The Atmospheric Imaging Mission for Northern Regions: AIM-North
Ray Nassar
Climate Research Division, Environment and Climate Change Canada, Toronto, ON,
Canada ([email protected])
Chris McLinden
Air Quality Research Division, Environment and Climate Change Canada, Toronto,
ON, Canada
Christopher E. Sioris
Air Quality Research Division, Environment and Climate Change Canada, Toronto,
ON, Canada
C. Thomas McElroy
Department of Earth and Space Science, York University, Toronto, ON, Canada
Joseph Mendonca
Climate Research Division, Environment and Climate Change Canada, Toronto, ON,
Canada
Johanna Tamminen
Earth Observation Research, Finnish Meteorological Institute, Helsinki, Finland
Cameron G. MacDonald
Department of Physics and Astronomy, University of Waterloo, Waterloo, ON, Canada
Cristen Adams
Environmental Monitoring and Science Division, Government of Alberta, Edmonton,
AB, Canada
Accepted to the Canadian Journal of Remote Sensing, special issue on Arctic and Northern Monitoring and Applications
June 2019
Céline Boisvenue
Pacific Forestry Centre, Canadian Forest Service, Natural Resources Canada, Victoria,
BC, Canada
Adam Bourassa
Institute of Space and Atmospheric Studies, University of Saskatchewan, Saskatoon, SK,
Canada
Ryan Cooney
Canadian Space Agency, Saint-Hubert, QC, Canada
Doug Degenstein
Institute of Space and Atmospheric Studies, University of Saskatchewan, Saskatoon, SK,
Canada
Guillaume Drolet
Direction de la recherche forestière, Québec Ministère des Forêts, de la Faune et des
Parcs, Québec, QC, Canada
Louis Garand
Meteorological Research Division, Environment and Climate Change Canada, Dorval,
QC, Canada
Ralph Girard
Canadian Space Agency, Saint-Hubert, QC, Canada
Markey Johnson
Air Health Science Division, Health Canada, Ottawa, ON, Canada
Dylan B.A. Jones
Department of Physics, University of Toronto, Toronto, ON, Canada
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June 2019
Felicia Kolonjari
Climate Research Division, Environment and Climate Change Canada, Toronto, ON,
Canada
Bruce Kuwahara
University of Waterloo, Waterloo, ON, Canada
Randall V. Martin
Department of Physics and Atmospheric Science, Dalhousie University, Halifax, NS,
Canada
Charles E. Miller
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California,
USA
Norman O’Neill
Université de Sherbrooke, Sherbrooke, QC, Canada
Aku Riihelä
Finnish Meteorological Institute, Helsinki, Finland
Sebastien Roche
Department of Physics, University of Toronto, Toronto, ON, Canada
Stanley P. Sander
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, U.S.A.
William R. Simpson
University of Alaska Fairbanks, Fairbanks, AK, U.S.A.
Gurpreet Singh
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June 2019
Department of Earth and Space Science, York University, Toronto, ON, Canada
Kimberly Strong
Department of Physics, University of Toronto, Toronto, ON, Canada
Alexander P. Trishchenko
Canada Centre for Remote Sensing, Natural Resources Canada, Ottawa, ON, Canada
Helena van Mierlo
Canadian Space Agency, Saint-Hubert, QC, Canada
Zahra Vaziri
Department of Earth and Space Science, York University, Toronto, ON, Canada
Kaley A. Walker
Department of Physics, University of Toronto, Toronto, ON, Canada
Debra Wunch
Department of Physics and School of the Environment, University of Toronto, Toronto,
ON, Canada
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June 2019
Résumé
AIM-North est une proposition de mission satellitaire visant à acquérir des observations de
l’hémisphère nord à une fréquence et une densité sans précédent pour le suivi des gaz à effets
de serre (GES), de la qualité de l’air (QA) et de la végétation. AIM-North serait constituée de
deux satellites placés dans une orbite hautement elliptique permettant d’observer les surfaces
situées entre 40° et 80°N à plusieurs reprises au cours d’une journée. Chaque satellite
transporterait un spectromètre imageur opérant dans le proche infrarouge et l’infrarouge de
courte longueur d’onde pour la mesure du CO2, CH4 et CO, ainsi qu’un spectromètre imageur
opérant dans l’ultraviolet et le visible pour mesurer la qualité de l’air. Ces deux instruments
mesureraient aussi la fluorescence induite par le soleil qui est émise par la végétation. De
plus, un imageur de nuages ferait des observations en temps quasi-réel, fournissant une
information permettant de pointer les satellites sur les régions les moins ennuagées. De
multiples satellites géostationnaires (GEO) pour la mesure de la QA et des GES sont prévus
pour la décennie 2020 mais ceux-ci ne couvriront pas les régions les plus nordiques comme
l’arctique. AIM-North pallierait à ce manque par des observations quasi-géostationnaires des
latitudes nordiques qui chevaucheraient la couverture des satellites GEO, facilitant ainsi
l’inter-comparaison et la fusion des divers jeux de données. Ces données amélioreraient notre
capacité à prévoir la QA des régions nordiques et à quantifier les flux d’espèces de GES et de
QA provenant des forêts, du pergélisol, de la combustion de biomasse et des activités
anthropiques, approfondissant notre compréhension scientifique de ces processus et appuyant
les politiques environnementales.
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June 2019
The Atmospheric Imaging Mission for Northern Regions: AIM-North
AIM-North is a proposed satellite mission that would provide observations of
unprecedented frequency and density for monitoring northern greenhouse gases
(GHGs), air quality (AQ) and vegetation. AIM-North would consist of two
satellites in a highly elliptical orbit formation, observing over land from ~40-
80°N multiple times per day. Each satellite would carry a near-infrared to
shortwave infrared imaging spectrometer for CO2, CH4, and CO, and an
ultraviolet-visible imaging spectrometer for air quality. Both instruments would
measure solar induced fluorescence from vegetation. A cloud imager would make
near-real time observations, which could inform the pointing of the other
instruments to focus only on the clearest regions. Multiple geostationary (GEO)
AQ and GHG satellites are planned for the 2020s, but they will lack coverage of
northern regions like the Arctic. AIM-North would address this gap with quasi-
geostationary observations of the North and overlap with GEO coverage to
facilitate intercomparison and fusion of these datasets. The resulting data would
improve our ability to forecast northern air quality and quantify fluxes of GHG
and AQ species from forests, permafrost, biomass burning and anthropogenic
activity, furthering our scientific understanding of these processes and supporting
environmental policy.
1. Introduction
The Arctic and adjacent northern high latitude regions comprise an area of Earth
undergoing rapid and unprecedented change. Temperatures in the northern high
latitudes have risen by about three times the global average increase over recent years
and this trend is expected to continue for the future (IPCC, 2013). These temperature
changes are closely-linked to changes in atmospheric composition and vegetation;
therefore, obtaining observations to monitor relevant atmospheric and vegetation
parameters to quantify changes and better understand the relevant processes is of high
importance.
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June 2019
Both in situ observations and atmospheric remote sensing from space have
improved our understanding of factors that determine global atmospheric composition,
such as surface sources and sinks as well as chemistry. However, in situ monitoring of
the Arctic and adjacent northern regions is a challenge related to the large spatial extent,
sparse infrastructure, remoteness and harsh weather conditions. Remote sensing of
northern regions from space has its own challenges; but many of these challenges can
be overcome with a mission that is optimized specifically for observing the North.
