Download GEO_BUV_AOMSUC_LEF_Poster_D3

Solar Backscatter Ultraviolet Instruments (BUV) on Satellites in L-1 and GEO Orbits
Lawrence Flynn1, Kelly Chance2, Jhoon Kim3, Ernest Hilsenrath4, Jay Herman4, Berit Ahlers5, and Ruediger Lang6
1NOAA
NESDIS, 2Smithsonian Astrophysical Observatory, 3Yonsei University, 4Joint Center for Earth Systems Technology UMBC, 5ESA/ESTEC, and 6EUMETSAT
Introduction
This poster describes four planned missions which will add Lagrange Point 1 (L-1) and geostationary (GEO) assets to the global BUV
satellite instrument complement and examines their relation to projects of the newly formed GSICS Research Working Group Ultraviolet
Subgroup (GRWG UVSG). Measurements from the new instruments / platforms will expand our capability to monitor the diurnal
variations of atmospheric constituents including ozone, UV absorbing aerosols, SO2, NO2 and other trace gases. The three instruments on
GEO platforms will provide opportunities to apply Low-Earth Orbit (LEO)/GEO comparison techniques (currently in use by GSICS
Research Working Groups for infrared and visible sensors) as measurements from these new instruments become available. This will lead
to improved calibration products for existing polar-orbiting BUV instruments. Even before the GEO instruments become available there
will be a new BUV/Visible instrument, the Earth Polychromatic Imaging Camera (EPIC), operating from L-1 opening new areas for
LEO/L-1 and GEO/ L-1 under-flight comparisons. Looking back in time, the BUV experience already includes SS/LEO [Space Shuttle
SBUV / NOAA Polar-orbit Operational Environmental Satellite (POES) SBUV-2] under-flight comparisons.
The first section of the poster describes the missions, measurements, products and applications for the four new instruments. The second
part looks at how these new measurements will be used with GSICS. The recently formed GSICS Research Working Group UV Subgroup
(GRWG UVSG) has started projects which will grow to include these new instruments. Some details on the projects are in the latest issue
of the GSICS quarterly available at http://www.star.nesdis.noaa.gov/smcd/GCC/newsletters.php . Additional information on how some of
the projects will include these new measurements is also given in the second part of this poster.
Geostationary Missions
By 2020, the geostationary orbits are expected to be filled with three
UV-visible spectrometers, the NASA Tropospheric Emissions:
Monitoring of Pollution (TEMPO) (P.I.: Kelly Chance, HarvardSmithonian Center for Astrophysics) over North America, the ESA
Sentinel-4 Ultraviolet Visible Near infrared (UVN) spectrometer over
Europe (Mission Scientist: Ben Veihelmann, ESTEC), and the KARI
GEMS over Asia (P.I.: Jhoon Kim, Yonsei University), with the
TROPOspheric Monitoring Instrument (TROPOMI) and Ozone
Mapping and Profiler Suite (OMPS) flying underneath in Low-Earth,
sun-synchronous, polar Orbits (LEO). Instrument attributes are
provided in the table to the right, and their expected coverage is
shown below.
Spatial coverage
Full Disk Obs. time
Onboard calibration
Volume (m3)
Mass (Kg)
Power (W)
Data rate (Mbps)
Ref.
TEMPO
290 – 490 / 540 - 740
0.6 (3 samples)
Sentinel-4 UVN
305-500 / 750-775
0.5 / 0.12
2.1 × 4.7
8 × 8 @ 45 N
19°N – 57.5°N
73°W – 130°W
1 hour
Solar
1.0 × 1.1 × 1.0
100
30°N – 65°N
30°W – 150°W
1 hour
Solar, cal light source
1 × 1 × 1.5
200
100
180
30
Kelly Chance
30
Ben Veihelmann
GEMS Instrument Description
DSCOVR EPIC Instrument
The GEMS(Geostationary Environment Monitoring Spectrometer) is
going to be launched into orbit at the end of 2018. It will be positioned
over Asia. The instrument is basically a step-and-stare scanning UVvisible imaging spectrometer, with scanning Schmidt telescope and
Offner spectrometer. A UV-enhanced 2-D CCD array takes images with
one axis spectral, and the other N-S spatial, with E-W scanning over
time. On-orbit calibrations are planned by making daily solar
measurements and weekly LED light source linearity checks. For the
solar calibration, there are two transmissive diffusers, a daily working
one and a reference diffuser used twice a year to check the degradation
of the working one. Dark current measurements are planned twice a day,
before and after the daytime imaging. In order to avoid dark current
issues and random telegraph signal (RTS), the CCD is cooled to
temperatures well below 0°C.
