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1
Comparing Spectro-radiometer Instruments for a Satellite
Mission to Detect Chemical Warfare Agents
Gary Sutlieff
University of Bristol
School of Civil, Aerospace, and
Mechanical Engineering
Queen’s Building, University
Walk
Bristol, BS8 1TR, UK
gary.sutlieff@bristol.ac.uk
Lucy Berthoud
University of Bristol
School of Civil, Aerospace, and
Mechanical Engineering
Queen’s Building, University
Walk
Bristol, BS8 1TR, UK
lucy.berthoud@bristol.ac.uk
Andrei Sarua
University of Bristol,
School of Physics, Tyndall
Avenue,
Bristol, BS8 1TL, UK
a.sarua@bristol.ac.uk
Abstract – Previous research and modelling utilised example
data from existing space instruments, indicating that it is
theoretically possible to detect chemical warfare agents, such as
sarin and sulphur mustard from space. This work used the
Infrared Atmospheric Sounding Interferometer (IASI) on ESA’s
MetOp satellite as an example of the current state-of-the-art
capability for such a task. However, the performed analysis
shows that limitations with spatial resolution and pixel size mean
that the likelihood of making a real-time detection with IASI of
chemical warfare agents is insufficient. This demonstrates the
need for a satellite mission that could achieve this capability,
either with an IASI-style instrument or an alternative. Such a
mission would be of value both to the monitoring of chemical
releases and to emergency services in responding rapidly to
chemical incidents and attacks, especially in remote and
inaccessible areas. In this paper, three potential instruments are
compared in a trade study, against IASI as a benchmark, and
requirements are established on the instruments’ parameters
which in turn, are derived from the operational scenarios in
which a potential future mission would operate. First is a Laser
Heterodyne Radiometer (LHR), second is the Tropospheric
Emission Spectrometer (TES), a spectrometer flown on NASA’s
AURA mission, and last is the High-resolution Anthropogenic
Pollution Imager (HAPI). The instruments were evaluated in a
trade study, comparing their parameters such as spatial, spectral,
and temporal resolutions as well as revisit time and instrument
needs such as Size, Weight and Power (SWaP) against the
determined mission requirements. It was determined that no
single instrument was able to meet all of the operational
requirements alone, in particular revisit time. This indicated that
the further study of combinations of instruments are necessary to
produce a mission that can fulfil the requirements, and two basic
mission concepts, each utilising a combination of a TES-style
spectrometer and multiple LHRs are suggested as candidates for
further research including orbital simulations of the selected
mission concepts and their instruments, assessing each concept’s
coverage and resolution capabilities. Other further work includes
improvements to the basis of this research, such as producing
refined instrument and orbital requirements for later use with the
orbital modelling through the simulations of observations and
releases of chemical agents.. That research will determine which
mission concept best meets the requirements of a chemical
weapon detection mission, leading to a mission design process.
978-1-6654-3760-8/22/$31.00 ©2022 IEEE
TABLE OF CONTENTS
1. INTRODUCTION................................................ 1
2. BACKGROUND OF INSTRUMENTS...................... 2
3. METHOD.......................................................... 4
4. RESULTS AND DISCUSSION ............................... 7
5. CONCLUSIONS & FUTURE WORK ................... 10
ACKNOWLEDGEMENTS...................................... 10
REFERENCES ..................................................... 11
BIOGRAPHIES .................................................... 12
1. INTRODUCTION
Chemical Warfare Agents, or CWAs, have seen
significant use in conflicts throughout the last few
decades, in particular during the recent Syrian Civil War,
where choking agents such as chlorine, along with blister
agents such as sulphur mustard gas and nerve agents such
as sarin have all seen use [1]. CWA attacks often have a
high casualty rate and present challenges in how they are
contained, managed, or in some cases detected. This
indicates a need for rapid and effective detection and
monitoring of incidents, to provide data to first
responders, in order to maintain their safety and aid them
with managing an incident, for exampling setting a
properly sized safety cordon around the affected area
[2][3]. There is also a need for effective, unambiguous
verification of when, where and whether an incident has
taken place.
Currently, there are two main strategies used for
verification of an incident. First is in-situ detection, for
example by analysing soil samples from the incident site,
taking eyewitness statements, or collecting physical
evidence such as the remains of shells used to deploy the
agent in question [4][5][6]. This method is conclusive
when applied but requires investigators and responders to
physically visit the incident site, which could be
dangerous if the incident occurred in a conflict area. This
method also cannot be used until well after an incident or
attack has ended, and thus will only improve safety for
First Responders on future incidents. The second strategy
involves remote sensing, though this is typically done
2
using either ground-based sensors or sensors mounted on
helicopters or Unmanned Aerial Vehicles (UAVs)
[4][5][6]. As such, monitoring an incident in progress
requires either an extremely rapid response or prior
knowledge of where an attack or incident will (or is likely
to) take place.
Using satellites for detection could in theory allow for
rapid identification of incidents as they occur, allowing
First Responders to know the situation ahead of time and
prepare accordingly, and a satellite can also be used to
monitor the incident as it develops, charting the spread of
the agent where applicable, which is valuable knowledge
both for managing an ongoing incident and modelling
future incidents [7]. However, more likely uses for
satellites with regards to CWA detection include
unambiguous verification of incidents, with the potential
to determine detailed information such as the amount of
agent released, as well as the precise location of the
incident’s origin, depending on the instrument used,
which could provide insight into who was responsible and
how an incident occurred. This could potentially be useful
in scenarios where opposing factions may blame the other
for the incident.