The Atmospheric Imaging Mission for Northern Regions (AIM-North) is a
proposed satellite mission that would provide observations of unprecedented frequency,
density and quality for monitoring greenhouse gases (GHGs), air quality (AQ), clouds
and vegetation productivity in northern regions. AIM-North would use a pair of
satellites in a highly elliptical orbit (HEO) formation, enabling dense observations over
land from ~40-80°N, multiple times per day. The project is a collaborative effort
between Environment and Climate Change Canada (ECCC) and the Canadian Space
Agency (CSA) with interest and involvement from other federal and provincial
government departments, Canadian academia and industry, and numerous international
scientists, with the potential for still broader participation.
AIM-North is undergoing Phase 0 studies for the CSA from 2019-2020. If
approved for further phases, launch would be feasible around 2026. The industrial team
for Phase 0 is composed of ABB Inc., Airbus Defense and Space and MDA Systems
Limited, with consultation support from NASA’s Jet Propulsion Laboratory. Although a
number of previous studies have already been conducted that have led to the AIM-North
mission concept presented here, some key questions regarding the design of the mission
remain unanswered at present. This paper describes the scientific and programmatic
motivation for the AIM-North mission, summarizes some key studies conducted to date,
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June 2019
describes the observing requirements to meet the scientific objectives, the evolving
mission concept (orbit, instruments and the general mission plan), the expected
validation capacity and finally, presents the link to decision support and the
international context for such a mission.
1.1 Arctic and Boreal Carbon Cycle
The boreal forests comprise ~30% of the world’s forest area and cover much of
North America and Eurasia (Brandt 2009). Boreal forests represent an important carbon
sink of about 0.5 Pg C yr−1 (Pan et al. 2011) equivalent to 17±6% of the global land
CO2 sink (Le Quéré et al. 2015), and they are an important driver of the seasonal cycle
in global atmospheric CO2 levels (Keeling et al. 2015; Forkel et al. 2016). Their
growing season has lengthened due to climate change (Graven et al. 2017; Pulliainen et
al. 2017), while at the same time disturbances such as fire and some types of insect
infestations are increasing (Bond-Lamberty et al. 2007; Veraverbeke et al., 2017; Seidl
et al. 2017; Natural Resources Canada, 2018) and restricting conditions on productivity
are changing. How this combination of factors will alter the carbon balance of boreal
forests in the future is unclear (Gonsamo et al. 2017, Helbig et al. 2017, Maaroufi et al.
2015; USGCRP, 2018).
Permafrost is ubiquitous in the northern high latitudes, spanning an area of
~18.8 million km2. The vast carbon content of the northern permafrost regions has been
estimated at 1672 PgC (Tarnocai et al. 2009), , 1100-1500 PgC (Hugelius et al. 2014) or
1330-1580 PgC (Schuur et al. 2015), with the higher estimates corresponding to almost
twice the mass of carbon in Earth’s atmosphere. As permafrost thaws, microbes
metabolize carbon and it is estimated that ~5-15% (Schuur et al. 2015) of the carbon
will be released to the atmosphere as CO2 or CH4 by ~2100, but the timing, spatial
distribution and CO2:CH4 ratio of future emissions is highly uncertain (Schneider von
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June 2019
Diemling et al. 2012; Schuur et al. 2015; AMAP, 2015a). This uncertainty is coupled
with changes to CO2 uptake related to changes in vegetation density (greening or
browning) in the Arctic, which have also been reported as the North warms (Lucht et al.
2002, Piao et al. 2006; Pearson et al. 2013; Bhatt et al. 2013; Fraser et al. 2014; Ju and
Masek, 2016; Edwards and Treitz, 2018; Treharne et al. 2018; USGCRP, 2018). It
remains important to reduce uncertainties and better understand these feedbacks on
future climate change.
Anthropogenic impacts on the carbon cycle are also increasing in the northern
high latitudes as a result of new and planned resource extraction projects and other new
anthropogenic activities enabled by a warming climate. Dense and frequent
observations of CO2, CH4 and solar induced fluorescence (SIF, an indicator of
photosynthetic activity) would improve our ability to monitor the carbon balance of
boreal forests and permafrost and enhance our ability to quantify anthropogenic
greenhouse gas emissions at various scales (e.g. Nassar et al. 2017)
AIM-North would provide observations to better quantify these biospheric and
anthropogenic changes and thus would provide support for the Government of
Canada’s ambitions to reduce greenhouse gas emissions and deliver on Canada’s
commitments under the United Nations Framework Convention on Climate Change
(UNFCCC) Paris Agreement.
1.2 Air Quality at Northern Latitudes
Canadian air quality is generally excellent and air quality has been improving over
North America over the past decade. Across North America, emissions of nitrogen
oxides (NOx), sulphur dioxide (SO2), and most other air pollutants (with ammonia being
the exception) have declined in recent decades as a result of emissions regulations on
cars and power plants, including, e.g., the use of flue gas de-sulfurization (FGD), and an
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June 2019
overall reduction in the use of coal (e.g. Kharol et al. 2017). As a result, there is a
decreasing trend in aerosol that can be detected from satellite observations of aerosol
optical depth (AOD) over south-eastern Canada (e.g. Boys et al. 2014; Keppel-Aleks
and Washenfelder, 2016). These changes in aerosols have led to significant health-
related improvements for Canadians (2000-2011) (Stieb et al. 2015). Meanwhile, trends
in ground-level ozone vary from region to region but tend to be small (Geddes et al.
2009; Pugliese et al. 2014; Tarasick et al. 2016). Nonetheless, exposure to air pollution
in Canada is responsible for a large number of premature mortalities. A 2018
Government of Canada study found 2.8% of all deaths in Canada are a result of outdoor
air pollution. A Canadian Medical Association study estimated that 21,000 deaths can
be attributed to air pollution annually, with an associated cost of $8B (CMA, 2008) and
by 2030, the accumulated cost is forecasted to be $250B (values in Canadian dollars).
The compounds thought to be responsible for the majority of these deaths are ground
level ozone, NO2, and particulate matter (PM). It is this realization that prompted
Canada to formulate an Air Quality Health Index (AQHI) (Gillet et al. 2004), which
uses the concentrations of these three pollutants to define a scale (ranging from 0 to
10+) to help Canadians understand what the air quality around them means to their
health. These pollutants, along with others such as SO2, formaldehyde, and carbon
monoxide (CO), result (directly or indirectly) from combustion. Anthropogenic sources
include fossil fuel combustion, including automobiles and industrial operations,
hydraulic fracturing (”fracking”), smelting, mining and oil sands operations. Perhaps the
world’s largest single source of SO2 is Norilsk, a collection of copper and nickel
smelters, located in Siberia at 69.38°N (Fioletov et al. 2016). Natural sources include
forest fires and volcanic eruptions.
Wildfires are particularly intense sources of air pollution in the spring and summer.
While variable from year to year, there has been an increase in the number and severity
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June 2019
of fires in Canada over the past half-century and this is expected to continue with the
warming climate (Gillet et al. 2004; Kirchmeier-Young et al. 2017; 2019). Changing
fire regimes in Canada’s boreal forests are expected to result in increasing fire
frequency and severity (Veraverbeke et al. 2017) as well as a northward march in large
fires with associated impacts to air quality, human health and decreased carbon storage
(Bond-Lamberty 2007; Brandt 2009; Kurz 2013). Forecasting the air quality from these
events using state-of-the-science models remains challenging and additional and
detailed observations are required to guide and verify improvements.
Although some air pollution in the Arctic is a result of local sources (Schmale et al.
