**MISSION NAME?**

**to Quantify and Track Global Changes in Atmospheric Composition,\
and Their Effects on Air Quality, Climate Change and Variability,\
and Ozone Layer Stability**

draft summary by

Nathaniel Livesey, Michelle Santee, Paul Stek, Joe Waters\
California Institute of Technology, Jet Propulstion Laboratory, USA

and

Pieternel Levelt, Hennie Kelder, Pepijn Veefkind, Marcel Dobber\
Royal Netherlands Meteorological Institute, The Netherlands

> This draft summary was produced using material already available from
> our CAMEO concept study, modified for an 830-km 53^o^ inclination
> orbit.
>
> We understand and appreciate that more consideration is needed on
> whether, overall, a precessing-local-time orbit (as discussed here) or
> a sun-synchronous orbit is most beneficial, overall, to atmospheric
> science and societal needs. Appendix 5 has coverage plots for both
> sun-synchronous and precessing orbits to provide information for
> helping with the choice of orbit.
>
> More material probably needs to be added on the use of measurements by
> other satellites (especially, NPOESS, and METOP) to help meet the
> goals of this mission. Appendix 4 shows the measurement overlaps with
> NPOESS/METOP for the orbit considered here.

**MISSION NAME?**

**to Quantify and Track Global Changes in Atmospheric Composition\
and Their Effects on Air Quality, Climate Change and Variability, and
Ozone Layer Stability**

Human activities are changing atmospheric composition in ways that can
adversely affect life on Earth. Unhealthful pollution episodes are
common. Increasing pollution worldwide can affect global air quality and
the atmosphere's ability to cleanse itself. Increasing greenhouse gases,
pollution and aerosol can affect climate. Global stratospheric chlorine,
now more than 5x the natural amount, has the potential in a changing
climate to cause more severe ozone loss before it decreases to
near-natural abundances in \~50 years.

Policy decisions related to atmospheric composition -- requiring
balances between economic and environmental concerns -- must be made and
continuously validated, but obtaining the necessary political consensus
is difficult. However, regulations on ozone depleting substances
demonstrate that consensus can be reached if sufficient scientific
information is produced. Such information requires global measurements
of atmospheric composition.

**MISSION NAME?** provides atmospheric composition measurements needed
for policy decisions related to air quality and climate change, and for
determining whether regulations on ozone depleting substances are
working. It quantifies the worldwide emissions and transport of
pollution -- essential for international policy to improve and protect
regional and global air quality. It globally measures the diurnal cycle
of convective deposition of water vapor and pollution into the upper
troposphere and the relationships among upper tropospheric water vapor,
pollution and clouds -- essential for improving predictions of climate
variability at all time scales. It globally tracks the vertical
distribution of stratospheric ozone, key chemicals affecting it, and
tracers that separate chemical and dynamical influences -- essential for
validating (and improving) our understanding of the ozone layer's
behavior in response to existing international regulations.

**MISSION NAME?** provides data that are essential for meeting NASA's
2006 Strategic Plan goal 3A.1 "Progress in understanding and improving
predictive capability for changes in the ozone layer, climate forcing,
and air quality associated with changes in atmospheric composition", and
contributes to goals 3A.2 "Progress in enabling predictive capability
for weather and extreme weather events" and 3A.5 "Progress in
understanding the role of oceans, atmosphere, and ice in the climate
system and in improving predictive capability for its future evolution."

**MISSION NAME?** remedies measurement deficiencies that have previously
limited our progress in science and applications directly linked to
societal needs and benefits, and continues needed key measurements, by
providing:

a)  an unprecedented suite of composition measurements and an
    unprecedented combination of temporal, vertical and horizontal
    resolution -- and global coverage -- for needed air quality and
    climate change measurements,

b)  upper tropospheric cloud ice and water vapor measurements with the
    temporal, vertical and horizontal resolution needed to improve the
    accuracy of global circulation models used for weather/climate
    prediction,

c)  stratospheric measurements needed to track stability of the ozone
    layer and detect changes well before they are evident in ozone
    itself.

