MAUVE/SWIPE: An imaging instrument
concept with multi-angular, -spectral,and -polarized capability for remotesensing of aerosols, ocean color, clouds,
and vegetation from space
R. Frouin1 , P.-Y. Deschamps2 , R. E. Rothschild3,
E. A. Stephan3, J. Riedi2
1Scripps Institution of Oceanography, University of California San Diego,

8810 La Jolla Shores drive, La Jolla, California 92037, USA

2Laboratoired’Optique Atmosphérique, Université des Sciences etTechnologies de Lille, 59655 Villeneuve d’Ascq, France

3Center for Astrophysics and Space Sciences, University of California San

Diego, 9500 Gilman Drive, La Jolla, California 92093, USA

Joint effort between the Scripps Institution of
Oceanography and the Center for Astrophysics
and Space Science at the University ofCalifornia San Diego, with the participation ofscientists from the University of Lille

-MAUVE: Monitoring Aerosols in the
Ultra-Violet Experiment- SWIPE: Short-Wave Infrared
Polarization ExperimentInstrument Concept-Based on the heritage of the POLDER
radiometer.
-Combines the merit of TOMS for observing
in the ultra-violet, MISR for wide field-ofview range, MODIS for multi-spectral aspects
in the visible and near infrared, and the
POLDER radiometer for polarization aspects.

Mission Scientific Objectives
Aerosols: To monitor aerosols globally (land andocean), including their properties (absorption,
scattering, vertical structure), sources, andtransport, to quantify their influence on climate,
and to assess their impact on our environment andhuman life.
Clouds: To determine cloud amount andcharacteristics (optical properties, type, altitude,
thermodynamic phase, 3-dimensional structure)
for radiation budget studies and climate modeling.

Mission Scientific Objectives (cont. 1)
Ocean Color: To observe accurately spectral
marine reflectance in both the open ocean andcoastal waters, determine inherent opticalproperties and water composition, and identifyfunctional phytoplankton types for studies ofglobal ocean dynamics, marine biosphereresources, and the role of biology in the global
carbon cycle.

Mission Scientific Objectives (cont. 2)
Land Surfaces: To identify land cover anddetect/monitor
itschange(anthropogenic,
natural), and to determine surface properties andcanopy parameters (bidirectional reflectance,
albedo, leaf area index, ratio of leaf size andcanopy height, etc.) for carbon cycle studies(sinks and sources of carbon dioxide) and climate
modeling.

Mission Requirements
Daily, global measurements with a ground
resolution of 1 km or better in the ultraviolet,
visible, near infrared, and short-wave infrared,
measurements at multi-angles, and measurementsof the polarized state of the incident light.

1. We must observe in the ultra-violet-Observing TOA reflectance in the ultraviolet
is the only way to get direct information on
aerosol absorption, because of the explicit andlarge influence of aerosol absorption on themeasurements.

Single Scattering Albedo, ωa

1

0.9

DDS
BDD
T70
M98

0.8

0.7

0.6

0.5
300

400

500

600

700

800

900

Wavelength (nm)Figure 1. Spectral variation of the aerosol single scattering
albedo for various aerosol models, i.e., maritime M98 andtropospheric T70 types with little absorption, and backgrounddust and desert dust storm types with high absorption.

Reflectance, R TOA

1

0.1
Mol.
DDS
BDD
T70
M98
0.01
300

400

500

600

700

800 900 1000

Wavelength (nm)Figure 2. Spectral variation of the top-of-atmosphere reflectance forthe same aerosol models as in Figure 1. The optical thickness is 0.5 at
865 nm, the altitude of the aerosol layer is 5 km, the groundreflectance is null, the solar zenith angle is 36 degrees, and the view
zenith angle is 70 degrees in the backward scattering plane.

-The decrease, ∆R, in TOA reflectance resultsfrom attenuation of the molecular scattering
below the absorbing aerosol layer, and it may be
written as:∆R = Rm – RTOA ≈ (1-ωa)τa(Ps - Pa)Rm
where Rm is the reflectance due to molecular
scattering, RTOA is the TOA reflectance, ωa is theaerosol single scattering albedo, τa is the aerosol
optical thickness, Ps is the surface pressure, andPa is the pressure level of the aerosol layer.