Satellite-based chemical detection is already possible,
with instruments such as IASI on the Metop satellites or
the TROPOspheric Monitoring Instrument (TROPOMI)
on Sentinel-5P monitoring the composition of Earth’s
atmosphere [8][9]. Instruments such as these also collect
data on the concentrations of various trace gases such as
ozone, methane, the sulphur and nitrogen oxides, carbon
dioxide, formaldehyde, and hydrogen cyanide. However,
these instruments are focused on detecting gases in the
upper troposphere and the stratosphere, as opposed to
lower altitudes at which CWA releases will occur.
Additionally, the scale of these measurements is very
large, typically several kilometres per data point over a
period of a few hours, while effective monitoring of CWA
incidents requires data of a much finer spatial and
temporal resolution on order of 10s to 100s of meters.
Indeed, while previous research indicated that an
instrument such as IASI could theoretically be used to
make detections of sarin gas at a level as low as 90 parts
per trillion by volume for up to an hour after a 100kg
release [10], resolution limitations due to the instrument’s
pixel size rendered an actual real-time detection unlikely.
Nonetheless, this capability is of great interest for
development, and an instrument dedicated to spotting
CWAs could be developed with the necessary resolution
improvements. However, there are multiple instruments
and types of instruments that could fulfil this purpose
besides an IASI-type instrument. This paper aims to
perform a trade study between them and to propose a
mission concept.
The paper is split into five sections. First is this section,
which has detailed the context for the research. Section 2
provides background information on each of the
instruments in the comparison. Section 3 describes the
methods used for the comparison, covering each stage of
the process, including a trade study. Section 4 contains the
trade study itself, assessing each instrument’s parameters
against the requirements and discusses the results and
their meaning, while Section 5 outlines the conclusions
drawn from the research and what future work might
entail.
2. BACKGROUND OF INSTRUMENTS
Discussed here are each of the instruments considered in
this comparison, outlining their functionality, and
detailing their specifications.
2.1. IASI
IASI, or the Infrared Atmospheric Sounding
Interferometer, was designed to produce atmospheric
spectra for meteorology, including numerical weather
prediction data, measuring profiles of atmospheric
temperature and water vapour levels. However, it is also
capable of atmospheric trace gas detection [8][11][12].
IASI operates in the thermal infrared region and is flown
on the MetOp satellite group. Figure 1 depicts the IASI
instrument aboard the MetOp satellite [13].
Figure 1. A representation from ESA of the IASI
instrument aboard the Metop satellite, depicting
infrared radiation being received by the instrument
[13].
IASI’s spectrometer functions as a Michelson
interferometer, using a moving mirror to produce an
interferogram. This is then Fourier transformed to produce
the absorption spectra for analysis. In IASI’s case, the
interferograms are processed and radiometrically
calibrated on board [11]. IASI also features a secondary
infrared imager, which is used to co-register the soundings
from IASI with those from the Advance Very High-
Resolution Radiometer, or AVHRR, another instrument
aboard the MetOp Satellites [11][12]. Currently, there is
also a plan for an IASI Next Generation Mission, or IASI-
NG, due to launch on three satellites from 2024 onwards
[14].
2.2. TES
The Tropospheric Emission Spectrometer, or TES, is an
infrared imaging Fourier transform spectrometer much
3
like IASI. TES flew on the Aura satellite launched in
2004, one of the three parts of NASA’s Earth Observing
System program [15].
The Fourier transform spectrometer used in TES is a
Connes-type four-port configuration, chosen since it
allows for the use of interchangeable filters to divide up
the spectral range of the instrument, controlling the
background and allowing for the detectors to be optimised
for each portion of the spectral range [15]. This reduces
the instrument’s data rate requirements. A depiction of
TES can be seen in Figure 2 [16].
Figure 2. An image of TES undergoing installation on
the Aura satellite at JPL prior to its launch [16].
TES targets gases involved in climate change, such as
methane, carbon dioxide, and water vapour, but the
primary targets are tropospheric ozone and the chemical
species that contribute to its production and removal. This
instrument’s capability to detect the spectral features of a
wide range of gases makes it of interest to this research.
TES has now been decommissioned due to a mechanical
failure, but the instrument’s capabilities warrant
consideration to determine whether a similar instrument
would be suitable if flown today [17]. The instrument’s
successors include IASI, which was launched 2 years later
[18].
2.3. LHR
Developed by a team at RAL (Rutherford Appleton
Laboratory) Space, the LHR is a passive sounding
instrument which utilises a laser as a local oscillator. The
incoming infrared signal to be analysed is superimposed
with the coherent signal from the laser, and the resultant
signal is then transferred to a photodiode mixer which
converts the spectral information to radio frequency for
analysis [19][20]. An example of an LHR is depicted in
Figure 3 [21].
Figure 3. A CAD Model of an LHR instrument
designed for MISO, a nanosatellite mission to detect
methane isotopologues by solar occultation [21].