**MISSION NAME?** uses advanced technology in three instruments of types
that have been proven extremely successful on previous missions:

(1) **SMLS**, the **S**canning **M**icrowave **L**imb **S**ounder, an
    advanced version of MLS on Aura, measures microwave limb emission in
    very broad spectral bands centered at 240 and 640 GHz. Major
    improvements from Aura MLS are (a) azimuth scanning -- simultaneous
    with the vertical scanning -- to give 50x50 km horizontal sampling
    (cross-track sampling 50x better than Aura MLS; along-track 3x
    better) needed for quantifying convective deposition of pollution
    into the upper troposphere and its resulting long-range transport
    and (b) in-orbit programmability of the suite of measurements. SMLS
    measures more molecules than Aura MLS while having a simpler signal
    chain and fewer radiometers.

(2) **TROPI**, the **TRO**pospheric **P**ollution **I**mager, an
    advanced version of OMI on Aura with long heritage from TOMS,
    measures reflected and backscattered solar radiation at ultraviolet
    and visible wavelengths Major improvements over OMI are (a) 10x10 km
    horizontal resolution (2-4x better than OMI) needed for resolving
    pollution sources and more cloud-free observations, and (b) bands in
    the near-infrared for improved measurements of cloud and surface
    properties which greatly improve the accuracy of the tropospheric
    observations

(3) **TIMS,** the **T**ropospheric **I**nfrared **M**apping
    **S**pectrometer, an advanced version of (? what can we say here?
    heritage from MOPITT or SCIA?) measures reflected and backscattered
    radiation in the short-wavelength infrared for lower tropospheric
    CO, CH~4~ and CO~2~ measurements. (A sentence or two more needs to
    be added here).

Appendices 1-3 give more information on these instruments.

**Fig. 1.** **MISSION NAME?** measurement suite. Yellow shows SMLS
measurements (where dotted lines indicate goals), cyan shows TROPI
measurements, and red shows TIMS measurements. Surface UV-B flux is also
measured by TROPI.

**MISSION NAME?** implements the international cooperation advocated by
the Earth Observation Summit held July 2003 in Washington, D.C. TROPI is
proposed to be developed by the Netherlands/ESA and is a modified
version of the TROPOMI instrument on the "Troposphere Composition and
Air Quality (TRAQ)" mission now in pre-phase-A studies by the European
Space Agency. Strong UK participation in SMLS is anticipated.

Figure. 1 shows **MISSION NAME?'s** measurement suite. SMLS measures
stratospheric profiles with a few percent precision and accuracy, and
temperature profiles with 0.1K precision and \~1K accuracy. The SMLS
upper tropospheric precision is addressed in Figure A1-4 of Appendix 1.
TROPI tropospheric column measurement accuracy is several percent for
O~3~, CH~4~, 10-20% for O~3~, NO~2~ and CO, and \~50% for CH~2~O and
SO~2~, depending on atmospheric conditions. This accuracy will be
improved for O~3~, CO, NO~2~ and CH~2~O by differencing TROPI total
columns and SMLS stratospheric columns. Differencing TROPI total column
from SMLS columns for the upper troposphere and above yields lower (or
mid-lower) tropospheric abundances for O~3~, CO, CH~2~O and potentially
NO~2~. Need to add similar statements for TIMS.

**MISSION NAME?'s** measurements are chosen to fulfill recognized needs
in the following scientific and applications areas (Decadal Survey
references are from interim panel reports made publicly available in
April 2006):

-   **Air quality.** **MISSION NAME?** makes all the priority
    tropospheric measurements (O~3~, CO, NO~2~, CH~2~O, SO~2~, aerosols)
    identified by the February 2006 "Community Workshop on Air Quality
    Remote Sensing from Space: Defining an Optimum Observing Strategy"
    and the NRC Decadal Study Panel on Human Health and Security. It
    provides the global measurements that are essential for obtaining
    international agreements on air quality issues. []{.mark}

-   **UV exposure.** **MISSION NAME?** makes all the NRC Decadal Study
    Human Health and Security Panel's 'UV exposure and skin cancer'
    priority chemical measurements (O~3~, H~2~O, HO~2~, NO~2~, ClO, BrO,
    HNO~3~, HCl, H~2~CO, HDO, H~2~^18^O, CO~2~, CO, NO~y~, N~2~O, CH~4~)
    -- except for OH (for which HO~2~ is a good proxy), IO (which cannot
    be done from space), and halogen source molecules (for which HCl and
    BrO give adequate information). Its additional measurements of
    H~2~O~2~, NO, HOCl, CH~3~Cl, and volcanic SO~2~ also help track
    stability of the ozone layer.