Spectral stability is required to be better than 0.02 nm over 24 hours,
stray light less than 2%, polarization sensitivity less than 2% at the
instrument level, and the instrument system level MTF better than 0.3.
Preflight calibration will be carried out by using the NIST standards.
Sentinel-4
(hourly)
TEMPO
(hourly)
Much of the material on given here on EPIC is taken from publically available materials contained in the posters and presentations of the
session ‘Earth Observations From the L-1 (Lagrangian Point No. 1)’ at the AGU 2011 fall meeting, San Francisco.
The Deep Space Climate Observatory (DSCOVR a joint NASA/NOAA project) will have a continuous view of the entire sunlit face of the
Earth. (http://www.nesdis.noaa.gov/DSCOVR/), DSCOVR will make unique space measurements from the first sun-Earth Lagrange point
(L-1). The L-1 point is on the direct line between Earth and the sun located 1.5 million kilometers sunward from Earth, and is a neutral
gravity point between Earth and the sun. The spacecraft will be orbiting this point in a six-month orbit with a spacecraft-Earth-sun angle
varying between 4 and 15 degrees.
The NASA Earth Polychromatic Imaging Camera (EPIC) instrument provides spectral images of the entire sunlit face of Earth, as viewed
from an orbit around L-1. EPIC is able to view the entire sunlit Earth from sunrise to sunset. EPIC's observations will provide a unique
angular perspective, and will be used in science applications to measure ozone and aerosol amounts, cloud height, vegetation properties
and ultraviolet reflectivity of Earth. The data from EPIC will be used by NASA for a number of Earth science developments including dust
and volcanic ash maps of the entire Earth. EPIC makes images of the sunlit face of the Earth in ten narrowband spectral channels. As part
of EPIC data processing, a full disk true color Earth image will be produced about every two hours. This information will be publicly
available through NASA Langley Research Center in Hampton, Virginia, approximately 24 hours after the images are acquired.
The Earth Polychromatic Imaging Camera (EPIC shown to the right) is a Cassegrain Telescope
imaging the disc of the Earth on a 2048 x 2048 pixel CCD array temperature stabilized at -40°C. The
irradiances pass through two filter wheels with six positions each (open hole plus five spectral
channels) providing ten narrow band channels in the UV and visible. Nominal exposure time is 40
msec for each channel providing signal to noise ratios for 250:1 at 80% well filling. Its temporal
resolution will depend on the final schedule of the data-downlink for all instruments on DSCOVR.
There is minimal overlap between EPIC’s and other satellites’ scattering angles. Therefore EPIC’s
observations from L-1 will provide a unique angular perspective and can be combined with other
measurements to obtain particle shapes, phase selection, optical depth, 3-D effects and stereo
heights.
Spectral ranges (nm)
Spectral resolution (nm)
Spatial Resolution
(NS km × EW km)
GEMS
300 – 500
0.6 (3 samples)
7 ×8 @ Seoul, 3.5 ×8 for
aerosol
5°S – 45°N
75°E – 145°E
30 min
Solar, cal light source
1.1 × 1.2 × 0.9
140
200 (on orbit) /100
(transfer)
40
Jhoon Kim
GEMS
(hourly)
TEMPO Science Traceability Matrix
Science Questions
Science Objective
Science Measurement Requirement
Observables
Q1. What are the
temporal and spatial
variations of emissions of
gases and aerosols
important for AQ and
climate?
Q2. How do physical,
chemical, and dynamical
processes determine
tropospheric composition
and AQ over scales
ranging from urban to
continental, diurnally to
seasonally?
Q3. How do episodic
events affect
atmospheric composition
and AQ?
Expected EPIC Data Products
Q4. How does AQ drive
climate forcing and
climate change affect AQ
on a continental scale?
Ozone: Total Column
Aerosol Properties: aerosol Index, optical thickness & height
Cloud & Surface Properties: surface albedo,
cloud fraction & height
Vegetation Properties: vegetation index & leaf area index
RGB: colored image of the Earth’s sunlit face
Q5. How can
observations from space
improve AQ forecasts
and assessments for
societal benefit?