This new signal, known as the intermediate frequency,
makes analysis simpler, since it can be amplified or
divided without any loss of signal-noise ratio. This means
that any fluctuations in the signal’s amplitude due to
absorption by atmospheric absorption can also be
amplified without an increase in noise, rendering these
fluctuations more easily discernible. This results in a very
fine spectral resolution of LHR, which is, however,
limited to measuring within a very small spectral region or
‘microwindow’ [19].
The LHR is not currently flown on any spacecraft and is
still undergoing developments to raise its Technology
Readiness Level (TRL), though designs have been made
for CubeSat and nanosatellite missions to conduct limb
sounding of the upper atmosphere for greenhouse gases
by solar occultation [21]. The LHR demonstrates potential
for significant improvements to spatial resolution
compared to current trace gas monitoring satellite
instruments.
2.4. HAPI
The High-resolution Anthropogenic Pollution Imager, or
HAPI, is an imaging radiometer developed as part of a
Centre for Earth Observation Instrumentation project to
detect and measure atmospheric NO2 for pollution
monitoring purposes. It utilises a discrete-wavelength
version of the Differential Optical Absorption
Spectroscopy technique, which measures solar radiation
backscattered by the atmosphere in 10 targeted narrow
bands [22].
HAPI is designed to image in the optical and near-ultra-
violet domains of the electromagnetic spectrum, so some
modifications would be necessary to produce an infrared
imaging version of the instrument, but its capability to
target highly specific spectral bands could be an equally
viable detection method as compared to monitoring a
wider, continuous spectral range if the bands were centred
at the peaks of a target agent’s absorption spectrum.
HAPI was also designed to be based on the OMNISAT
platform [22]. Figure 4 depicts the HAPI instrument [23].
4
Figure 4. A 3D CAD model of HAPI [23].
3. METHOD
3.1. Methodology
This section details the process used to compare each of
the instruments in this research as well as in the further
studies planned. Each of the stages will be described in
more detail below, but a summary of the process is
provided in Figure 5.
Figure 5. A flowchart summarising the methodology
process followed in this research. The green boundary
marks areas covered in this paper, while the red
marks areas that will be developed in further study.
3.2. Operational Scenarios
The first step was to determine operational scenarios in
which these instruments would be compared. These
scenarios allow for the definition of requirements for
instrument and orbital parameters for later use in the trade
study and in future modelling research. These
requirements depend on the CWAs being targeted as well
as the size and duration of incidents.
To gather information on operational scenarios and use
cases that would be of the most value, several reports
from the Organisation for the Prohibition of Chemical
Weapons, or OPCW, Fact-Finding Missions and the
Investigation and Identification Team, or FFM and IIT
respectively, were used together with verbal
communications to contribute towards the determination
of operational scenarios [24][4][5][6].
The FFM and IIT reports detail the OPCWs efforts to
verify that alleged chemical weapons attacks did occur,
then to identify the agents involved, assess the facts of the
incident, and attempt to identify the party most likely to
be responsible, though it has no authority to assign legal
or criminal responsibility. The FFM gathered information
with the goal of verifying the incident and determining the
agent involved [4], and the IIT has a mandate to follow up
on the findings of the FFM where it is likely that an
incident did in fact occur to determine the suspected
perpetrator of the incident [5][6].
With a major objective of OPCW missions being to verify
an incident and to identify the agent or agents used, this
presents a clear operational use for a CWA detection
satellite, which could provide scientific proof of an
incident’s occurrence while also identifying the agent
used without reliance on witness testimonies and
information from varies state parties, which could be
subject to bias. While the IIT does make use of requested
satellite imagery already in its procedures, this is for the
purposes of verifying weapon impacts and similar visual
evidence [5][6]. Satellite verification of the involved
chemicals themselves would either quicken the procedure
or augment the evidence from current investigative
procedures.
In particular, the need for identification indicates that the
optimal operational scenario would feature an instrument
capable of detecting a variety of possible chemical agents
simultaneously. Such a mission would also likely have a
wide enough spectral range to detect other gases of
interest to Earth observation, such as greenhouse gases or
toxic industrial chemical leaks, making the mission multi-
purpose.
Another potential scenario to consider would be the
detection of a single more commonly used agent, such as
sarin, based on the peaks in its absorption spectrum.
However, this operational scenario places fewer
requirements on the instruments and is therefore more
likely to be viable with a variety of instruments.
With regards to the spatial coverage a mission would
require, there are two main options to consider. The first
and most optimal scenario involves global coverage, since
an incident or attack could conceivably happen anywhere.
However, there is little need to image the highest and
lowest latitude regions, so coverage of the region between
±60° would be a suitable alternative to full global
coverage, since 95% of the world’s population lives
between these bounds [25]. Alternatively, it may also be
practical to focus only on conflict areas where chemical
5
weapons use is more likely, as this could allow for more
creative orbit design to increase dwell time over specific
locations.
3.3. Instrument Parameter Requirements
The next step is to define the requirements for the key
parameters of each instrument such as SWaP (Size,
Weight and Power), spectral range, spectral resolution,
and field of view. These requirements would then be
compared with the existing parameters of the instruments,
establishing whether and how sufficiently each instrument
can meet the requirements.