-   **Climate variability and change. MISSION NAME?** makes measurements
    for climate variability and change that have been identified as
    crucial by many studies, including the NRC Decadal Study Panel on
    Climate Variability and Change. It provides global and simultaneous
    measurements of pollution (both gaseous and aerosol) and upper
    tropospheric cloud ice needed for progress in understanding how
    changes in composition may affect cloud properties and, thus,
    climate. Its simultaneous measurements of water vapor, cloud ice and
    temperature -- made with resolution needed for progress in
    quantifying the effects of convection -- improve model predictions
    of seasonal to interannual climate variability and of long-term
    natural and anthropogenic-induced climate changes. Itmeasures
    convective deposition of trace gases that affect ozone in the upper
    troposphere, where ozone's radiative forcing of climate is greatest.

-   **Global circulation models used for climate variability and weather
    predictions.** **MISSION NAME?'**s measurements of cloud properties
    (height, fraction, ice mass and particle size information), and
    their diurnal variations, improve model parameterizations of cloud
    processes that affect prediction skill. **MISSION NAME?'s** temporal
    resolution improves worldwide extreme event forecasting and hazard
    assessment. Its novel data sampling enhances the performance of data
    assimilation systems and regional forecast models.

**Fig. 2**. Temporal and global coverage of **MISSION NAME?** in 830 km,
53^o^ inclination, orbit. [Everywhere]{.underline} within a given color
is measured that number of times per 24-hour period. Measurements are
made in 'spurts' of \~1.7 hour intervals (approximately the orbit
period) followed by a several-hour gap. The temporal pattern precesses
through all local times during every 36-day period. The SMLS thermal
emission measurements (left column) have a frequency pattern that is
nearly constant throughout the year. The TROPI measurements (reflected
and backscattered sunlight) have a pattern that varies annually, for the
UV/Vis bands as shown in the bottom right panel: mid-latitudes are
measured 3-5 times per day over month-long periods that switch between
north and south (GOME-2 on METOP will provide UV/Vis measurements -- but
with poorer spatial resolution -- that fill the TROPI gaps with
once-per-day measurements). The top right panel shows the TROPI
measurement frequency in mid-July, a time of peak urban pollution in the
northern hemisphere. Need to add coverage plot for TIMS.

![](media/image1.png){width="2.9618055555555554in"
height="2.4444444444444446in"}Figure 2 shows the **MISSION NAME?**
temporal and global coverage for 830 km orbit height (that of NPOESS)
and 53^o^ orbit inclination. SMLS makes − for every 24-hour period
[everywhere]{.underline} in both northern and southern hemispheres − 6
or more measurements at 25-60^o^ latitudes, 4-6 measurements at 0-25^o^,
and measurements to 80^o^ on each orbit. TROPI makes 3-5 measurements
per day [everywhere]{.underline} over a 30-60^o^ latitude band that
covers the dominant regions of anthropogenic pollution emission, shown
in Figure 3, and which alternates between northern and southern
hemispheres on a monthly basis. TROPI also makes 1-2 measurements per
day everywhere at \~±30^o^ latitudes. Although the TROPI mid-latitude
coverage gaps are certainly undesirable, NASA's A-train is already
providing a tremendous multi-year record of continuous
[once-per-day]{.underline} global composition measurements, and GOME-2
on EUMETSAT's METOP (to be operational 2007-2020) will provide
[once-per-day]{.underline} global UV/Vis measurements of column O~3~,
NO~2~, SO~2~, and perhaps others -- but with poorer spatial resolution
than TROPI. **MISSION NAME?** gives unprecedented
[multiple-per-day]{.underline} measurements needed globally to improve
understanding of hourly-scale processes that critically affect our
ability to predict climate change and global air quality. Coincidences
between **MISSION NAME?** measurements and those of relevant sensors on
operational missions planned for the same time period are shown in
Appendix 4: there are always multiple daily coincidences with SMLS at
mid and high latitudes, and coincidences with TROPI for about 50% of the
time. (An alternative sun-synchronous orbit provides TROPI measurements
without the alternating mid-latitude gaps, but with only 1-2
measurements per day, as shown in Appendix 5.)