2018) a significant fraction is the result of long-range transport from both natural and
anthropogenic sources (AMAP, 2015b). Modelling studies show that European and
Asian sources are important for near-surface pollution, while transport from Asia and
North America may occur at higher altitudes (Walker et al. 2012; Xu et al. 2017). Arctic
Haze is a pan-Arctic pollution phenomenon lasting from late winter to early spring (e.g.
Herber et al. 2002). Further, with the accelerating disappearance of multi-year ice,
Arctic sea routes are more readily negotiated by large container ships. In the coming
years, increased marine traffic through the Arctic, in an attempt to reduce fuel used in
long journeys, could impact air quality by increasing ozone, NO2, SO2, and increasing
the abundance of aerosols (Gong et al. 2018) which will likely affect cloud formation.
1.3 Mission Objectives
The broad objectives of AIM-North are:
• Monitor greenhouse gas (GHG) and air quality (AQ) species over northern regions to
quantify natural and anthropogenic sources/sinks in support of policies to mitigate
climate change and improve air quality.
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• Address the gap in northern AQ and GHG spatial and temporal coverage from the
emerging international virtual constellations and provide overlap with geostationary
(GEO) observations over North America, Europe and Asia.
These objectives closely align with the priorities and mandate of the
Government of Canada and drive a number of specific mission science objectives
outlined in the AIM-North Mission Objectives Document (Nassar et al. 2018) and
restated here in Table 1.
2. Observing Requirements
To obtain atmospheric measurements that provide information on processes occurring at
the Earth’s surface-atmosphere interface, such as emissions or uptake, observations with
some sensitivity to the lowest part of the atmosphere are required. For AQ,
observations are typically made of sunlight reflected off the surface of the Earth in the
ultraviolet-visible (UV-vis) spectral range where these gases absorb light with a
characteristic spectral signature. For GHGs, absorption spectra from reflected sunlight
in the near-infrared and shortwave infrared (NIR-SWIR) are used. For both the UV-vis
and NIR-SWIR, only observations during daylight are obtained and the signal strength
is related to the surface albedo. In addition to observing in the correct spectral region,
precision, accuracy, coverage, rapid revisit rates and spatial resolution are all key
considerations in meeting the science objectives identified for AIM-North. Based on
experience from existing greenhouse gas and air quality satellite missions, available
requirements for upcoming missions and some targeted studies, a set of observing
requirements has been identified for AIM-North to direct Phase 0 studies. These
requirements are summarized in Tables 2 to 5, where inn most cases both a ‘Goal’
requirement and ‘Threshold’ requirement are given. The goal is the desired quantity to
achieve if practical, whereas the threshold is a more easily achievable minimum
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June 2019
requirement.
For all GHGs, AQ species and SIF, the requirement for pixel size is 2x2 km2
(goal) and 4x4 km2 (threshold) with a revisit time of better than 90 minutes (goal) or
180 minutes (threshold) during daylight, when clouds permit, over a mission duration of
3 (goal) to 5 (threshold) years. AIM-North observing requirements are linked to
international standards and/or aligned with the capabilities of future missions planned
by international space agencies. Justification or the origin of each requirement is
provided in the AIM-North Mission Objectives Document (Nassar et al. 2018). The
rapid revisit rate requirements with diurnal sampling (Table 2) will enhance our
understanding of carbon cycle and air quality processes and support monitoring. The
precision, accuracy and spatial resolution requirements (Tables 2-3) will be challenging
to achieve, but will enable both regional-scale applications and anthropogenic point
source or urban emission quantification north of ~40°N.
Perhaps the most demanding requirements are those for the precision and
accuracy of CO2 (Table 3). The requirements for the column-averaged dry-air mole
fraction of CO2 (XCO2) are a 1-σ single observation precision of ~1 ppm or 0.25% (G =
goal) or ~3 ppm or 0.75% (T = threshold). If the single observation precision meets the
threshold but not the goal, improved precision would still achievable by spatial or
temporal binning of observations. The CO2 accuracy requirement goal is a bias of less
than ~0.2 ppm or 0.05%, with a threshold of ~0.6 ppm or 0.15%. Achieving such a low
bias will very likely require a bias correction at the post-processing stage. These
stringent XCO2 requirements are linked to (but not identical to) the requirements for
Essential Climate Variable (ECVs) identified for the Global Climate Observing System
(GCOS) (CEOS-CGMS, 2015). Similarly, the AIM-North AQ observing requirements
are also aligned with international missions.
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June 2019
AIM-North’s primary SIF observations would be obtained near 758 nm with
high spectral resolution to resolve solar Fraunhofer lines (Frankenberg et al. 2011), but
lower spectral resolution SIF data from the UV-vis may also be acquired, both yielding
multiple observations spanning the daylight diurnal cycle. Observations of GHGs, AQ
species or SIF spanning the diurnal cycle are most informative of the underlying natural
or anthropogenic processes driving the fluxes and furthermore can help to disentangle
the different contributing factors.
Spatially and/or temporally averaging AIM-North data can improve precision
beyond target values for some applications. Alternatively, sequentially combining
multiple images can yield animations of evolving atmospheric composition. No other
satellite mission ever formally proposed could offer equivalent data for atmospheric
composition or vegetation in northern regions.
3. Orbits
Obtaining frequent and sustained space-based observations of the northern high
latitudes presents orbital design challenges.
A number of different Low Earth Orbit (LEO) variations are commonly used by
atmospheric satellites, which generally provide global sampling; however, with a single
satellite, revisit times are on the order of days to weeks. Sentinel 5P TROPOMI
observes multiple AQ species and CH4 from LEO with roughly daily global coverage
(before loss of data due to clouds) using an extremely wide (~2700 km) swath
(Veefkind et al., 2018). With CO2 observations being more demanding, swath widths
tend to be narrower. The CO2 Monitoring Mission (CO2M), a high priority candidate
under consideration in the European Copernicus Programme (Meijer et al. 2018) is the
widest swath LEO CO2 mission currently planned, with competing designs delivering
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June 2019
swaths of ~200-250 km, thus requiring multiple satellites to achieve revisits of 2-3 days.
Wider swath LEO CO2 satellites remain of interest and the technology needed to
achieve this continue to be studied (Miller et al., 2017), but even with a hypothetical
~1000 km swath, three satellites would be needed to achieve daily revisit rates and far
more for diurnal coverage (~3 hourly during daylight).
Geostationary orbit (GEO) enables much more frequent revisit rates from a
single satellite, but only over a limited region. Modern weather forecasting relies on a
constellation of LEO and GEO satellites to obtain observations with dense and frequent
coverage. At present, satellite observations of atmospheric composition have only been
made from LEO, but by the early 2020s, three GEO missions for air quality: NASA’s
Tropospheric Emissions: Monitoring Pollution (TEMPO) (Zoogman et al. 2016),
Europe’s Sentinel-4 (Ingmann et al. 2012) and Korea’s Geostationary Environmental
Monitoring Spectrometer (GEMS) (Kim et al. 2016; 2017) will be operating,
complemented by a number of LEO missions. This virtual constellation will consist of
satellites from multiple countries coordinated by the Committee on Earth Observation
Satellites (CEOS) Atmospheric Composition Virtual Constellation (AC-VC) group
(CEOS-ACC, 2011). A coordinated constellation architecture is also emerging for
GHGs (Crisp et al. 2018), with many existing and planned LEO missions, NASA’s
GeoCarb as the first GEO GHG mission to launch around 2022 (Moore et al., 2018) and
missions such as the Geostationary Emission Explorer for Europe (G3E /
ARRHENIUS) (Butz et al. 2015) considered by other space agencies. A coordinated
GHG constellation would enable major advances in carbon cycle monitoring, including
both natural and anthropogenic sources, in support of the UNFCCC Paris Agreement.