Q6. How does transboundary transport affect
AQ?
A. High temporal resolution
measurements to capture changes
in pollutant gas distributions. [Q1,
Q2, Q3, Q4, Q5, Q6]
B. High spatial resolution
measurements that sense urban
scale pollutant gases across GNA
and surrounding areas. [Q1, Q2,
Q3, Q5, Q6]
C. Measurement of major elements
in tropospheric O3 chemistry cycle,
including multispectral
measurements to improve sensing
of lower-tropospheric O3, with
precision to clearly distinguish
pollutants from background levels.
[Q1, Q2, Q4, Q5, Q6]
D. Observe aerosol optical
properties with high temporal and
spatial resolution for quantifying and
tracking evolution of aerosol
loading. [Q1, Q2, Q3, Q4, Q5, Q6]
E. Determine the instantaneous
radiative forcings associated with
O3 and aerosols on the continental
scale. [Q3, Q4, Q6]
F. Integrate observations from
TEMPO and other platforms into
models to improve representation of
processes in the models and
construct an enhanced observing
system. [Q1, Q2, Q3, Q5, Q6]
G. Quantify the flow of pollutants
across boundaries (physical &
political); Join a global observing
system. [Q2, Q3, Q4, Q5, Q6]
Spatially imaged &
spectrally resolved,
solar backscattered
earth radiance,
spanning spectral
windows suitable for
retrievals of O3, NO2,
H2CO, SO2 and C2H2O2.
[ A, B, C, E, F, G ]
Measurements at spatial
scales comparable to
regional atmospheric
chemistry models. [ A,
B, C, D, F, G ]
Multispectral data in
suitable O3 absorption
bands to provide vertical
distribution information.
[ A, B, C, E, F, G ]
Spectral radiance
measurements with
suitable quality (SNR) to
provide multiple
measurements over
daylight hours (solar
zenith angle < 70°) at
precisions to distinguish
pollutants from
background levels.
[ A to G ]
Spatially imaged,
wavelength dependence
of atmospheric
reflectance spectrum for
solar zenith angles
<70°.[ B, D, E, F, G ]
Physical Parameters
Instrument Function Requirements
Parameter
Required
Predicted
Baseline#* Trace gas column densities (1015 cm-2) hourly @ 8.9 km x 5.2 km
Species
Precision
Band
Signal to Noise
O3: 0-2 km
10 ppbv
O3:Vis (540-650 nm)
≥1413
1765
O3: FT
10 ppbv
O3: UV (290-345 nm)
≥1032
1247
O3: SOC
5%
O3: Total
3%
NO2
1.00
423-451 nm
≥781
2604
H2CO
17.3
327-354 nm
≥742
2266
SO2
17.3
305-330 nm
≥1100
1328
C2H2O2
0.70
433-465 nm
≥1972
2670
Property
AOD
AAOD
AI
CF
COCP
Baseline#* Aerosol/Cloud properties hourly @ 8.9 km x 5.2 km
Precision
Band
Signal to Noise
0.10
0.06
354, 388 nm
≥1414
2158
0.2
0.05
346-354 nm
≥1200
2222
100 mb
Spectral Imaging Requirements
Relevant absorption bands
for trace gases & windows
for aerosols
Spectral Range (nm)
Spectral Resolution (nm)
Spectral Sampling (nm)
290-490, 540-740
≤0.6
< 0.22
Solar irradiance and Earth
backscattered radiance
spectrally resolved over
spectral range
Wavelength-dependent Albedo
Calibration Uncert. (%)
Wavelength-independent
Albedo Calibration Uncert. (%)
Spectral Uncertainty (nm)
Polarization Factor (%)
290-490, 540-740
0.6
0.2
Radiometric Requirements
≤1
0.8
≤2
2.0
< 0.02
<5 UV, <20 Vis
< 0.02
≤4 UV, <20 Vis
Spatial Imaging Requirements
Observations at relevant
urban to synoptic scales
and multiple times during
daytime
Revisit Time (hr)
FOR
Geolocation Uncertainty (km)
IFOV*: N/S × E/W (km)
E/W Oversampling (%)
MTF of IFOV*: N/S × E/W
≤1
CONUS
<4.0
≤2.2 × ≤5.2
7.5 ± 2.5
≥0.16 × ≥0.30
1
GNA
2.8
2.2 × 5.2
7.5
0.16 × 0.36
Investigation
Requirements
UVN Calibration Sources
Mission lifetime:
1-yr (Threshold),
20-mon (Baseline),
10-yr (Goal)
Orbit Longitude °W:
90-110 (Preferred),
75-137 (Acceptable)
GEO Bus Pointing:
Control <0.1°
Knowledge <0.04°
On-orbit Calibration,
Validation,
Verification
FOR encompasses
CONUS and
adjacent areas
Provide near-realtime products to
user communities
within 2.5-hr to
enable assimilation
into chemical
models (NOAA &
EPA) and use by
smart-phone
applications
Distribute and
archive TEMPO
science data
products
These figures on either side show how
the absorption patterns for trace gases
are revealed in the Earth BUV
radiances. The figure on the left
compares spectral albedos (normalized
radiance / irradiance ratios) for four
scenes. The figure on the right isolates
the optical depth spectra for select
atmospheric constituents. Some
signals/retrieval estimates can be
obtained by using simple pairs of
weakly and strongly absorbing channels
while other require very high signal to
noise measurements over spectral
intervals.