The requirements on spectral range are defined by the
range of the absorption spectra of the target chemicals, as
seen in Table 1.
In this research, four chemicals of interest are taken as
examples. These are sarin and sulphur mustard gas, a
nerve agent and a blister agent respectively, both of which
have seen recent use in the Syrian civil war [5]. Next are
diisopropyl methylphosphonate, or DIMP, and dimethyl
methylphosphonate, or DMMP. These are not CWAs
themselves, but are often used as simulator agents for
CWAs, and are also precursor agents produced in the
manufacturing process for sarin [26]. As such, the
detection of these chemicals is a valuable capability since
it allows for the identification of chemical weapons
manufacturing sites. Table 1 details the positions of up to
four peak absorption lines in the absorption spectra of the
four chemicals [26][27][28][29].
As standard, the requirement on spectral resolution can be
produced by simulating observations of the target agents
with each of the instruments, accounting for the line
widths of each of the key peaks in the absorption spectra
and in the spectra of interfering/background atmospheric
species. This will inform the sampling, noise and
background parameters as well as any possible trade-offs.
However, the accurate measurement simulations required
for this are outside the scope of this work and would need
to be conducted in the future for each particular
instrument configuration. Based on previous simulations
of sarin measurements from IASI which indicate that it is
spectrally capable of retrieving sarin [10], the spectral
resolution of IASI, 0.25 cm-1, can be selected as an
example requirement for the spectral resolution.
Furthermore, an absorption spectrum region of all four
agents stretches at least from 720 to 1326 cm-1 defining an
example spectral range if the instrument were to be
capable of detecting all four agents.
For instruments with a narrow spectral range, it may be
possible to detect a single chemical based on one or more
of its spectral lines, which splits the single-agent
operational scenario into two scenarios – one based on the
detection of a single agent centred on a single, dominant
spectral peak, and another scenario based on detecting
multiple spectral lines.
Considering the relationship between the field of view
between the swath width, and the narrower instantaneous
field of view to the spatial resolution, it is appropriate to
let the requirement on these parameters be driven by the
requirement on spatial resolution, as discussed in the next
subsection on the requirements for the orbit-driven
parameters.
Table 1. The positions of up to four absorption lines in
the spectra of the target agents in cm-1.
For the instruments’ size, weight, power, and data rate, the
lower these parameters are, the better.
For comparison against the established requirements,
detailed here are the key parameters of each of the
instruments in this comparison that are not affected by the
orbits used. Table 2 details each parameter for the four
instruments [11][12][15][22][19]. Note that there are
some gaps where information was not available or could
not be determined. For example, the LHR’s data rate
could not be determined, since no model of it suitable for
heavy molecule detection has been developed, and the
same is true for the LHRs size, weight and power, where
examples have instead been provided from CubeSat
models, though these are likely to be less than the
requirements of an LHR suitable for CWA detection.
Chemical
Agent
Sarin
Sulphur
Mustard
DIMP
DMMP
Absorption
Band 1
Position
(cm-1)
1021
720.58
995.1
1050.1
Absorption
Band 2
Position
(cm-1)
1305
1207
1267.07
1280.6
Absorption
Band 3
Position
(cm-1)
1326
1299.4
-
-
Absorption
Band 4
Position
(cm-1)
-
1444.15
-
-
6
Table 2. Summary of Instrument Parameters
3.4. Orbital Parameter Requirements
It is useful to establish requirements for orbit-driven
parameters, such that the nominal resolution capabilities
of the instruments can be considered in the trade study.
These orbit-driven parameters include spatial resolution,
swath width, dwell time, latency, and temporal resolution.
It is necessary to place requirements on some of these
values to ensure that the chosen instruments provide an
improvement over existing capabilities. Specifically for
the purposes of this paper, requirements are needed for the
spatial and temporal resolution. For example, the spatial
resolution requirement would be determined based on
how precisely it is necessary to locate an incident,
whether that is to the scale of a town or street.
An IIT report refers to incidents taking place over scales
of tens to hundreds of metres, with munitions making
impact around 200 metres away from a location where
several victims were affected in one incident involving
sarin [5]. In another incident, chemical samples were
taken at 50 and 150 metre ranges from one impact crater.
The same report also describes cylinders of the agent used
in the attack falling between 50 and 200 metres away
from the first in various directions, for a total ‘area of
interest’ of around 200 metres by 200 metres [30].
Another IIT report indicates that where weapon impact
sites need to be determined, visual satellite imagery is
already utilised in conjunction with other information
such as video recordings and statements from witnesses,
and is sufficient to locate craters from munitions impacts
or other origin points for an agents release [6]. With visual
satellite data reaching resolutions as fine as 15 cm, far
finer than can be achieved with chemical detection
instruments, it is clear that precisely locating an exact
source point using a chemical detection satellite is not a
necessary capability for investigative purposes [31],
placing the focus solely on identification, verification, and
assessing the amount of agent released where possible.
Combined, this information suggests that a spatial
resolution of 50-200 metres would be useful for the
purposes of the operational scenarios described
previously. This would allow an instrument to detect an
incident within a single instantaneous field of view, or
over a small number of IFOVs.