Since GEO satellites are stationed at a fixed longitude over the equator and
synchronized with Earth’s rotation, they have the ability to view within a region defined
by some viewing zenith angle (VZA) limit. VZA limits differ for the retrieval of
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June 2019
different constituents, and can range from ~55-70° (Trishchenko et al., 2019a). For
typical a VZA limit of 62°, regions poleward of ~48-54°N/S (depending on longitude)
are out of range. Although increased use of GEO observing has many advantages, it
creates a serious gap in equivalent high frequency coverage of high-latitude areas,
encompassing most of Canada, Russia, Alaska and northern Europe.
Satellite observations from a highly elliptical orbit (HEO) can address this gap,
since near the highest point of an elliptical orbit (the apogee), the satellite moves very
slowly relative to the Earth. A properly designed 2-satellite constellation can thus
provide quasi-geostationary observations of the North. Figure 1 depicts GEO and HEO
viewing geometries using the Alberta oil sands (~57°N) as an example. From GEO, the
VZA for a point at this latitude is at least 65° and the observed atmospheric column is
far from vertical, which causes distorted pixels and is problematic for data quality. From
a HEO near the critical inclination (i=63.435°N), the VZA is more favourable when the
satellite is within a few hours of apogee. Any longitude offset of the location observed
from the satellite longitude also increases the VZA further, compounding the difficulty
of high-latitude viewing from GEO, where the high Arctic is completely out of range.
The World Meteorological Organization (WMO) has long recognized the role
for HEO in providing meteorological observations of the Arctic with its Vision for the
Global Observing System in 2025 (WMO, 2010). This has been expanded in the Vision
for the WMO Integrated Global Observing System in 2040 (WMO, 2018). CEOS has
recognized that HEO missions can address this high-latitude gap for GHG (Crisp et al.
2018) and AQ (CEOS-ACC, 2011) observations and they would have an important role
in future global GHG or AQ constellations. One of the most unique features of AIM-
North is its proposed plan to observe GHGs and AQ from HEO.
The use of HEOs for Earth observation was first suggested by Kidder and von
der Haar (1990). Canada explored the possibility of a HEO mission for the Polar
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Communications and Weather (PCW) mission concept (Garand et al. 2014) as far back
as 2007. The Polar Highly Elliptical Orbit Science (PHEOS) Weather, Climate and Air
quality (WCA) instrument suite (McConnell et al. 2012; LaChance et al. 2012) was
considered by CSA as an enhancement to PCW. PCW and PHEOS-WCA completed
parallel Phase 0 and A studies in 2012. Although PCW and its possible enhancements
are no longer under consideration, these studies spurred interest in the potential for
atmospheric composition observations from HEO that continues today.
A mission concept study (2016-2018) involving CSA, ECCC and an industry
team of ABB Inc., Airbus Defense and Space and MDA Systems Limited explored a
standalone HEO mission for AQ and GHG, which built upon the PHEOS-WCA studies
and this concept has been recognized as AIM-North since November 2017. The
European Commission has resumed studies on a HEO meteorological concept through
the European Space Agency, with Canadian participation, that has many similarities to
the meteorological component of PCW.
HEOs are an entire class of orbits with many possibilities (Trichtchenko et al.
2014; Trishchenko et al. 2016) and the specific HEO option for AIM-North has not yet
been chosen. The Molniya orbit is the most well-known HEO, due to its use by Soviet
communication satellites since the 1960s. The Molniya orbit was investigated for PCW
(Trishchenko and Garand, 2011) and is one possible candidate for AIM-North. A
Molniya orbit with a period of ~12 hours, an inclination of 63.435° (the critical
inclination), an apogee altitude of ~39,800 km and a perigee of ~500 km or higher, has
two apogee positions separated by 180° longitude. Adding a second satellite in the same
orbital plane, 6 hours behind, gives a second ground-track, which together yield apogee
positions every 90° longitude. A three apogee (TAP) orbit has a ~16-hour period and a
ground-track with three apogee positions separated by 120° longitude, which is another
viable option and has the advantage that the two satellites share a single ground track
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with a repeat cycle of 2 days. Figure 2 shows a polar view of potential Molniya and
TAP ground tracks and apogee positions. Although TAP apogees are slightly higher
(43,493 km for eccentricity = 0.55) than for Molniya orbits, the higher perigee (~5000-
10000 km) reduces exposure to proton radiation, since the satellite does not cross
Earth’s van Allen belts (Trishchenko et al. 2011) as with a lower perigee. Other HEO
possibilities include multiple apogee (MAP) orbits, such as a 14-h orbit which has 7
apogees over a period of 12 days, a 15-h orbit giving 5 apogees over a period of 7 days
and an 18-h orbit giving 8 apogees over 3 days (Trichtchenko et al, 2014; Trishchenko
et al, 2016). Additional recommendations on how to reduce total ionizing dose and
therefore extend the mission lifetime for 14-h, 15-h and 16-h HEO constellations are
presented by Trishchenko et al. (2019b).
For each HEO, favourable observations of the North can be made for roughly
2/3 of the orbit (i.e. 8 h of a 12-h orbit or ~10.7 h of a 16-h orbit, etc.) but the exact
observing period depends on orbital parameters and a VZA limit of 60°, which is
similar to the maximum VZA assumed for current and future LEO and GEO missions
for GHGs and AQ. Since the GHG and AQ observations require reflected sunlight,
there is also a solar zenith angle (SZA) requirement of ~80°, and VZA and SZA
together define the path length of the light through the atmosphere (airmass). Potential
coverage from Molniya and TAP orbits (for a given date/time) can be discerned from
Figure 2.
The final selection of a HEO for AIM-North requires consideration of the timing
of apogee with respect to solar illumination. Since the local solar time of the apogee
changes over the course of a year, maintaining appropriate timing with solar
illumination at priority observing times is crucial. When the apogee is near local
midnight, there is a period of a few weeks with few if any viable observations. The
most sensible approach is to set this “blackout” period to occur during the northern
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winter, perhaps January or February. However, for a Molniya orbit, the date of apogee
for a given local time drifts by ~317-329 days per cycle (Trishchenko and Garand,
2011), while for the TAP orbit the drift is ~348-358 days and for an 18-h orbit the drift
can exceed 360 days. An orbit where this cycle is close to one year (repeatable
annually) would offer an advantage. The advantages of HEO for high latitude viewing
are well-established and several HEO orbit options could meet the needs of AIM-North,
however, more detailed studies weighing all the relevant factors for AIM-North, are
required in order to determine an optimal HEO solution.
4. Instruments
Each of the two AIM-North satellites would have two main instruments: an Ultraviolet-
Visible Spectrometer (UVS), which primarily measures air quality parameters; a near-
infrared and shortwave infrared (NIR-SWIR) spectrometer, which primarily measures
GHGs and SIF; plus a small cloud imager. Each of these instruments would have their
own input optics and pointing systems as described in the subsections below, followed
by some potential mission enhancements.
4.1 Ultraviolet-Visible Spectrometer (UVS)
AIM-North would use a nadir-viewing ultraviolet-visible spectrometer (UVS) to
measure reflected sunlight to retrieve O3, NO2, BrO, HCHO, SO2, OClO, CHOCHO,
aerosols and other species for air quality research and operational forecasting. The UVS
would be a dispersive spectrometer spanning 290-786 nm with about 2000 spectral
elements, giving a spectral sampling of ~0.25 nm (before binning 3 spectral samples).