-Obviously, the inversion problem is not easy
since aerosol scattering and surface reflectancealso affect the measurements, as well as thealtitude of the aerosol layer. Furthermore,
knowledge of τa is require to deduce ωa from theabsorption optical thickness (1 - ωa)τa.

Figure 3. Global map (ocean only) of the aerosol absorption effect at 443 nm,
basically the product of the absorption optical thickness and the altitude of
the aerosol layer, obtained from POLDER-1 imagery at multiple angles andaveraged over the period 11-20 March 1997. High values correspond to major
sources of absorbing aerosols out of Africa and South Asia.

2. We must observe in the visible, near infrared,
and short-wave infrared-Knowledge of not only aerosol absorption, butalso aerosol scattering properties is required forthe applications envisioned.
-Over the ocean, this is usually achieved using
bands in the red and near infrared. Spectral
bands in the middle infrared are useful, sincesensitive to the relative importance of the aerosol
fine and coarse size modes, whatever theabsorption.

-Over land, Dense Dark Vegetation algorithms use
shorter wavelengths, i.e., in the visible, but theyrequire measurements in the short-wave infrared
to estimate surface reflectance.
-Surface reflectance must be derived to correct
its effect on the retrieval of aerosol absorption in
the ultra-violet, especially over the ocean wherereflectivity can be high.
-Spectral bands in the blue and green allow one toretrieve a model of ocean optical properties thatcan be extrapolated to the ultra-violet.

-The average altitude of the aerosol layer, or
bettertheverticalprofileofaerosolconcentration, should be determined for aerosol
studies and to derive accurately marinereflectance in the presence of absorbing aerosols.
-This can be accomplished by differential
absorption, using dual spectral bands locatedinside and outside the oxygen absorption bandaround 763 nm, by exploiting the coupling between
gaseous absorption and aerosol scattering.

Figure 4. Aerosol layer altitude ZA (km) as a function of the reflectanceratio using the oxygen A-band channels of the POLDER instrument,
centered on 763(±10) nm and 765(±40) nm. Simulations are presented forsolar zenith angles of 0, 30, and 60o, a viewing zenith angle of 30o, and a
relative azimuth angle of 0o, assuming a urban aerosol model an optical
thickness of 0.3 at 550nm.

Table 1. Theoretical accuracy (km) on the aerosol layer altitude ZA.
The accuracy (i.e., root-mean-square-error) has been estimatedfrom simulations assuming an uncertainty of ±1% on thereflectance ratio, and, various sun and view zenith angles.

Figure 5. (Left) POLDER image of a dust plume off Africa on September
17, 2003 and (Right) estimation of the altitude of the aerosol layer frommeasurements in the two spectral bands of the POLDER instrument in theoxygen A-band.

Figure 6. (Left) Relative error on chlorophyll-a concentration, [Chl-a], whenatmospheric correction algorithms suppose that aerosol concentration follows
an exponential law with a scale height of 2 km whereas aerosols are actually
between 4 and 5 km. (Right) Relative error when the aerosol altitude is overestimated or under-estimated by 0.5 km. Aerosols are strongly absorbing(urban-type) with optical thickness of 0.1 and 0.6. The solar zenith angle is 30o.
Relative azimuth angles are 90o and 180o. The actual [Chl-a] is 0.3 mg m−3.

-Spectral information in the ultra-violet, visible,and near infrared is needed to derive ocean
optical properties, determine water composition,and identify functional phytoplankton types.

Figure 7. Geographic distribution of the most frequent phytoplankton
functional types for January (right) and June (left) 2001, determined from
SeaWiFS spectral bands between 412 and 555 nm. Haptophytes are in blue,
Prochlorococcus in green, Cyanobacteria in yellow, and Diatoms in red
Phaeocystis and coccolithophorids blooms are in light and dark grey,
respectively (unvalidated). Unidentified pixels are in black. (After Alvain etal., 2006.)

3. We must observe polarization-Polarization measurements contain a wealth of
information on aerosol scattering properties.The
polarized scattering matrix depends largely on
the refractive index of the particles, giving moreclues on the aerosol type.
-Over the ocean, they complement multi-spectral
measurements in differentiating aerosol models.
-Over land, they allow aerosol retrievals overnon-DDV surfaces that are weakly polarized.