Effective Reflectivity and Aerosol Indices
Recognized by the Committee on Earth Observing Satellite
(CEOS) Atmospheric Composition Constellation (ACC), the
geostationary constellation of UV-visible spectrometers will
enlighten us on the global distribution of ozone, aerosol, and
their precursors. To integrate the datasets for global
measurements, harmonized data quality is very important. Thus
the inter-calibration among the three different UV-visible
satellite instruments is very important, in addition to the quality
of the data processing. Therefore, the standardization of data
products and pre-calibration/ post-calibration / validation
methods are being discussed. The GSICS Research Working
Group UV Sub Group has a project on “Best Practices” for BUV
calibration. The GSICS Quarterly has an article by Ruediger
Lang, “In-flight Characterization of the Solar Diffuser of
GOME-2 on Metop-A” with some lessons learned.
Images provide by C. Seftor, SSAI.
To L1
Schematic for L-1 &
and
LEO matched viewing
the
conditions at Equinox.
Sun
Matches shift north or
south seasonally
“following” the sun. Local
Match for viewing
geometry
Solar
Noon
LEO
Orbital
Equator Track
or
LEO Cross
Track
FOV
Great Circle aligned
with Cross-track FOV
Sunlit side of
the Earth
The schematic above shows a Simultaneous View Path (SVP) match up between DSCOVR EPIC
at 0º offset with the Earth/Sun line and Suomi-NPP OMPS. Similar matches will be present for
any BUV instrument on a GEO platform with one in a LEO orbit as the LEO orbital tracks pass
near the GEO sub-satellite point. An even more extensive set of matches between L-1 and GEO
observations will occur as the GEO sub-solar point moves through local solar noon for latitudes in
the sun/L-1 line. The GEMS instrument with its coverage extending across the Equator will offer
the best match ups with EPIC and LEO instruments. The TEMPO and UVN match up geometries
will often require allowances for differences in scattering or viewing path angles depending on the
season cycle of solar angles, although TEMPO at 19ºN is only 3º off nadir. Matched viewing
conditions will depend on the DSCOVR-Earth-Sun angles, the seasonally varying tilt of the Earth
relative to the sun, and each GEO instrument’s Field-of-Regard (FOR). The best match ups for the
northern hemisphere will occur in the late fall and early spring.
Comparisons of Solar Spectra
The GRWG UV Subgroup has a project on
comparisons of Solar Spectra from satellite-based
instruments. The two initial actions are to collect and
compare high spectral resolution solar data sets for the
UV, and to collect solar measurements from BUV
sensors with sufficient information on their spectral
bandpasses and wavelength scales to allow
comparisons. The recent GSICS Quarterly article, “Use
of Solar Reference Spectra for Satellite Instruments” by
Matthew DeLand, provided a survey and summary of
several choices for reference spectra. We plan to add
the new instruments’ solar spectra to the data set for
comparisons.