An order sorter filter would be used to deal with the wide spectral range. The input
aperture of the instrument would have a diameter of 70 mm. The UVS would image the
above species with pushbroom scanning, for example, by acquiring ~500 simultaneous
observations every 2.8 seconds using one dimension of a space-qualified focal plane
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array (FPA), imaging the field of regard every ~60-180 minutes of daylight.
Observation pixels of ~3x5 km2 at the sub-satellite point from apogee, are being
considered, but pixels become smaller when then satellite is below apogee and distort in
each dimension for non-zero VZAs up to a factor of ~2 for VZA=60°. Figure 3 shows
an example of a simplified viewing pattern for the UVS (assuming an apogee around
midday and a TAP orbit), in which the UVS could cover each coloured region in three
east-to-west sweeps.
4.2 Near-infrared and Shortwave Infrared (NIR-SWIR) Spectrometer
AIM-North would image CO2, CH4 and CO by observing spectra of reflected
shortwave infrared (SWIR) and near infrared (NIR) solar radiation. This could
potentially be carried out with either a dispersive spectrometer (similar to the UVS) or
with an imaging Fourier Transform Spectrometer (IFTS). During the mission concept
study, the baseline design was for an IFTS with 4 NIR-SWIR spectral bands using
separate but identical FPAs and the same fore-optics. Preliminary estimates for the SNR
requirements are stated in Table 4, but such values are highly dependent on the specific
spectral band selected, spectral resolution and type of spectrometer (via the instrument
lineshape), so changing any of these factors as the instrument design evolves will also
change the SNR requirements. New air quality and greenhouse gas retrieval studies are
currently underway, which could also lead to refinements to these numbers as Phase 0
progresses.
The interferometer, which is the core of an IFTS, would use two moving mirrors
on a pivot arm, rather than a traditional Michelson interferometer design (one fixed and
one moving mirror). With the two mirrors moving in opposite directions, only half the
displacement is needed to achieve a maximum optical path difference of 4 cm and
spectral sampling of 0.25 cm-1 (resulting in a spectral resolution of ~0.3 cm-1). The IFTS
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interferometer design has LEO spaceflight heritage in Canada’s Atmospheric Chemistry
Experiment (ACE, Bernath et al. 2005), Japan’s Greenhouse Gases Observing Satellite
(GOSAT, Kuze et al. 2009), GOSAT-2 and other missions; however, imaging would be
a new application facilitated by the possibility for longer stare times from a HEO or
GEO vantage point. Internationally, space-based IFTS instruments are already planned
for the mid-IR and longwave IR, including the GEO operational meteorological
Meteosat Third Generation Infrared Sounder (MTG-IRS) from Europe (Tjemkes et al.
2016) and the Geosynchronous Interferometric Infrared Sounder (GIIRS) from China
(Yang et al. 2017). CSA is currently investing in raising the technology readiness level
of Canadian IFTS technology by supporting research by Canadian industry and
academic partners in consultation with ECCC.
Potential AIM-North IFTS pointing and scanning patterns are still being
investigated, with multiple factors such as input aperture size, the dimensions and frame
rate of the FPA and the integration time in the trade space. A recent IFTS configuration
under consideration would use a 200 mm diameter input aperture to image 128×128
pixels on each FPA with a ground sampling distance of 4x4 km2 (at the sub-satellite
point from apogee), then step to a new position. In many ways, this configuration
resembles the design of the proposed GEO-FTS instrument studied by NASA (Key et
al. 2012, Xi et al. 2015).
With an integration time of 60 seconds, the AIM-North IFTS as configured
above would meet the threshold SNR requirements, while integrating for ~180 s would
meet the goal. With a larger input aperture, integration times could be reduced for the
same SNR, but this would lead to a larger instrument overall. Selecting the integration
time must balance the need for acquisition of data with sufficient signal-to-noise ratio
(SNR), with the fact that longer integration times give measurements subject to more
satellite motion relative to the Earth as well as atmospheric motion (clouds and trace
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gases). The present plan is to select a baseline integration time, but build flexibility into
the mission design such that it can easily be changed during operations, perhaps even
modified for different viewing conditions or observing priorities.
ESA’s SCIAMACHY (Buchwitz et al., 2005), NASA’s Orbiting Carbon
Observatory 2 (OCO-2) (Crisp et al. 2004) and ESA’s Tropospheric Monitoring
Instrument (TROPOMI) have demonstrated the capabilities of dispersive/grating
spectrometers for measuring CO2, CH4, CO and SIF from LEO and NASA’s GeoCarb
will use a grating spectrometer to observe these species from GEO (Moore et al. 2018).
A grating spectrometer is also a possibility for AIM-North and in fact, our dispersive
band selection and spectral sampling in Table 4 is based directly on plans for GeoCarb
(O’Brien et al. 2016). Advantages of a grating spectrometer would include
simplification of the mission design with more commonalities in scanning and hardware
between the UVS and NIR-SWIR spectrometer and even the potential for sharing fore-
optics or merging the two instruments. Advantages of an IFTS relate to the more direct
imaging approach by using both dimensions of an FPA to image, and more importantly
the greater potential for cloud avoidance due to the shape of the field of view (FOV).
Large gains could be obtained with implementation of an intelligent pointing approach,
which our simulations indicate yields a greater improvement for a square FOV than a
narrow and elongated linear one (i.e. pushbroom scanning) with the same number of
pixels. Intelligent pointing depends on cloud data to inform pointing decisions. The
most reliable method of obtaining such data for the North would likely be to include a
cloud imager in the mission, as discussed below.
4.3 Cloud Imager and Intelligent Pointing
At any given moment, about 70% of the Earth is covered by cloud (Stubenrauch et al.
2013). However, since the clearest regions of the world tend to be dry desert regions of
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the tropics and low latitudes, the northern mid- and high latitudes are cloudier than the
global average. During the Arctic summer, the monthly mean cloud cover may reach as
high as 85% (Kay et al. 2016).
Measurements of atmospheric GHGs and AQ are sensitive to clouds to varying
degrees. Retrieval of CO2 with its high precision and accuracy requirements is among
the most sensitive and thus aggressive filtering is applied to remove data that are
contaminated by clouds for current missions. Clouds can have a direct effect if they are
in the light path, but other cloud effects (scattering into the light path, shadows, etc.) can
also interfere with observations. In OCO-2 version 8 data, only 7-12% of observations
in a given month pass all of the cloud and other data quality filters (Crisp et al. 2017),
meaning that ~90% of observations are lost due to cloud and other factors. Data loss for
GOSAT is similar (Yoshida et al. 2013), but GOSAT-2 uses an intelligent pointing
approach where a cloud imager provides real time information on clear locations within
the field of regard, where the GHG observations have the best chance of success.
GOSAT-2’s intelligent pointing from LEO improves the yield of cloud-free data from
partially cloudy regions. However, a cloud-informed intelligent pointing approach is
expected to be much more powerful from GEO or HEO, since with the larger
instantaneous field of regard due to a much higher satellite vantage point, more pointing
options are available at any given instant. A cloud imager with the capability to observe
the entire Earth disk north of 40°N, every two hours or better, could provide sufficient
information to inform pointing, which would greatly improve the yield of clear-sky
observations of GHGs and possibly also AQ species.
Figure 4 outlines the basic approach to intelligent pointing and its potential.
Figure 5 illustrates the coverage obtained with this approach for a 90-minute period for
an IFTS with some assumed parameters. Studies are underway to better quantify the
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impact of intelligent pointing from HEO rather than a simple pointing approach.
