EFFORT
Operational software will be installed at the Space Forecast Center (SFC). This software will be used to automatically
convert the SSUSI data into environmental parameters. Figure 13 shows how the SSUSI Sensor Data Records (SDR) are
converted into Environmental Data Records (EDR). This approach does not require that "research" or "first principles" codes
and be supported at Space Forecast Center or any other operational environment. The first product from the ingest process are
summary images based on the SDR. The SDR is SSUSI data that have been time-tagged, calibrated14,15, and geolocated. Some
software is, of course, required to map these data onto a display. To do this an SDR from a single orbit is regridded onto what we
call a "user grid". These radiance maps could be archived and would then provide an immediate resource for making qualitative
comparisons different orbits, days, seasons, portions of the solar cycle, etc. Driven by user requirements for resolution and
accuracy, we put the radiance data onto another user grid (this time by region: day, night, or aurora). This regridded data is then
the direct input to the simple algorithms that are tailored to specific regions.
After the gridding, the region-specific algorithms can operate on the data. These are shown in a top-level view in Figure
13. Each element has been described in detail2. Note that the three algorithm processes can occur concurrently and that each is
designed to be capable of independently producing EDR. The software developed for the calculation and display of environmental
parameters determined from the SSUSI instrument will be an integrated, interactive system. It will provide a tool for visualizing
the remote sensing products defined in this document. The software will support strong interactions with the system, data, and
user. The environmental parameters routinely produced by SSUSI are summarized in Table 4. The SSUSI algorithms are slated for
delivery by January 1995.
AURORA
NIGHT
DAY
boundary specification
midlatitude Nm F2
solar EUV flux index, Qeuv
HI 1216 geocoronal background
midlatitude hm F2
O/N2 ratio on disk
proton flux, Qp
midlatitude wind field
exospheric temperature, Texo
electron flux, Qe
low latitude Nm F2
neutral density profiles (NDP): O,
N2 , and O2 on limb
characteristic electron energy,
low latitude hm F2
Ee
E-layer electron density profile,
NDP for O, N2 , and O2 on
disk
low latitude winds
midlatitude EDP
critical frequency, fo E
low latitude E field
low latitude EDP
height of layer, hm E
midlatitude trough location
EDP
TABLE 4. SSUSI Products
SDR's for
one orbit
Calibration
factors
Calibration
(1)
Gridding
(data and LOS information
on user grids)
(2)
LOS info
LOS info
LOS info
LOS info
LOS info
Gridded
auroral disk
images
Gridded
nighttime
disk images
Gridded
nighttime
limb images
Gridded
daytime disk
images
Gridded
daytime limb
images
Databases
containing
model
results
Auroral
algorithms
(3)
Databases
containing
model
results
Nighttime
algorithms
(4)
Databases
containing
model
results
Daytime
algorithms
(5)
Solar &
geomagnetic
indices
SSUSI single
sensor auroral
EDR's
SSUSI single
sensor nighttime
EDR's
SSUSI single
sensor daytime
EDR's
Figure 13 An overview of the algorithm development scheme. The numbers in parentheses correspond to more detailed
flowcharts which are not reproduced here but may be found in an APL report 2.
5. ACKNOWLEDGEMENTS
This work has been supported through DMSP under Task LBJ at the Applied Physics Laboratory. We acknowledge
assistance at APL from A.G. Bates, R.E. Gold, P. Hemler, K.H. Sanders, and H.H. Wright. SSG, Inc is fabricating the SIS. The
project team at SSG includes D. Wang (SSG SIS Program Manager), A. Mastandrea, L. Gardner, P. Hadfield, H. Luther, P.
Cucchiaro, R. Glasheen, and W. Brady. The algorithm development effort includes contributions from David N. Anderson at
Phillips Laboratory. Donald E. Anderson performed the LOWTRAN calculations of Figure 11. J. Scott Evans, Ray Barnes, and
Robin Cox were instrumental in providing computational support for this work. The authors appreciate the comments of the
Conference Chair, Robert E. Huffman, on an earlier version of this paper.
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