-Accurate retrieval of aerosol scattering
properties from polarization measurements,
however, requires some flexibility on the viewing
geometry to achieve a desired set ofcharacteristic scattering angles.
-The same target must be viewed under multiple
angles.

-The polarization measurements are mosthelpful in the red, near infrared, and shortwave
infrared where molecular scattering is relatively
ineffective.
-In the ultraviolet, the polarization signature
originates mostly from molecular scattering.
Nevertheless, polarization data remain useful inthis spectral range to discriminate aerosol
models.

QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.

Figure 8. Example of spectral, directional, and polarized measurements
over a land pixel. Knowing the pixel characteristics (altitude, surface
classification), the molecular and the surface polarized radiances arecomputed in the 865 and 670 nm channels, for the viewing directions.
Given an aerosol model, the optical thickness is adjusted to fit thepolarized radiances, and the measured ones. (After Deuze, 2004.)

QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.

Figure 9. Aerosol optical thickness at 865 nm for June 2003 derived fromPOLDER data. Only the accumulation mode (small spherical particles) is
detected using polarized radiances. This picture shows the regions where
the anthropogenic aerosol loadings are high. Extreme values are found overSouth-East Siberia, Korea, and North China. High values appear on Central
Africa, Central America, and Canada, corresponding to biomass burning or
pollution events.

4. We must observe at multiple angles-Multi-angular observations are essential
exploit polarization information effectively.

to-Like polarized observations, however, they aresensitive to the scattering phase function. They
allow, even without polarization information, abetter determination of the aerosol model by
fitting the multi-angular measurements in thevisible and near infrared.

-Furthermore, the effect of aerosol absorption in
the ultraviolet can be greatly enhanced comparedto the effect of aerosol scattering at specificangles, namely at backward large viewing angles(i.e., above 60o).

-Using multi-angular observations of differential
absorption in the O2 band may also give more clues
on the vertical profile of aerosol concentration.
-When the viewing angle increases, so do the air
mass, and the associated vertical profile of theatmospheric weighting function for aerosol
scattering changes, and will have a maximum at ahigher altitude.
-Combining spectral differential absorption andmulti-angular observations is a new and exciting
field that has not been explored yet for practicalapplication to atmospheric sounding.

Figure 10. Vertical weighing functions in the oxygen absorption band. (Left)
nadir viewing, centered at 764,2 nm, different spectral bandwidths. (Right)
spectral bandwidth of 20 cm-1 or 0.7 nm, different air masses corresponding
to off nadir viewing.

-Over the oceans, multi-angular observations canbe used to improve atmospheric correction in thepresence of absorbing aerosols.
-In this approach, after a standard correction foraerosol scattering is effected, the estimatedwater reflectance in all viewing directions islinearly regressed versus an absorption predictor,
i.e., a function representing the directional effectof an absorbing aerosol, essentially the molecularreflectance.

Rw‘ = ≈ (1-ωa)τa(Pa - Ps)Rm + Rw

Figure 11. Estimated marine reflectance at 443 nm versus absorption
predictor. The aerosol layer, τ = 0.2 at 865 nm, is located at 3.76 nm. Sun
zenith angle is 36o and view zenith angle varies from 0 to 75o, backward, inthe principal plane. The marine reflectance is 0.02.

AOT, 865 nm

ρw(0+), 443 nm

No Absorption Correctionρw(0+), 443 nm

Absorption Correction

Figure 12. Application of the multi-angle algorithm to POLDER imagery
over the Mediterranean Sea during a dust outbreak from Africa.

5. Daily global coverage/Twin satellite concept-Daily global coverage is needed for theapplications envisioned. From a sun-synchronouspolar orbiting satellite at the altitude of 800 km,
this requires a cross-track field-of-view of about± 60o in order to avoid gaps between orbits at theEquator on a daily product.
-The instrument system could be installed aboard
two satellites on interlaced orbits to ease global
coverage. The orbit of the second satellite would
be positioned to fill the gaps of the first satellite.

-The equatorial crossing time of the twosatellites could be adjusted to allow simultaneous
observations of the overlapping zone, i.e.,observations in a more complete range ofscattering angles and stereoscopic viewing.

Figure 13. Twin view concept from adjacent orbits using POLDER data.
Center: daily coverage for stereoscopic viewing if the Equator Crossing
Time (TU) is the same.