Preliminary simulations suggest that more than 50% of AIM-North observations would
be cloud-free using intelligent pointing, but a number of factors impact the results, such
as the size and shape of the FOV and the number FOVs per repeat cycle. . Figure 6
shows that for simulated observations from a TAP Orbit (e = 0.55, i = 63.435°, and
apogee at local noon on July 25) a square 120x120 IFTS FOV with 4x4 km2 ground
pixels results in more cloud-free observations than 40 scans with a 360-pixel 5.333 km
x 1080 km dispersive instrument FOV, covering the same total area and number of
pixels in the same amount of time. A more detailed description of our intelligent
pointing studies will be given in a future publication, but this example suggests better
efficiency for a FOV with a low aspect ratio (~1:1) for observing between clouds, hence
the advantage of intelligent pointing for a dispersive instrument may not be as large as
for an IFTS.
Although data to inform pointing would be the main objective of a cloud imager,
higher spatial resolution cloud data could enhance AQ retrievals, hence requirements
for these different applications are separated in Table 2. Clouds are of course also
important for both weather and climate. AIM-North observations of cloud coverage and
potentially also cloud optical thickness and cloud-top height, with frequent revisit and
good spatial resolution would have a number of applications related to weather and
climate. Cloud data could be of use for weather and climate model evaluation, process
studies, or for initialization of forecasts in numerical weather prediction or solar
irradiance forecasts for the rapidly expanding renewable energy industry (Mathiesen et
al. 2018).
4.4 Potential Mission Enhancements
GHG and air quality observations are the primary observing objectives of AIM-North;
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however, an enhancement to the mission above the baseline is also possible by adding
one or more detector arrays and a cryo-cooler to cover the longwave and mid-wave
infrared regions as proposed for PHEOS-WCA (McConnell et al. 2012, Lachance et al.
2012). This would enable measurements of temperature, water vapour and atmospheric
motion vectors in northern regions, along with numerous AQ species and CO2 and CH4
with mid- to upper tropospheric sensitivity during days and nights and during all
seasons, to support weather forecasting, air quality studies, studies of vegetation carbon
cycling and a number of other applications.
A number of other instruments for Earth observation (or even other applications
like communications or search and rescue) could benefit from access to HEO. Although
these other ideas are not being pursued directly by the AIM-North team, the
accommodation of additional/hosted payloads may be possible and could potentially
strengthen the rationale for the overall mission.
5 Validation Capacity
Validation is a key component of ensuring high accuracy and precision.
Different forms of validation exist, including surface in-situ and surface remote sensing,
aircraft observations and LEO and GEO satellite missions. For validation, the most
direct comparisons can be made with observations that measure the same fundamental
quantities as AIM-North, vertically integrated abundances. Canada currently operates
two certified sites for satellite XCO2, XCH4 and XCO validation in the global Total
Carbon Column Observing Network (TCCON) (Wunch et al. 2011) that use high-
spectral-resolution, solar-viewing ground-based Fourier Transform Spectrometers
(FTSs). Canada has an Arctic site at Eureka, Nunavut (80°N, 86.4°W) and a boreal site
at East Trout Lake, Saskatchewan (54.4°N, 105°W). The FTSs at these sites also
measure a number of other species, while Eureka hosts a wide range of other
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instruments for both science and validation. Northern high-latitude TCCON sites in
Europe such as Sodankylä (67.4°N, 26.6°E) and others will also contribute to AIM-
North validation. Additional XCO2, XCH4 and XCO validation can be carried out with
lower resolution FTSs, and there are a growing number of such instruments in Canada
and Europe as well as one in Alaska (Frey et al. 2018).
Air quality validation is becoming more standardized with observations from the
Network for the Detection of Atmospheric Composition Change (NDACC) and
Pandonia Network. NDACC uses ground-based FTSs and UV-visible spectrometers for
remote sensing of the atmosphere from a number of sites across the globe. The
Pandonia network carries out ground-based remote sensing of species including NO2
and SO2 using Pandora/Pandora-2S instrumentation. Both networks have a number of
high northern latitude sites in Canada and Europe. For the validation of aerosol optical
depth and some other aerosol properties, Aeronet, and in particular the Aerocan sub-
network with roughly 20 sites well distributed throughout Canada, is well established.
Aircraft observations are also important for validation of air quality data, and in
particular to understand vertical and horizontal gradients, but also in the validation of
more derived products such as emissions from industrial operations.
Existing northern TCCON, NDACC, Pandonia, Aeronet and other validation
sites along with some new sites will be required to assess data quality and ensure that
accuracy targets are met. These ground-based validation stations are critical to mission
success, especially for the carbon cycle, since small spurious gradients can infer
significant fluxes (Miller et al. 2007). Individual stations need to remain in good
working order throughout the entire mission lifetime to assess drifts and systematic bias.
The networks also require a mechanism for ensuring that the measurements are
calibrated to the same absolute scale. TCCON accomplishes this with aircraft and
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AirCore (Karion et al. 2010) in situ overflights, which are difficult in the Arctic, but
other options such as co-located portable FTSs may provide a path forward.
6. Decision Support and International Context
Anthropogenic emission of CO2 and CH4 related to fossil fuel extraction, combustion or
leakage and emissions due to land use change, have perturbed the natural carbon cycle
elevating atmospheric CO2 and CH4 concentrations above pre-industrial levels.
Developed countries have been obliged to report their emissions annually under the
United Nations Framework Convention on Climate Change (UNFCCC), signed in 1992.
. Emission self-reporting is done according to internationally agreed upon guidelines
(IPCC, 2006) and in the case of fossil fuel combustion, emissions are calculated
primarily using statistical activity data and emission factors.
Atmospheric concentration measurements currently have an extremely limited
role in national inventory reporting, but there is a growing interest in including
observation-based estimates to provide complementary data on emissions. The
UNFCCC Paris Agreement of 2015, expanded the sphere of countries obligated to
report emissions, although detailed reporting requirements (especially for developing
countries) are still being determined. Quantification of anthropogenic CO2 and CH4
emissions using observations from space has the potential to support national emission
reduction goals and the transparency framework of the Paris Agreement. Over 60 CEOS
member agencies CEOS agreed to the Declaration of New Delhi in 2016, which
identified the need for better space-based GHG observations to support emissions
monitoring for the Paris Agreement.
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Although national reporting under the Paris Agreement will continue in
accordance with approved best practices (IPCC, 2006) and future approved
methodological refinements, satellite observations of XCO2 and XCH4 can play a key
role in quantifying the spatial, temporal and sectoral distribution of CO2 and CH4
emissions and in tracking evidence of emission reductions or the effectiveness of
policies to reduce emissions on the scale of large facilities, municipalities or
provinces/territories. The use of space-based data is consistent with the transparency
framework of the Paris Agreement and could support the Global Stocktake, intended to
track progress towards achieving the goals of the agreement.
The link between space-based observations and GHG emission reduction
strategies is of high interest in Europe (Ciais et al. 2015; Pinty et al. 2017). The
Copernicus CO2 Monitoring Mission (CO2M), a constellation of 3-4 LEO satellites
aiming to be operational by 2025 (Meijer et al. 2018), is a high priority candidate for
Sentinel expansion. In the longer term, Europe’s vision is for an optimized GHG
constellation that includes LEO, GEO and HEO (Pinty et al. 2017), consistent with the
more detailed constellation architecture recommended by CEOS AC-VC (Crisp et al.
2018).