The improved TROPI resolution, compared to that of OMI on Aura, provides
considerably more cloud-free data and more accurate determination of
pollution sources and sinks (e.g., by using inverse modeling
techniques). The temporal resolution of TROPI gives essential
information on the diurnal cycle of [worldwide]{.underline} boundary
layer pollution. As an example, Figure 4 shows the variation of NO~2~
pollution in the southeastern United States between 1 and 7 pm on 17
August 2000 as would be sampled with the TROPI spatial and temporal
resolution. The much larger NO~2~ pollution abundances that occur later
in the day, as compared to mid-day when previous global measurements
have been made (e.g., those in the left panel of Figure 3), are clearly
evident. Understanding, and quantifying by measurement, the global
inventory of tropospheric pollution requires global observations over a
range of local times, which **MISSION NAME?** provides for the first
time.

![](media/image4.png){width="1.1279527559055118in"
height="1.1744094488188976in"}![](media/image5.png){width="1.0980314960629922in"
height="1.1755905511811024in"}![](media/image6.png){width="1.0937007874015747in"
height="1.1791338582677164in"}![](media/image7.png){width="1.0791338582677166in"
height="1.1771653543307086in"}

![](media/image8.jpeg){width="1.5700787401574803in"
height="1.213779527559055in"}![](media/image9.jpeg){width="1.6653543307086613in"
height="1.2192913385826771in"}![](media/image10.png){width="1.543700787401575in"
height="1.15in"}![](media/image11.png){width="1.552755905511811in"
height="1.15in"}

Figure 4 shows the dramatic improvement in SMLS horizontal resolution
over that of Aura MLS, and the value of this improved resolution for
tracking intercontinental transport of pollution in the upper
troposphere. Figure 5 illustrates the typical spatial and temporal
scales for convective deposition of boundary layer air into the upper
troposphere. Figure 6 shows the diurnal variation of convection, and how
**MISSION NAME?** greatly supplements the information now being obtained
from NASA's "A-train" of satellites. **MISSION NAME?'s** SMLS provides
the first global data set for quantifying the extent to which convection
"short-circuits" the upper troposphere to the boundary layer. Such
"short-circuiting" of the upper troposphere to the boundary layer has
serious implications for climate change, long-range transport of
pollution, and the stability of the ozone layer. The SMLS measurements
of H~2~O, H~2~^18^O and HDO allow unique identification of air masses
associated with convection. These, along with the simultaneous
measurements of cloud ice and temperature, provide unprecedented and
needed new information -- especially on diurnal variations -- to improve
the parameterization of convection and cloud microphysics in general
circulation models (GCMs) that are used both for weather and climate
change predictions.

**MISSION NAME?**, in addition to producing data files that are archived
and publicly available for scientific research, will produce
direct-broadcast and near-real-time data that can be used for
operational purposes.

![](media/image12.png){width="2.9779527559055117in"
height="1.551968503937008in"}

![](media/image13.wmf){width="3.2881889763779526in"
height="2.0220472440944883in"}

**Appendix 1. The Scanning Microwave Limb Sounder (SMLS)**

SMLS has been under development at JPL for several years. It combines
(a) ultra-sensitive SIS (superconductor-insulator-superconductor)
radiometers, which have been used in atmospheric measurements for \~30
years and are now integrated in the Japanese SMILES flight instrument
planned for the Space Station and (with JPL SIS devices) in the HIFI
instrument for the European Herschel astrophysics mission, and (b) a
novel azimuth-scanning antenna whose principle has been demonstrated by
functional models, with geometric and physical optics designs already
performed specifically for the SMLS instrument. Flight-qualified
coolers, of the general type required for the SIS devices, are already
integrated in the Japanese SMILES instrument and being used in the
European Planck astrophysics mission. U.S. industrial sources exist for
the SMLS cooler, which is expected to be similar to those being
developed for the James Webb Space Telescope. The SMLS design requires
\<180 W, including all inefficiencies, for the cooler drive power.

Advanced SIS radiometers that cover more than 100 GHz bandwidth, and
have been demonstrated by Caltech in radio astronomy applications, are
used in SMLS. The sensitivity of these radiometers is \~30× better than
Aura MLS, as needed (1) for useful precision with the short measurement
times available at each azimuth position, and (2) for obtaining
measurements of additional trace chemical species. SMLS measures more
molecules than Aura MLS, while having a simpler signal chain and fewer
radiometers. It contains radiometers in two broad bands: 180-280 GHz
(both polarizations) mainly for the troposphere, and 580-680 GHz mainly
for the stratosphere. The National Radio Astronomy Observatory is
implementing an array of 200 SIS sideband-separating radiometers, as
used by SMLS, around 230 GHz. NASA's Advanced Component Technology (ACT)
program is supporting development of 200 GHz SIS radiometers for
specific application to needed new tropospheric measurements and
potential deployment in SMLS (*Ultra Low Noise Radiometers for
Tropospheric and Stratospheric Limb Sounding* -- Karen Lee, JPL,
Principal Investigator). Stratospheric measurements (less difficult than
tropospheric measurements) by SIS in the \~580-680 GHz region have been
done from both aircraft and balloon.