The time scales on which incidents take place between
first impact of munitions and initial collection of samples
from the incident site take place over several hours,
though not longer than this. One incident timeline has the
collection of debris and soil chemical samples recorded as
having taken place 9 hours after the attack was recorded
as having occurred [6]. Other details in the reports include
the collection and recording of samples a few hours after
an attack, and specifically 24 hours [5]. This suggests a
need for regular observations on the scale of much less
than a day, down to a few hours. Among CWAs, nerve
agents such as sarin are often non-persistent, dispersing
and losing effectiveness within hours, and while blister
agents such as sulphur mustard are described as persistent,
this refers to deposition and contamination on surfaces
rather than a tendency to remain in the air [32]. This also
suggests a need for regular temporal coverage or revisit
every few hours. A satellite with a very low revisit time
could collect data earlier after an attack than the standard
sample collection procedure. To achieve this capability, a
revisit time of 6 or fewer hours would be necessary.
For comparison with the requirements, nominal examples
of these orbit-driven parameters for particular platforms
compatible with each instrument can be found where
applicable in Table 3. The platforms used for each
satellite are MetOp-A for IASI [11], Aura for TES [15], a
LEO CubeSat for the LHR [19], and the OmniSat
constellation (design only) for HAPI [22].
3.5. Trade Study of Instruments
With the requirements established, the next step is to
compare each of the instruments’ parameters in more
detail against the requirements and establish what the best
solution might be.
Parameter
IASI
TES
LHR
HAPI
(Omnisat)
Field of
View
±48°
20’
45°
cone
A few
hundred
metres
17.5°
IFOV
14.65
mrad
0.075 x
0.75
mrad
0.1 mrad
0.0175°
Spectral
Resolution
0.25 cm-
1
0.0592
cm-1
0.001-
0.02
cm-1
1.7-1.85 nm
Spectral
Range
645-
2760
cm-1
650-
3050
cm-1
1-100
cm-1
10 discrete
bands
Mass
236 kg
385 kg
3.9 kg
for
CubeSat
model
3 kg
Volume
1.1 x 1.1
x 1.2 m
1.0 x
1.3 x
1.4 m
1.5 U
for
CubeSat
model
207 x 154 x
129 mm
Power
Draw
(peak)
240 W
361 W
25 W for
CubeSat
model
-
Data Rate
(Peak)
1.5
Mbps
6.2
MBPS
-
0.41 Mbps
7
Table 3. Summary of Nominal Orbit-Driven
Parameters for Each Instrument and a Compatible
Platform
3.6. Selection of Instruments
The trade study examines how well each instrument
matches the requirements, particularly those on spatial,
temporal, and spectral resolution to assess performance, as
well as spectral range to assess which instruments are
suitable in the majority of the defined operational
scenarios. Then the instruments size, weight, power, and
data requirements will be compared, since lower values
will allow for the instruments to be used with versatility
on a variety of platforms. The selection of the instruments
based on the trade study’s results can be found in Section
5.
4. RESULTS AND DISCUSSION
4.1. Trade Study
Each of the instruments are discussed here in turn,
comparing their parameters first to the requirements as
defined in Section 2, and then to each other. Finally, a
suitable mission concept was selected.
IASI – In spectral terms, IASI acts as a benchmark for the
required range and spectral resolution, as described in
Section 2. The instrument has a sufficient spectral range
to cover the absorption spectra of all four target agents,
and sufficient spectral resolution to determine from
measurements whether an agent is present or not.
However, this is not the case for IASI’s spatial and
temporal resolution.
IASI uses a scanning mirror for a total field of view equal
to 48 degrees and 20 minutes, equivalent to a swath width
of 2200 km for the MetOp satellites’ 817 km orbit, though
the instantaneous field of view of the instrument itself is
3.3 degrees, corresponding at nadir to 50 km on the
ground – this is composed of two by two 12 km diameter
circular pixels from MetOp. The MetOp satellite orbits
allow for a 1-3 day revisit and data latency between 130
and 65 minutes at best [12][33].
This spatial and temporal resolution is not ideal for the
detection of CWAs, since incidents are likely to take place
on a much smaller scale than 12 km, and the revisit time
means that fortunate timing would be needed to be able to
monitor an incident! These issues could be addressed with
a constellation, though cost would be a major drawback.
This would also be difficult due to IASI’s SWaP. The
instrument is relatively large at 1.1 x 1.1 x 1.2 m, with a
mass of 236 kg, and a power draw of 240 W [34]. These
requirements are manageable for a large satellite, but
difficult for a Smallsat or CubeSat to support. The data
rate of IASI is 1.5 Mbps, roughly in the middle of the four
instrument options [34].
As a result of all this, IASI, as it stands fails to meet the
necessary spatial and temporal resolution requirements for
the mission of 50 to 200 metres and 6 hours or fewer,.
However, its spectral range is sufficient to feasibly detect
all four of the target agents outlined in the operational
scenarios, or each of the absorption peaks in a single
target agent’s spectrum. IASI also has the spectral range
for the detection of other relevant gases such as
greenhouse gases or toxic industrial chemicals, a purpose
for which the instrument already sees use.