Signal variations in the UV from changes in solar
activity (both on the 11-year solar cycle of 27-day solar
rotation time scales) complicate comparisons of solar
spectra. Recent work by Marchenko and Deland (2014)
“Sun as a Star with Aura OMI: Spectral Changes in the
on-Going Cycle 24”, provide very accurate
determination of the spectral patterns associated with
this activity. These results, shown to the right, will help
to identify changes associate with instrument from
those do to measurements at different times.
DSCOVR-EPIC
Band
Channel λ
FWHM
Primary Application
No.
(nm)
(nm)
1
Ozone, SO2
317.5±0.1
1±0.2
2
Ozone
325±0.1
2±0.2
3
Ozone, Aerosols
340±0.3
3±0.6
4
Ozone, Aerosols,
388±0.3
3±0.6
5
Aerosols, RGB
443±1
3±0.6
6
Aer,Veg, RGB
551 ± 1
3±0.6
7
680 ± 0.2
2±0.4 Aer, Veg, Cloud, RGB
8
Cloud Height
687.75 ± 0.2 0.8±0.2
9
Cloud Height
764 ± 0.2
1±0.2
10
Clouds
779.5 ± 0.3
2±0.4
Suomi-NPP
Band
Channel λ
(nm)
OMPS (NM)
317
OMPS (NM)
325
OMPS (NM)
340
OMPS (NM)
360-380
VIIRS-M2
444
VIIRS-M4
551
VIIRS-M5
672
VIIRS-M5
672
VIIRS-M6
745
FWHM
(nm)
1
1
1
1
19.8
14.3
20
20
14
The table above shows DSCOVR-EPIC versus Suomi-NPP OMPS and VIIRS Band Comparison.
Suomi-NPP OMPS Nadir Mapper (NM) has spectral coverage every 0.42-nm from 300 to 380 nm at 1nm FWHM. Selections and aggregations of the 196 spectral measurements can be used to approximate
the three EPIC UV channels (1st to 3rd channel for EPIC) very well. Extrapolation of the
radiance/irradiance ratios for channels in the 360 to 380 nm range can be used to estimate the 388 nm
values. VIIRS has 2 bands that include the EPIC VIS/NIR (5th -6th) channels well. The 7th to 9th
VIS/NIR channels of EPIC can be calibrated with extrapolation and radiative transfer modeling of
VIIRS M5 and M6 channels. Stability of all EPIC channels can be monitored with Validation Time
Series (VTS) at vicarious sites.
The GRWG UVSG has a project to compare effective reflectivity and aerosol index products for
BUV measurements in spectral regions with little trace gas absorption, primarily from 340 nm to
405 nm. All four instruments will make measurements within this spectral range and create these
products. The GSICS quarterly has an article by Omar Torres, a primer on the properties of UVAbsorbing Aerosol Incides (AAI) and their relationship to inter-channel calibration. The
statement within it—“For a well calibrated sensor the AAI is close to zero in the absence of
aerosols and clouds”—can be inverted to give a check on inter-channel calibration for regions
with known clear sky and low aerosol loading. This UVSG project’s goals are to develop
vicarious calibration methods and provide comparisons for monitoring the BUV measurements
for these channels. Approaches for checking reflectivity channel calibration will follow the work
in Jaross and Krueger (1993), where they looked at using ice radiances to characterize timedependent changes in reflectivity channel performance. Additional target regions have been
identified and used by BUV practitioners since then.
The image immediately to the left shows an Aqua Moderate Resolution Imaging
Spectroradiometer (MODIS) Red-Green-Blue image over the Arabian Peninsula region for
January 30, 2012. The image to its left shows a false color map of the OMPS effective reflectivity
(from a single Ultraviolet channel at 380 nm) with 5×10 km2 FOVs at nadir for the same day. The
color scale intervals range from 0 to 2 % in dark blue to 18 to 20 % in yellow. Products like these
from LEO instruments can use the corresponding EPIC or GEO products as transfers comparison
standards.
Space Shuttle Under-flights of LEO
Flights of the SSBUV on the Space Shuttle demonstrated the value of comparisons for match
up observations with LEO instruments. Hilsenrath et al. (1995) describes corrections of prelaunch calibration for the NOAA-11 SBUV/2 by using in-orbit comparisons of ultraviolet
geometric albedos measured by shuttle SBUV (SSBUV) and the NOAA 11 SBUV/2.
Geometric albedo comparison data were further corrected using a radiative transfer code to
account for the small difference in observing conditions between the two spacecraft.