All SMLS measurements are of thermal emission, and thus made both day
and night. Vertical profiles are obtained for all measurements. An
important feature of the MLS technique is its ability to make
measurements in the presence of ice clouds that degrade or prevent
measurements by shorter-wavelength techniques.

A major advance of SMLS over Aura MLS -- needed for progress in
quantifying key upper tropospheric processes affecting climate change,
global air quality, and ozone layer stability -- is its dramatic
improvement in horizontal coverage illustrated in Figure A1-1. Also, its
greatly improved sensitivity can provide many more chemical measurements
than possible with Aura MLS. SMLS measures many more molecules than Aura
MLS while having a simpler signal chain and fewer radiometers.

![](media/image8.jpeg){width="2.048031496062992in"
height="1.456299212598425in"}![](media/image9.jpeg){width="2.0098425196850394in"
height="1.5192913385826772in"}

The size of the SMLS antenna is determined, primarily, by (1) the
desired width of the measurement swath, which sets the azimuth scan
range and overall 'width' of the antenna system, and (2) the desired
vertical resolution which sets the 'height' of the antenna primary
reflector. For the desired SMLS swath width (\~5000 km from \~800 km
orbit, corresponding to ±65^o^ azimuth scan range), a \~2 m 'wide'
antenna is required, which is not expected to be a

problem for spacecraft accommodation. For the desired vertical
resolution (1.5 km, \~2x better than Aura), a \~4 m 'high' primary
reflector is required. If a 4 m primary reflector cannot be accommodated
or afforded, then a smaller one can be considered (down to \~2 m as
flown on Aura MLS -- but as large a reflector as can be accommodated is
extremely desirable. Figure A1-2 shows vertical resolution versus
precision for 4 m and 2 m primary reflectors.

![](media/image14.wmf){width="1.8in"
height="1.547244094488189in"}![](media/image15.wmf){width="1.5771653543307087in"
height="1.561023622047244in"}![](media/image16.wmf){width="1.5618110236220473in"
height="1.5618110236220473in"}

Figure A1-3 shows a drawing of SMLS instrument and its signal flow block
diagram. The antenna system, consisting of primary, secondary, tertiary
and scan mirrors, is scanned in the vertical by \~1^o^ rotation, as on
Aura MLS,. A vertical scan is performed every \~10 s, to give
along-track sampling of 50-100 km. The conical azimuth limb scan is
repeated every \~0.5 s during the vertical scan, by ±65^o^ rotation of a
small (\~10-cm diameter) reflector. A measurement is made every \~3 ms
to give 50-100 km cross-track sampling to match the along-track
sampling. 'Overscan' in the vertical accounts for Earth's oblateness and
changes in vertical scan range with azimuth. A multipoint calibration is
performed between each azimuth scan. All mechanisms have 5-year (or
longer) operational lifetime. Table A1-1 gives estimates of the SMLS
mass, power and data rate. Momentum compensation of the SMLS antenna
scan is not included; it will be studied later if needed to meet
spacecraft attitude stability requirements.

![](media/image17.png){width="3.9993055555555554in"
height="3.0229166666666667in"}

**Fig. A1-3**. Signal-flow block diagram of SMLS (left) and drawing
(right). The SMLS signal exits the scanning antenna system along the
vertical scan axis, as on previous MLS

**Table A1-1**. SMLS mass, power and data rate estimates.

  ------------------------------------------ ----------------------------
  overall mass                               500 kg

  full-up power                              400 W

  data rate                                  10 Mb/s
  ------------------------------------------ ----------------------------