TES - Despite launching 2 years prior to IASI, TES has a
wider spectral range and finer spatial resolution at nadir,
with pixels measuring 0.53 by 5.3 km from the Aura
satellite’s 705 km orbit [15]. However, it has a slightly
smaller field of view at 45 degrees to IASI’s 48 degrees
and somewhat greater requirements for Size, Weight, and
Power. The exact revisit time of TES is also significantly
longer at 16 days for the Aura satellite, though the
instrument does have a near-revisit on a time-scale similar
to that of IASI at 2 days. It is important to consider,
however, that this near-revisit likely involves off-nadir
pointing, which results in poorer spatial resolution up to
pixels of 2.3 by 23 km size [15].
Where IASI was designed to collect atmospheric spectra
for the construction of temperature and water vapour
profiles, TES was designed to produce 3-dimensional
measurements of Ozone and of physical and chemical
Parameter
IASI
(Metop-
A)
TES
(Aura)
LHR
(LEO
CubeSat)
HAPI
(Omnisat)
Spatial
Resolution
(nadir)
12 km
0.53x5.
3 km
pixel
50-70m
600 m
(200m
sampling,
3x3 pixel
bins)
Swath
Width
2200 km
cross-
track
5.3 x
8.5 km
nadir
-
200 km
Temporal
Resolution
29 days
exact; 1-3
days near
16
days
exact;
2 days
near
-
2 hr targeted;
13.8 hr
global
Sampling
Time
8s scan,
216 ms
sample
4s
scan,
208s
max
dwell
0.15-5s
-
Latency
130-65
mins
Dataset
every
2-3
days
-
2 hr
Orbital
Altitude
817 km
705 km
700 km
650 km
Eccentricity
0.000136
2
0.0001
164
-
0
Inclination
98.4642°
98.215
5°
-
86.4°
8
factors that drive the production, transport, and removal of
Ozone [15].
Compared to IASI, TES’s improvements in spatial and
spectral resolution indicate that TES would be a more
suitable selection for a potential mission, since like IASI it
meets the spectral requirements for an observation while
improving on IASI in terms of both spatial and spectral
resolution, as well as spectral range, without significant
losses in field of view. However, compared to the
requirements set out previously, TES still fails to meet the
requirements on spatial resolution, albeit by a less
significant margin than IASI at 530 m (by 5.3 km) against
the 200 m requirement.
Additionally, the instruments are similar enough in
function that the practicalities of implementing TES
would likely be similar to that of IASI, but TES could
prove slightly more problematic due to its increased
requirements for SWaP and data rate. While the two
instruments are of similar size with TES measuring 1.0 x
1.3 x 1.4 m, TES is heavier at 385 kg, has a higher data
rate at 6.2 MBps, and a greater power draw at 361 W [15].
Despite the increase in platform requirements, TES is an
improvement in terms of spectral range over IASI.
Nonetheless, it still fails to meet all the requirements. The
increased SWaP also limits opportunities for constellation
development due to cost, just as IASI did.
LHR – The LHR offers an improvement in spectral
resolution over both IASI and TES, with resolution
ranging between 0.001 and 0.02 cm-1. However, as
described previously, this instrument operates within a
narrow spectral microwindow [19].
An LHR’s microwindows are often very narrow when
compared to the spectral ranges of a typical spacecraft
instrument, being on the scale of 0.1 cm-1. This is
insufficient for detection of broadband absorbers such as
CWAs, the spectra of which can range over thousands of
wavenumbers. However, a broader spectral range can be
achieved depending on the laser source used for the local
oscillator [19]. While a Quantum Cascade Laser, or QCL,
which is understood to produce the best spectral
resolution for the instrument, has a microwindow range of
0.1 cm-1 [19][20][35], an external cavity laser could
provide spectral ranges of up to 100 cm-1 and beyond –
still a narrow range compared to typical space-borne
sounders, but potentially feasible for detection depending
on the absorption spectrum of the target agents, by
targeting one peak on a single target agent’s spectrum
[36]. This limits the potential use cases for the LHR to the
less-than ideal operation scenario of detecting a single
agent, and a single peak in that agent’s absorption
spectrum. However, the resolution capabilities of this
instrument could make such a sacrifice worthwhile.
Alternatively, a satellite could fly multiple LHRs in a
single payload, which could between them cover the
necessary spectral range to target multiple agents. The
absorption peaks of the four target agents considered in
this research are spread across a spectral range of around
606 cm-1, so 6 or 7 external cavity laser LHRs with a 100
cm-1 range would be sufficient to cover this, provided
their SWaP requirements could be met.
An LHR’s FOV is determined by the maximum tolerable
angular mis-alignment of the incoming infrared beam and
the local oscillator’s beam, due to the requirement of
matching wavefronts for the two beams’ fields during the
superimposition process. The field of view is inherently
very narrow at around 0.1 mrad, which corresponds to a
scale of around 70 metres for an LEO satellite at 700-800
km, but this comes with improved spatial resolution. An
LHR on a LEO satellite within this altitude range could
offer a resolution of 50-70 metres, as opposed to the 12
km pixel size of IASI at 817 km [19]. With appropriate
knowledge and selection of target sites for monitoring, or
prediction of where an incident may occur, the satellite’s
pointing could be managed so as to produce results
regardless of the limited field of view. However, pointing
off-nadir would decrease the spatial resolution of the
measurements.