Northern ecosystems are a massive carbon reservoir (Le Quéré et al. 2015) and
the multitude and complexity of interacting processes make accurate carbon budgets
difficult to estimate (Delpierre et al. 2012); however, the storage or release of this
carbon can affect the global GHG balance (Pan et al. 2011). The potential for dense and
frequent observations of SIF, as enabled by HEO offers an unprecedented monitoring
opportunity, to improve our ability to quantify the photosynthetic carbon uptake of
vegetation (Brown 2018) and stability of ecosystems at high latitudes to characterize the
state of the northern carbon reservoir.
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AIM-North observations of trace gases with high spatial heterogeneity are of
value to detect emission sources (e.g. McLinden et al. 2016a), for air quality assessment
(e.g. Martin, 2008), and for top-down constraints on emissions (e.g. Streets et al. 2013)
by direct analysis or by use in data assimilation. AIM-North observations of NO2, O3,
and aerosol optical depth could also be provided in near-real-time to forecasting centres
such as ECCC and the European Centre for Medium-Range Weather Forecast
(ECMWF) to be assimilated with models in order to improve forecasts of air quality
indicators, such as the AQHI and the UV-index.
Copernicus and ESA have also commissioned studies on Arctic observing needs
and requirements. These studies have focused mainly on meteorological observations,
as in Canada’s past PCW studies, but observations of GHGs and AQ species are
peripherally being considered. Finland has also been a strong proponent of HEO
missions to observe the North based on their statements while chair of the Arctic
Council. With multiple European studies on Arctic observing or HEO missions recently
completed or at various stages of maturity, a partnership may be possible. Scenarios
include hosted payloads in HEO or even merging missions for a more integrated
partnership, which might reduce total cost. Sharing a HEO platform with a
meteorological imager would also remove the need for AIM-North to have its own
dedicated cloud imager, if the data from the full meteorological imager could be
available in real time to support intelligent pointing.
7. Discussion
AIM-North is currently undergoing Phase 0 studies for the CSA in partnership with
ECCC. Although numerous details of the final technical design and the resulting data
remain to be determined, a key aspect of the mission is the plan for a pair of satellites in
a HEO formation. With this approach, AIM-North would observe GHGs, AQ species,
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clouds and SIF with unprecedented frequency, density and quality over northern land
regions (~40-80°N). These observations would help to improve our ability to quantify
sources and sinks of GHGs and AQ species for Arctic and boreal science,
anthropogenic emissions monitoring and assist in operational air quality forecasting.
With AIM-North’s objectives closely aligned with Government of Canada priorities,
data from this mission would also support evidenced-based decision making for Canada
and its northern partners, in an era where the importance of space-based Earth
observation data continues to gain increasing recognition as an essential method of
monitoring our planet.
Acknowledgements
We thank S. Polavarapu and M. Neish for providing EC-CAS simulations that were
used in this manuscript and related AIM-North simulation studies. A portion of the
research reported in this paper was performed at the Jet Propulsion Laboratory,
California Institute of Technology, under contract with the National Aeronautics and
Space Agency (NASA). We thank NASA for making their MERRA-2 cloud data
publicly available. Lastly, we acknowledge the significant effort of Professor Jack
McConnell from York University, who led the PHEOS-WCA Phase 0 and Phase A
studies, before he passed away in 2013. AIM-North would not exist today without his
past leadership in pursuing a HEO mission to measure atmospheric composition.
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Accepted to the Canadian Journal of Remote Sensing, special issue on Arctic and Northern Monitoring and Applications
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Accepted to the Canadian Journal of Remote Sensing, special issue on Arctic and Northern Monitoring and Applications
June 2019
Table 1. Specific mission science objectives for AIM-North.
Greenhouse Gas and Carbon Cycle Objectives
GHG-1
Improve our ability to quantify natural and anthropogenic CO2 and CH4 sources and sinks in the Arctic and northern mid-latitudes (~40-80°N) using imaging observations of column CO2 and CH4
a) Improve our understanding of forest CO2 fluxes and the net carbon balance of northern forests
b) Reduce uncertainties in the spatial, temporal and sectoral attribution of CH4 surface emissions
c) Detect and quantify potential acceleration in CO2 and CH4 emission from permafrost
d) Improve estimation of northern anthropogenic CO2 and CH4 emissions at the scale of a municipality or large industrial source
GHG-2 Improve our understanding of northern vegetation health, processes and carbon flux in a changing climate using solar induced fluorescence (SIF) observations
GHG-3 Better quantify emissions and separate CO2 emissions from anthropogenic versus biospheric sources (respiration and wildfires) with the help of CO, NO2, SIF or other supporting observations
GHG-4 Improve representation of the carbon cycle in climate prediction models through an improved knowledge of current carbon fluxes and processes
Air Quality Objectives AQ-1 Better quantify anthropogenic (including agricultural) and wildfire
emissions and their impact on northern air quality (~40-80°N), including understanding the relative contribution from local sources versus long range transport
AQ-2 Better monitor and predict surface air quality (including UV) over Canada, including understanding how episodic events impact air quality and in particular the Air Quality Health Index (AQHI)
AQ-3 Understand how air pollution influences climate change in the Arctic/Subarctic and the extent to which climate change impacts Arctic/Subarctic pollution
AQ-4 Monitor stratospheric ozone and ozone-related compounds in the North
Cloud Objectives C-1 Obtain observations of clouds to inform the pointing of the Greenhouse
Gas and Air Quality instruments
C-2 Obtain high spatial resolution cloud information to be used in determining cloud fractions for trace gas retrievals and to identify small clouds to allow for more accurate aerosol retrievals
Accepted to the Canadian Journal of Remote Sensing, special issue on Arctic and Northern Monitoring and Applications
June 2019
Table 2. Pixel Size and Revisit Requirements
Parameter Pixel Size Temporal Revisit
AQ and GHGs species, SIF 2x2 km2 (G), 4x4 km2 (T) 60 min. (G), 180 min. (T)
Clouds for pointing 2x2 km2 (G), 10x10 km2 (T) 30 min. (G), 120 min. (T)
Clouds for AQ retrievals 0.25x0.25 km2 (G), 1x1 km2 (T) 15 min. (G), 60 min. (T)
(G) = Goal (An ideal requirement above which further improvements are not necessary) (T) = Threshold (A minimum requirement to ensure mission objectives can be met) Table 3. Precision and Accuracy Requirements
Species Single Observation Precision (1σ)
Accuracy (1 σ maximum allowed bias)
Nominal Spectral Region
Primary CO2 (X)1 0.25% (1 ppm) (G),
0.75% (3 ppm) (T) 0.05% (0.2 ppm) (G), 0.15% (0.6 ppm) (T)
~1600 nm ~2060 nm ~760 nm
CH4 (X)1 0.5% (9 ppb) (G), 1.5% (27 ppb) (T)
0.1% (2 ppb) (G), 0.3% (6 ppb) (T)
~2340 nm ~760 nm
CO (C or X)1 5% (G) 15% (T)
5% (G) 15% (T)
~2340 nm ~760 nm
O3 (SC) 3% (G)
5% (T) 2% (G) 3% (T)
290-345 nm 540-650 nm
O3 (TC) 3% (G) 5% (T)
20% (G) 30% (T)
290-345 nm 540-650 nm
NO2 (SC) 3% (G) 5% (T)
10% (G) 15% (T)
400-470 nm
NO2 (TC) 1.0x1015 cm-2 (G) 1.5x1015 cm-2 (T)
15% (G) 20% (T)
400-470 nm
Aerosol AOD (C)
0.03 + 15% (G) 0.05 + 20% (T)
0.03 (G) 0.05 (T)
(1) 354, 388, 440, 555, 675nm (2) O2 A-band
Cloud COD (C) NR NR Vis (day) IR (day/night)
Secondary Solar Induced Fluorescence (SIF)
0.30 Wm-2 sr-1 µm-1 (G) 0.90 Wm-2 sr-1 µm-1 (T) – requires averaging
NR ~758 nm (high res) 500-780 nm
SO2 (C) 1.0x1016 cm-2 (G) 1.5x1016 cm-2 (T)
2x1015 cm-2 (G) 3x1015 cm-2 (T)
305-345 nm
HCHO (C) NR NR 325-360 nm BrO (C) NR NR 340-370 nm OClO (C) NR NR 360-390 nm CHOCHO (C) NR NR 420-465 nm
Shading denotes species may be required in near real time (X) = column-averaged dry air mole fraction (C) = total vertical column (TC+SC) (TC) = tropospheric vertical column density (SC) = stratospheric vertical column density NR = no requirement
Accepted to the Canadian Journal of Remote Sensing, special issue on Arctic and Northern Monitoring and Applications
June 2019
Table 4. NIR and SWIR Spectral Bands, Species and Requirements
Species FTS wavelength range (nm)
FTS wavenumber range (cm-1)a
Preliminary FTS SNR
requirement
Dispersive wavelength range (nm)
Dispersive resolution
(nm)
O2 758-762 13118-13192 88 (G), 30 (T) 757.9-772.0 0.0474
CO2 1598-1618 6180-6258 119 (G), 40 (T) 1591.5-1621.2 0.101
CO2 2042-2079 4810-4897 116 (G), 40 (T) 2045.0-2085.0 0.136
CO, CH4b 2301-2380 4195-4345 130 (G), 40 (T) 2300.6-2345.6 0.153
a FTS spectral sampling is 0.25 cm-1 for all bands, which gives a spectral resolution of ~0.30 cm-1 b CO and CH4 are both retrieved from this band, but the SNR requirements are driven by the CO precision requirements.