Both the SMLS azimuth and vertical scans are programmable to enable
efficient use of measurement time and fulfill various scientific
objectives. A 'continuum' of scan modes, easily implemented by
programming the scan ranges, is available, and the measurement time
(which determines the measurement signal-to-noise) depends upon the scan
mode. The SMLS measurement time needed for an upper tropospheric
spectral line radiance signal-to-noise of 10 for various species is
shown in the Figure A1-4. A 'maximum global mode' with measurements at
20 points in the vertical and every 50x50 km in the horizontal over a
5000 km cross-track width allows 0.003 s for each individual
measurement; a 'regional study mode' with, for example, measurements at
5 points in the vertical and every 50x50 km in the horizontal over a
cross-track width of 2000 km allows 0.05 s for each measurement; a
'local study mode' with, for example, measurements at 3 points in the
vertical and every 50x50 km in the horizontal over a 200 km cross-track
width allows 1 s for each measurement. The available measurement time
can be increased by degrading the spatial or temporal resolution: a
factor of 4 increase, for example, is gained by changing from 50x50 km
to 100x100 km horizontal resolution, which can be done in ground data
processing. Because of the large dynamic range of manyabundances, useful
measurements can be obtained with a signal-to-noise of 3, which reduces
the required measurement time 9x from that shown here. Figure A1-5 shows
the fraction of the troposphere over which MLS is expected to make trace
gas measurements; this is the upper 40-50% for most of the globe

![](media/image19.wmf){width="7.099212598425197in"
height="2.925196850393701in"}The SMLS measurement suite is also
programmable, simply by programmable tuning of local oscillators within
the instrument. One set of measurements is made continuously (e.g.,
H~2~O and perhaps its isotopes, cloud ice, temperature, O~3~, CO); other
sets of related measurements are time-shared to reduce the required
number of spectrometers. Time-sharing modes can be changed, in
principle, as often as each vertical scan. The programmability and
spectral coverage allow measurements of additional species: in
principle, nearly any species with a dipole moment. SMLS can, as needed,
make spectral surveys to test the completeness of our knowledge of
atmospheric composition.

**Fig. A1-4.** SMLS measurement time needed for an upper tropospheric
spectral line radiance signal-to-noise of 10 for various species. Colors
of the bars give representative abundances for different situations.
Blue is for typical or minimum abundances; pink for enhanced abundances
that have been observed or inferred; brown for enhanced boundary layer
abundances that can be convectively transported to the upper
troposphere; grey for soluble species that may reach the upper
troposphere less easily. Because of the large dynamic range in many
species abundances, useful measurements can be obtained with a
signal-to-noise of 3, which reduces the required measurement time 9x
from that shown here. This plot is for a tropical 'background'
atmosphere and 9 km height; it includes 2-3x attenuation of the target
molecule signals by water vapor continuum, and spectral line wings of
'interfering' gases. For the tropical troposphere above 12 km, a region
of considerable current interest, SMLS signals are typically 2-3x
stronger than indicated here. All these measurements are made in the
180-280 GHz region, from spectral lines selected by considerations of
(a) line strength, (b) freedom from interference, and (c) instrument
simplification. Cloud ice measurements from 'continuum' emission, and
temperature from O^18^O, are obtained with good signal-to-noise in
milliseconds.

![](media/image20.wmf){width="1.8665354330708661in"
height="1.5688976377952757in"}![](media/image21.wmf){width="2.044488188976378in"
height="1.550787401574803in"}**Fig. A1-5.** Percent of the troposphere
measured by SMLS. These maps give the climatology, from MLS measurements
for the percent of the troposphere (in height from the tropopause down)
where 200 GHz limb radiances are \<160K, indicating optical depth small
enough for composition measurement. The effects of clouds are included.

**Appendix 2. The TROpospheric Pollution Imager (TROPI)**

TROPI is a modified version of the TROPOMI instrument on the
Tropospheric Air Quality (TRAQ) mission[^1] now in pre-phase A study by
the European Space Agency. It measures reflected and scattered solar
radiation in ultraviolet (UV), visible (Vis) and near-infrared (NIR)
bands. TROPI consists of two modules: the UV/Vis/NIR module and the
electronics module. Table A2-1 gives the atmospheric measurements and
other characteristics for the TROPI spectral bands. Figure A2-1 is a
drawing of the UV/Vis/NIR module.

> **Table A2-1**. Atmospheric measurements and other characteristics for
> the TROPI spectral bands. Signal-to-noise values are for small solar
> zenith angles and low surface albedo, which corresponds to the minimum
> input radiance scenario in which the absorption structures have a
> small amplitude.