Specifications-Measurements of spectral radiance: Between 350
and 2130 nm in 16 spectral bands centered on
350(±5), 380(±5), 412(±5), 443(±5), 490(±5),
555(±5), 620(±5), 670(±5), 710(±5), 750(±5),
763(±1.5), 865(±10), 1040(±20), 1240(±20)‚1600(±20), and 2130(±20).

-Measurements of polarization (first three Stokes
parameters), i.e., analysis with 3 orientations ofpolarization: In specific spectral bands: 350, 412,
670, 865, 1040, and 1600 nm;-Multi-angular measurements: In a field-of-view of120 degrees;-Signal-to-Noise Ratio: 1000 for an albedo of 1
with the sun at zenith for spectral and polarizedradiance;
Noise
Equivalent
Differential
Reflectance: <0.0005 at low signal level for the sun
at zenith; Polarization accuracy: 0.5%;

-Dynamic range of radiometry: Up to analbedo of 1 with the sun at zenith;-Ground resolution: 1 km at nadir; and-Frequency of measurements: Spectral every
20s, polarization every 10s.

Instrument Concept/Principle-Two complementary camera systems, MAUVE(350-1050 nm) and SWIPE (1600-2130 nm),allowing a 120-degree field-of-view with adequate
resolution (1 km) from satellite altitude.
- MAUVE and SWIPE have four identical cameras
with a field-of-view of ±30 degrees and theoptical axis oriented at 30 degrees from nadir,forward and backward with respect to thesatellite motion.

-Multi-angle viewing is achieved by the along-track
migration at spacecraft velocity of the twodimensional field-of-view.
-Two filter wheels are installed between opticalassembly and detector array, one carrying
spectral filters, the other polarizing filters (plusopen and closed positions), allowing measurementsof the first three Stokes parameters I, Q, and Vof the incident radiation in the selected spectral
bands. The filter wheels do not rotate steadily,but step-by-step to allow greater flexibility inobserving modes.

-Compared with the POLDER radiometer, thefield-of-view is larger, the ground resolution
better, and the spectral range extended to theultraviolet and short-wave infrared.
-Radiometric quality is improved thanks to the
4-camera system, that reduces stray light andpolarization of the optics at large viewing angles.

Figure 14. MAUVE (SWIPE) camera system.

Optical Elements-The design is telecentric, with focus adjustment
onto the 2-dimensional detector array using aspectral filter/polarizer combination.
-It assumes that the corners of the detector
array are illuminated, and that the ±30-degree
field-of view are mapped onto a 1024x1024x18µm
detector array.

a. MAUVE Optics. Effective focal length:24.5mm; F#: 4.0; Field-of-view: ±31.0 Deg;
Resolution: 1 mRad; Format: ±12.75mm(illuminated corners of the detector array;
Overall length (from front surface of element
1 to focal plane): 150.0mm, Diameter: ~33.2mm;
Transmission: 0.8, except at 350 nm (0.65);
Mass Estimate (glass only, to clear aperture):
40g; Image performance: near diffractionlimited, MTF > 0.8; Back Focal Length: 28.0mm;
Front element substrate is SiO2 for radiation
control; Tele-centric image plane.

b. SWIPE Optics. Effective focal length:24.5mm; F#: 4.0; Field-of-view: ±31.1Deg;
Resolution: 1 mRad; Format: ±12.75mm(illuminated corners of the detector array);Overall length (from front surface of element 1
to focal plane): 150.0mm, Diameter: ~33.5mm;
Transmission: 0.85; Mass Estimate (glass only,
to clear aperture): 69g; Image performance:near diffraction-limited, MTF > 0.6; Back Focal
Length: 26.0mm; Front element substrate is
rad-hardK5G20 for radiation control; Telecentric image plane.

Figure 15. MAUVE Optical design.

Detector Arrays-The focal plane detector arrays are theTCM8020A/Si-PIN and TCM8020A/HgCdTe
products from Rockwell Scientific, Inc.
-They are composed of an optical detector
combined with a read out integrated circuitthat allows the electronic access of every
pixel in the array.