Comparison of data from three SSBUV flights, which occurred about one year apart, with
concurrent SBUV/2 data provided an independent check of the time-dependent change derived
from the in-flight calibration data. The SSBUV instrument was returned to the laboratory after
each flight check its performance. More recently, LEO/LEO comparisons were used to set the
calibration of the Total Ozone Unit (TOU) instrument on FengYun-3 (W. Wang, 2014).
GEO / LEO Comparison Example
The two figures to the right give information on a comparison of GOME-2
Metop-A with Seviri / MSG measurements. There are two steps to create the
match ups. First there is a spatial collocation where the average of all Seviri
measurements (~4km) in one GOME-2 ground pixel (40 by 80 km) is computed.
Match up must be within ±15 minutes. The second step is to use the high
resolution measurements from GOME-2 to approximate the Seviri channel
bandpass. The figure immediately to the right shows the spectral coverage. Match
up collected from 1st Jan to 22nd Nov 2013 are show in the far figure to the right.
The red symbols are two week averages. The results find an ~8% gain in
reflectivity for Severi Channel 1 compared to GOME-2
Time-averaged irradiance differences: (mid-y2012+y2013) vs.
(mid-y2007+y2008+mid-y2009)
Summary
The four new BUV instruments in L-1 and GOE orbits will provide exciting new measurements and products for atmospheric composition
researchers and practitioners. The GRWG UV Subgroup is already starting projects that will lead to improved measurements and products not
just from these instruments but from from the whole constellations of space-based BUV instruments.
References
Bhartia, P.K., et al., 1995, “Applications of the Langley Plot Method to the Calibration of SBUV Instrument on Nimbus-7 Satellite,” J. Geophys. Res., 100, 2997-3004.
Cebula, R.P., & DeLand, M.T., 1998, “Comparisons of the NOAA-11 SBUV/2, UARS SOLSTICE, and UARS SUSIM Mg II solar activity proxy indexes,” Solar Physics, 177, 117–132.
Chance, K., 2014, Overview of TEMPO status. TEMPO Science Team Meeting, Hampton, VA.
Committee on Earth Observation Satellites Atmospheric Composition Constellation (CEOS ACC), 2011, A Geostationary Satellite Constellation for Observing Global Air Quality: An
International Path Forward, position paper.
CEOS http://www.ceos.org/images/ACC/AC_Geo_Position_Paper_v4.pdf
GSICS Quarterly, Special Issue on Ultraviolet (2014), 8(2) doi: 10.7289/V5N29TWP
http://docs.lib.noaa.gov/noaa_documents/NESDIS/GSICS_quarterly/v8_no2_2014.pdf
Use of Solar Reference Spectra for Satellite Instruments by Matthew DeLand, SSAI
The Absorbing Aerosol Index by Omar Torres, NASA
Ozone Measurements from FY-3A by Weihe Wang, CMA
Measurement of Atmospheric Composition using UV-visible Spectrometers from Geostationary Orbits by Jhoon Kim, Department of Atmospheric Sciences, Yonsei University, Seoul,
Korea
Hilsenrath, E., R. P. Cebula, M. T. DeLand, K. Laamann, S. Taylor, C. Wellemeyer, and P. K. Bhartia(1995), Calibration of the NOAA 11 solar backscatter ultraviolet (SBUV/2) ozone
data set from 1989 to 1993 using in-flight calibration data and SSBUV, J. Geophys. Res., 100(D1), 1351–1366, doi:10.1029/94JD02611.
Jaross, G. and Krueger, A. J., 1993. Ice radiance method for BUV instrument monitoring. Proc. SPIE Int. Soc. Opt. Eng., 2047, 94–101.
Kim, J., 2014, Overview and status of GEMS. TEMPO Science Team Meeting, Hampton, VA.
Marchenko, S., and DeLand, M.T. , “Sun as a Star with Aura OMI: Spectral Changes in the on-Going Cycle 24”, submitted to The Astrophysical Journal, January 2014.
Veihelmann, B., 2013, Sentinel-4 status. CEOS ACC-9 meeting, Darmstadt, Germany.
A bibliography of papers on SSBUV underflights of the SBUV(/2) instruments can be found at disc.sci.gsfc.nasa.gov /ozone/documentation/publications/sensor/ssbuv_pubs.shtml.