+-------------------------+-----------+--------+--------+------------+
|                         | U         |        |        |            |
|                         | V/Vis/NIR |        |        |            |
|                         | module    |        |        |            |
+-------------------------+-----------+--------+--------+------------+
| spectral band           | UV1       | UV2    | V      | Near IR    |
|                         |           |        | isible |            |
+-------------------------+-----------+--------+--------+------------+
| measurements            | str       | O~3~\  | NO~2~, | cloud\     |
|                         | atosphere | SO~2~\ |        | optical    |
|                         | and upper | H~2~CO | O~2    | thickness, |
|                         | tr        |        | ~-O~2~ | pressure,  |
|                         | oposphere |        | cloud  | fraction   |
|                         | O~3~      |        | fr     |            |
|                         |           |        | action | aerosol    |
|                         |           |        | and\   | height\    |
|                         |           |        | pr     | di         |
|                         |           |        | essure | stribution |
|                         |           |        |        |            |
|                         |           |        |        | H~2~O      |
+-------------------------+-----------+--------+--------+------------+
| spectral range (nm)     | 270-320   | 3      | 3      | 710-775    |
|                         |           | 00-400 | 80-490 |            |
+-------------------------+-----------+--------+--------+------------+
| spectral                | 1.1/0.3   | 0.4    | 0.5    | 0.45/0.11  |
| resolution/sampling     |           | 5/0.11 | 2/0.13 |            |
| (nm)                    |           |        |        |            |
+-------------------------+-----------+--------+--------+------------+
| signal-to-noise         | 1         | 1000   | 1500   | absorption |
|                         | 00@270nm\ |        |        | 100\       |
|                         | 1         |        |        | continuum  |
|                         | 000@310nm |        |        | 500        |
+-------------------------+-----------+--------+--------+------------+
| horizontal pixel size   | 20x40     | 10x10  |        |            |
| (km x km)               |           |        |        |            |
+-------------------------+-----------+--------+--------+------------+
| cross-track swath width | 2600      |        |        |            |
| (km)                    |           |        |        |            |
+-------------------------+-----------+--------+--------+------------+

![](media/image22.png){width="2.8208333333333333in"
height="2.5701388888888888in"}

**Fig. A2-1.** Drawing of the TROPI UV/Vis/NIR module.

The TROPI design, by TNO and Dutch Space in The Netherlands, has a
strong heritage from the Aura Ozone Monitoring Instrument (OMI) \[Levelt
et al, 2005\] that was designed and built by the same Dutch industry
expected to build TROPI. The compact design has been optimized to give a
wide swath, high spatial and spectral resolution, high S/N and
polarization scrambling . Two-dimensional detector arrays are used, with
one axis of the array giving spectral information and the other giving
cross-track spatial information.

The UV/Vis/NIR module consists of the optical bench (telescope and
spectrograph) and a detector for each channel (i.e. four detectors in
total). The optical bench is operated at room temperature and the
detectors at 233 K. To accommodate the temperature gradients between the
optical bench and the detectors a "labyrinth" construction is used,
similar to that in the Scanning Imaging Absorption Spectrometer for
Atmospheric Chartography (SCIAMACHY) on the ESA Envisat satellite. The
cold parts are passively cooled. The temperatures of both the detectors
and the optical bench are actively stabilized. An important element in
the optical design is the polarization scrambler. Results from OMI
demonstrate that this concept works very well and avoids the need for
polarization measurement devices to correct for the polarization state
of the incident light.

In-orbit calibration is done by regular solar irradiance measurements
using various types of diffusers. The optical design, like that of OMI,
is optimized to minimize the differences in the light paths for radiance
and irradiance measurements in the instrument. This design feature
cancels any remaining calibration errors and minimizes the impact of
in-flight degradation. Other on-board calibration sources are light
emitting diodes near each detector and a white light source to monitor
potential detector, electronics and optical degradation.

Si-based CMOS (Complementary Metal Oxide Semiconductor) detector
technology will most likely be used. Compared to the frame-transfer CCDs
employed in OMI-Aura, the CMOS detectors have better radiation hardness,
require less off-chip circuitry and consume less power. Each detector
has a 14 bit analogue-to-digital converter with digital interface to the
electronics module, which allows co-adding of the measured spectra up to
5 bits.

The electronics module provides the interface for the UV/Vis/NIR module
to the satellite bus. This module also controls the detector readout,
heaters and mechanisms, handles the video and thermistor data and
provides power to the UV/Vis/NIR module. It is a high-density box in
terms of power consumption and operates at about 310 -- 315 K and,
therefore, needs to be located at some distance from the TROPI
UV/Vis/NIR module. The temperature of the electronics box will be
actively stabilized.