Accepted to the Canadian Journal of Remote Sensing, special issue on Arctic and Northern Monitoring and Applications
June 2019
Table 5. UV-Vis Spectral Bands, Species and Requirements
Species Fitting Window Wavelength (nm)
Wavelength (nm) for SNR requirement
SNR Goal
SNR Threshold
Primary Species (single observation SNR requirements are given)
O3 290-345 330 239 144
NO2 400-470 416.4 765 536
O3 540-650 544 650
177 123
106 74
AOD 675 675 36 12
Secondary Species (SNRs can be achieved by binning in space and/or time)
SO2 305-321 320.8 2090 1447
HCHO 325-360 330 2394 1596
BrO 340-370 366 3084 2159
Accepted to the Canadian Journal of Remote Sensing, special issue on Arctic and Northern Monitoring and Applications
June 2019
Figure 1. (Left) A 16-hr HEO (Three Apogee orbit) with an inclination of 63.435°,
eccentricity of 0.50 and apogee and perigee altitudes of ~41,885 km and ~9709 km. The
number of hours relative to apogee for the satellite in orbit is indicated, showing that for
more than 10 hours of the 16-hr period, the satellite would have a favourable view of
the north. (Right) Lines on the figure show the nadir and ±60° latitude from the sub-
satellite point for GEO and HEO (i=63.435°), while the red dot indicates the Alberta oil
sands at ~57°N.
Accepted to the Canadian Journal of Remote Sensing, special issue on Arctic and Northern Monitoring and Applications
June 2019
Figure 2. Examples of AIM-North coverage for two potential HEO scenarios. (Left)
The orbit track (black line) for a two-satellite Molniya constellation assuming an
eccentricity of 0.75 and the critical inclination (63.435°N). The satellites reach the four
apogee points at local noon on June 21. The satellite positions (circled dots) and the
viewable region are shown for June 22 at 16:15 UTC. (Right) The orbit track for a two-
satellite TAP orbit constellation assuming an eccentricity of 0.55 and the critical
inclination. For the TAP orbit, both satellites share a ground track and each reach the
three apogee points near local noon on July 15. The satellite positions and viewable
regions are shown for July 16 at16:00 UTC. The viewable region shown in green is that
where both viewing zenith angle (VZA) and solar zenith angle (SZA) are within their
limits of 60° and 80° respectively. Areas with only one of the two requirements satisfied
are shown in yellow (VZA) and orange (SZA) respectively, while for areas in red,
neither requirement is satisfied at this moment in orbit, but would be observed at other
times.
Accepted to the Canadian Journal of Remote Sensing, special issue on Arctic and Northern Monitoring and Applications
June 2019
Figure 3. A simplified scanning pattern for AIM-North assuming the satellite reaches
apogee around midday and a TAP orbit. The UVS could cover each coloured region in
three west-to-east sweeps while the IFTS could cover the coloured region with a step
and stare approach and a variety of different possible scanning patterns. Since both
instruments observe reflected sunlight, only observations during daylight, over land and
sufficiently cloud-free would be successful.
Accepted to the Canadian Journal of Remote Sensing, special issue on Arctic and Northern Monitoring and Applications
June 2019
Figure 4. (Left) The red boxes show 124 possible positions for a 128x128 pixel IFTS
field-of-view (FOV) with ground sampling of ~4x4 km2 from AIM-North at an apogee
at 95°W, accounting for pixel growth with larger viewing angles. (Centre) The 45 FOV
locations that AIM-North could point its GHG instrument in a 90-minute period based
on cloud coverage (NASA MERRA2 0.5°x0.67° cloud fractions greater than 0.1 in
white to grey), solar illumination and viewing angles. The satellite position and orbit
tracks are also shown. (Right) An indication of the average number of clear-sky solar-
illuminated hours per day in June 2015, which suggests that the atmosphere over
essentially all northern land is observable if pointing occurs at the proper time, which
would be facilitated by the flexibility of an intelligent pointing approach from HEO, but
not necessarily with the rigid observing schedule obtained from LEO overpass times.
Accepted to the Canadian Journal of Remote Sensing, special issue on Arctic and Northern Monitoring and Applications
June 2019
Figure 5. a-c) Model XCO2, XCO and XCH4 from the Environment Canada Carbon
Assimilation System (EC-CAS) forward simulations (Polavarapu et al. 2016). d) NASA
MERRA2 cloud data (0.5°x0.67°) showing cloud fractions greater than 0.1 (white to
grey) at the specified time, along with the AIM-North satellite position, orbit track,
illuminated/dark areas of the Earth disk and red squares to show the locations of 45
stares by the AIM-North NIR-SWIR FOV in a 90 minute period. e) The resulting
observational coverage (red) for XCO2, XCO, XCH4 and SIF that would be obtained in
this 90 minute period with an intelligent pointing approach, after accounting for
pointing, clouds and the loss of data over water due to its low albedo.
Accepted to the Canadian Journal of Remote Sensing, special issue on Arctic and Northern Monitoring and Applications
June 2019
Figure 6. The number of XCO2 observations per month for AIM-North based on an
Observing System Simulation Experiment (OSSE) that assumes two satellites in a TAP
orbit using intelligent pointing with an IFTS and a 120x120 pixel focal plane array with
4x4 km2 ground pixels and a dispersive spectrometer giving 40 scans of 360 pixels,
each 5.33x3 km2, each observing over 120 seconds.
Accepted to the Canadian Journal of Remote Sensing, special issue on Arctic and Northern Monitoring and Applications
June 2019