-The TCM8020A/Si-PIN, sensitive in the
spectral range 350-1050 nm, and theTCM8020A/HgCdTe, sensitive in the spectral
range 900-2500 nm, will be used for MAUVEand SWIPE, respectively.
-The total number of pixels is 1024x1024.
-The charge storage capacity is adjustable fromone picture to the next, with 0.7, 1.5, and 3
million electrons (standard product), but thevarious gains can be optimized for the MAUVEand SWIPE measurement configurations.

-Quantum efficiency is above 70%, except at
1040 nm (30%). An electronic shutter isactivated in readout mode.
-Read noise is 500, 1000, and 2000 electrons at
5MHz, respectively.
- Dark current is negligible.

-A typical observation will consist of 29 and 9
sequences for MAUVE and SWIPE, respectively,
where the spectral filter and/or the polarizer
wheels move to the desired positions with theelectronic shutter closed.
-An image will then be collected with the shutter
open, and after 20 to 40 ms, the shutter isclosed and the image is read.
-The cycle then repeats for the next image
acquisition.

-The desired spatial resolution on the ground
dictates the integration time and how often thecycle occurs, and places requirements on thedetector array readout and the filter wheel
motion.
-Presently, we have, as a baseline, 5 images per
second, i.e., it will take 5.8s and 1.8s to acquire,
for a given gain, a full set of MAUVE and SWIPE
data, respectively.
-Two gains will be used, one for high-reflectancesignals (cloudy atmosphere, sun glint), and one forlow-reflectance signals (ocean, clear atmosphere).

Radiometric Calibration-The basic plan is a deployable diffuser like the
SPECTRALON diffuser onboard MERIS.
-This solution applied to the MAUVE andSWIPE instruments demands further study
mainly because of the large field-of-view of the
instruments.
-Indirect techniques using molecular scattering,sun glint, clouds, etc.

Cooling System-Passive cooling of the detector arrays should be
adequate.
-Operating temperatures for the detectors would
be approximately -10C.
- If spacecraft or mission dynamics cannot support
this approach, simple Peltier cooling of thedetector arrays can be implemented and this has
been included as eventual in the power estimate.

Data Rate
-Without implementing a compression scheme,
the 1x106 pixels converted to 12 bit resolution
requires 12x106 bits per readout.
-If readouts are done each 200 ms, or 5 per
second, this produces 5x12x106 bits per second,or 60 Mb/s for each detector.

-Therefore, we have 240 Mb/s for MAUVE and240 Mb/s for SWIPE. An additional 10%
increase in rate can be estimated to allow forencoding structure and housekeeping data.
-However, it will be sufficient, for the scientificrequirements of the mission, to transmit thedata at a rate of 1 image per second, i.e., 48
Mb/s for MAUVE and 48 Mb/s for SWIPE, oreven less if necessary.

Power-Detector and detector electronics (1 detector+ electronics = 5W) x 8 = 20W/0.8 (efficiency) =50W.
-(Central Electronic Unit = 8W) x 2 = 16W/0.8(efficiency) = 20W. Note that two additional
CEUs are planned for redundancy.

-Peltier cooling (eventual): 10W are budgeted for
MAUVE and 10W for SWIPE.
-Motor power is estimated at 5 watts per motor, 4motors = 20W.
- Active thermal control is estimated to require an
average of 10% of the power consumption, so thiswould be 50+20+20+20=110, 110x0.10 = 11W.

Summary of Characteristics-Altitude: 800 km-Ground velocity: 6.6 km/s
-Swath: 2800 km-Pixel size: 800 m at nadir-Nb. of pixels: 1024x1024-Nb. of cameras: 8-Nb. of bands: 16 spectral, 6 with 3 orientations ofpolarization-Multi-angle measurements: In a field-of-view of120oSummary of Characteristics (cont.)-Digitization: 12 bits
-Frequency of measurements: Spectral every 20s,
polarized every 10s-Maximum integration time: 40 ms-Signal-to-noise ratio: 1000 for an albedo of 1 andthe Sun at zenith-NEDR: <0.0005 at low signal level for the Sun atzenith-Polarization accuracy: 0.5%
-Data rate: 96 Mb/s
-Mission duration: 3-5 yearsBudgets-Total power consumption during imaging phase:
121W with active cooling and 101W without active
cooling.
-Total weight: 35 kg.
-Dimensions: 0.6mx0.5mx0.25m.
-Estimated cost: $25M.

Figure 16. MAUVE/SWIPE assembly.