Table A2-2 gives the estimated size, mass, power and data rates for the
three TROPI modules.

> **Table A2-2**. Estimated size, mass, power and data rate for the
> TROPI modules. Studies are underway to obtain more accurate values for
> mass and data rate.

+-------+---------+-----+-----------+--------------+----------------+
| **mod | *       | *   | **power\  | **data rate\ | **notes**      |
| ule** | *size** | *ma | (W)**     | (Mby         |                |
|       |         | ss\ |           | tes/s)^1)^** |                |
|       |         | (kg |           |              |                |
|       |         | )** |           |              |                |
+-------+---------+-----+-----------+--------------+----------------+
| UV/Vi | 3       | 80  | 60        | 1.5 -6       | Need           |
| s/NIR | 8x45x56 |     | nominal\  |              | unobstructed   |
|       | cm^3^   |     | 100 peak  |              | cross track    |
|       |         |     |           |              | view +/- 58^o^ |
|       |         |     |           |              | from nadir,    |
|       |         |     |           |              | unobstructed   |
|       |         |     |           |              | view for the   |
|       |         |     |           |              | solar port,    |
|       |         |     |           |              | and cold space |
|       |         |     |           |              | view for the   |
|       |         |     |           |              | passive        |
|       |         |     |           |              | coolers.       |
|       |         |     |           |              |                |
|       |         |     |           |              | Perform daily  |
|       |         |     |           |              | calibration    |
|       |         |     |           |              | views of the   |
|       |         |     |           |              | sun.           |
+-------+---------+-----+-----------+--------------+----------------+
| e     | 0.04    | 20  | 40        | negligible   | Shape can be   |
| lectr | m^3^    |     | nominal\  |              | modified as    |
| onics |         |     | 50 peak   |              | needed         |
+-------+---------+-----+-----------+--------------+----------------+
| total |         | 100 | 100       | 1.5 -6       |                |
| mass, |         |     | nominal\  |              |                |
| p     |         |     | 150 peak  |              |                |
| ower, |         |     |           |              |                |
| and   |         |     |           |              |                |
| data  |         |     |           |              |                |
| rate  |         |     |           |              |                |
+-------+---------+-----+-----------+--------------+----------------+

^1)^ The nominal average data rate will be 1.5 Mbytes / s, the peak data
rate can be up to 6 MBytes / s. The peak data rate will only be used
during special events or calibration campaigns (max 2 weeks) and during
the Launch and Early Operations Phase.

**Appendix 3. The Tropospheric Infrared Mapping Spectrometer (TIMS)**

**Material to be added.**

**Appendix 4. Measurement coincidences with some relevant sensors on
operational satellites**

The figure below quantifies the coincidences between **MISSION NAME?**
(in 830 km high, 53^o^ inclination, orbit) observations and thermal
infrared observations from NPOESS/METOP. The top plot shows that, of the
6-8 daily SMLS observations made [everywhere]{.underline} from 30-60º
latitude, 1-3 are coincident (within 30 minutes) with an observation by
CRIS on NPOESS (13:30 and 17:30 equator crossing times) or by IASI on
METOP (21:30 crossing time) [every day]{.underline}. In the tropics,
SMLS and CRIS/IASI coincidences occur in \~1 month periods with \~1
month gaps. The lower plot shows those observations that are
[also]{.underline} coincident with TROPI observations. This pattern
naturally follows the TROPI coverage, shown in figure 2 of the text of
this document, with one or more coincidences occurring about 50% of the
time at mid-latitudes.

![](media/image23.png){width="4.5152777777777775in" height="5.1625in"}

**Appendix 5. Comparing coverages from precessing and from
sun-synchronous orbits**

The figures below compare global and temporal coverage of SMLS and TROPI
UV/Vis in an 830-km 53^o^-inclination precessing-local-time orbit (top),
and in an 830-km 99^o^-inclination sun-synchronous orbit (bottom).
Everywhere within a given color is measured that number of times per
24-hour period.

![](media/image1.png){width="5.091666666666667in"
height="2.9791666666666665in"}

![](media/image24.png){width="5.168055555555555in"
height="3.0104166666666665in"}

[^1]: Pieternel Levelt lead investigator, Claude Camy-Peyret, co-lead
    investigator.