Currently, the instrument is still undergoing modifications
to produce a spaceflight model. Developments are focused
on miniaturisation through methods such as integration of
the instrument into a hollow waveguide [37], and the
design of a model that could be flown on a 1.5U Cubesat
[21], though a larger instrument would be required for the
detection of CWAs. In any case however, it is likely that
an LHR would have lower size, weight and power
requirements than a spectrometer such as IASI or TES,
with the CubeSat model drawing 25 W compared to
IASI’s 240 W and TES’ 361W. While a LHR suitable for
CWA detection would have higher requirements than
these, this means that if a suitable instrument could be
developed, it would be simpler to implement on one of a
range of platforms than either of the heavier
spectrometers.
Other known issues include that the scanning mode
required for a wider spectral range has a long integration
time, as high as 5 seconds, and analysis suggests that for a
cross-track scanning, nadir-pointing configuration
mirroring that of IASI, the results produced by an LHR
would have prohibitively high noise. There is the
possibility of a multiplexing mode, which integrates all
channels simultaneously to reduce integration time, but
this is still under development [19].
Furthermore, the relatively narrow spectral range of the
LHR compared to other instruments produces difficulty in
detecting heavier molecules and broad-band absorbers,
which include the CWAs targeted in this study. The
improvements required to detect lighter molecules from a
satellite include development of the multiplexing and
back-end spectral analysis capabilities, and the detection
of heavier molecules would also require development of
more capable detectors than the microchannel plates
currently in use.
The highest TRL model LHR is a ground-based solar
occultation system for measuring the concentrations of
lighter molecules such as Methane and Carbon Dioxide
[38], but the spatial resolution capabilities of this
instrument are unprecedented compared to its
9
counterparts, so TRL raising is of high importance.
Despite the challenges associated with developing an
LHR for CWA detection, the instrument still demonstrates
potential for the near future once the necessary
developments have been made, and chemical detection
with this instrument is an area of interest for expansion.
The LHR’s spatial and spectral resolution are more than
sufficient to meet the requirements. However, its limited
FOV means that the revisit time on any single satellite
would be too long to meet the requirement on temporal
resolution. An interesting prospect would be the
implementation of the LHR on a constellation of small
satellites.
HAPI – In most aspects, HAPI presents a middle ground
between the larger spectrometer instruments and the LHR.
In combination with the OMNISAT constellation platform
(a design only at this stage) for which it was designed,
which would orbit at 650 km, it has the potential for 200
metre spatial resolution, and HAPI’s field of view would
then be 17.5 degrees, which corresponds to a 200 km
swath from that platform [22]. This nominal spatial
resolution would be suitable to meet the requirement.
HAPI also has low requirements on SWaP and data rate. It
measures at 207 x 154 x 129 mm and has a mass of just 3
kg. It also has a low data rate of 0.41 Mbps. This means
that it could be implemented on a range of platforms,
including the constellation of 24 satellites that it was
intended for [22].
However, the revisit time of HAPI would still fail to meet
the requirement with just one satellite, since its FOV is
smaller than that of IASI and TES, both of which also had
insufficient revisit time. With the OMNISAT
constellation, though, the potential revisit time for this
instrument is 2 hours, which would be sufficient to meet
the requirement.
Unfortunately, there is one other issue with HAPI: it was
designed for the detection of NO2 in the visible and near
ultra-violet domain, as opposed to the infrared. As such,
the instrument’s spectral resolution is 1.7-1.85 nm [22].
This is insufficient for detecting any of the target CWAs,
and does not meet the requirement for spectral resolution.
There is also the matter of modifying the instrument to
observe in infrared. HAPI’s discrete bands of observation
are defined by the filters used, so with different filters, the
instrument could in theory image in infrared. However,
depending on the width of the peaks and on the bandwidth
of the filters used, the discrete wavelength observation
method may not be viable. If a target agent’s absorption
peaks are wider than the maximum filter bandwidth, then
the discrete observation method would be unsuitable for
detecting that peak. The fact that the instrument was
designed for imaging in visible and near UV also means
that it is difficult to discern what the possible spectral
resolution of the instrument would be if converted to
infra-red.
Additionally, some alterations to the detectors may be
necessary, in particular to alter the spectral resolution to
meet the requirements, and it is not clear how difficult this
would be, and the nature of such modifications is well
outside the scope of this research. One modification that
would clearly be necessary is the addition of cooling
technology to the detectors, something that is necessary
for IR detectors to reduce noise, but isn’t required for
visual detectors. This would have the additional downside
of increasing the SWaP requirements of the payload.
As such, in order to proceed with HAPI as the chosen
instrument it would be necessary to assume that the
necessary modifications could be made, which does not
seem reasonable given the potential complexity involved
in them. This is unfortunate, since HAPI would otherwise
be one of the instruments with the most potential due to its
suitable spatial resolution and excellent spectral
resolution.
Selection of Instrument
The first thing revealed by the trade study is that none of
the instruments can meet the requirement for temporal
resolution with just one satellite. Though this is much as
expected, this means that the requirement for temporal
resolution will need to be largely disregarded in future
orbital modelling research unless constellations of
satellites are being considered to improve revisit time, and
the revisit time will instead have to be optimised within
the limitations of a single satellite against the other
requirements. It also highlights the importance of using
multiple satellites or even constellations to achieve
sufficient revisit time, for example the OMNISAT
constellation for HAPI. This will be particularly relevant
in future research, as discussed in Section 5.
More significantly, the trade study confirms that none of
the instruments alone can meet all of the requirements,
and none of the instruments alone are sufficient for all of
the operational scenarios. IASI and TES possess sufficient
spectral resolution, and a wide enough spectral range for
the multi-peak detection of all four target chemicals, but
their spatial resolution is unsuitable to meet the
requirements.
The LHR has potential for better-than sufficient spatial
and spectral resolution in exchange for the narrowest FOV
of the four instruments, corresponding to a view of a few
hundred metres at ground level from LEO, which is only
enough to view an incident with none of the surrounding
area, as well as a very limited spectral range that only
allows for the analysis of a single peak in the spectrum of
one target agent, limiting the operational scenarios the
instrument can viably perform in.
Lastly, HAPI is designed with a constellation platform in
mind, with which it could in theory offer sufficient spatial
resolution as well as effective global coverage and rapid
revisit times, but its unique detection mechanism means
that it can only be targeted to match the peaks in the
absorption spectrum of one target chemical. This prohibits
the ideal operational scenario of monitoring for multiple
agents, though unlike the LHR, HAPI does at least have
enough bands to cover each of the targeted chemical’s
peaks. However, all of this capability is based on the
assumption that HAPI can be modified to detect in
infrared as opposed to the visible and ultra-violet
10
domains, which is not a reasonable assumption given the
complexity of the modifications and the low TRL of the
instrument, meaning that HAPI cannot be selected without
further dedicated research.
The fact that no single instrument is sufficient also means
that alternative solutions must be considered, as suggested
in the method. Given that this research is only considering
a single satellite, there may be a benefit to utilising
multiple of different instruments on a single satellite. For
example, if it is possible to achieve a resolution of around
1 or 0.5 kilometres with a spectrometer such as IASI or
TES at a lower altitude while maintaining a sufficiently
wide field of view and swath, then such an instrument
could be paired with an LHR (or several) as a narrow-
field instrument. The LHR’s low SWaP and data needs
suggest that this may be feasible, though with around 6
being necessary to meet the spectral range requirement,
this is not certain.
Alternatively, it may be possible to consider utilising a
satellite with a single instrument such as an LHR, but
flying in tandem with an existing satellite featuring one of
the larger spectrometer instruments, such as the Metop
Satellites, which include IASI on their payloads, or the
upcoming second generation Metop satellites with IASI-
NG. If the spectrometer on the existing mission had
sufficient resolution to make wide-field detection, the
smaller satellite with the LHR could follow up with a
narrow-field view at higher spatial resolution. However,
the orbit for this mission would be pre-defined by the
existing satellite in the tandem, and as such the
spectrometer may not be able to achieve sufficient spatial
resolution for the wide-field detection.
This presents two possible mission concepts to be
assessed by orbital modelling in further research. First, a
small satellite with a payload of multiple LHRs in tandem
with an existing or upcoming mission which defines the
orbit for the small satellite. Second, a large satellite
featuring a payload of multiple LHRs and a large
spectrometer, to detect in the wide and narrow fields
simultaneously, but in a more ideal orbit specifically
suited to this task, the altitude of which can be varied to
optimise coverage and revisit time.
5. CONCLUSIONS AND FUTURE WORK
5.1. Conclusions
This work has looked at the possibility of using space
instruments to detect and monitor chemical weapons agent
use on Earth. Operational scenarios based on real-life
intelligence have been used to derive requirements for
temporal and spatial resolution and four example agents
have been taken to give spectral requirements. Four
existing instruments IASI, TES, LHR and HAPI have
been compared against these requirements in a trade
study. The conclusions of the trade study point to two
possible mission concepts: a satellite with multiple LHRs
flying in tandem with an IASI type mission or a larger
single satellite with a TES type spectrometer and multiple
LHRs.
5.2. Future Work
The focus of further research carrying on from this work
will be on conducting orbital modelling of the suggested
concepts, comparing the coverage, revisit times, and
spatial resolution achievable with each concept to
determine which is best suited for development into a
mission. This process will likely include testing the effects
of changing the satellite’s altitude, inclination, and other
orbital parameters on the results, and could also include
the evaluation of coverage and revisit times with
constellations of satellites as opposed to just one satellite.
establishing how many satellites bearing each instrument
would be needed to achieve the desired coverage. This
would naturally result in a re-evaluation of the trade study
with regards to the costs and practicalities involved with
each concept. A constellation would also provide
coverage and temporal resolution much more in line with
the requirements for detecting these short-duration
incidents, so investigating each option within that concept
would be a valuable direction to progress in.
Naturally, such research could also lead into the mission
design process for the selected concept, finalising each of
the mission, instrument, and orbital parameters,
potentially even leading to the design of satellites bearing
the chosen instruments, fulfilling the long-term purpose of
research such as this.
However, it will first be necessary to produce more
refined requirements on the instrument and orbital
parameters, to validate this comparison and ensure that the
correct constraints can be set in the orbital model. This
could be achieved through the simulation of the release
and observation of the target chemical agents utilising a
radiative transfer model and a model for the agent
propagation. This will provide more information both on
the capabilities of the instruments to observe each agent
and will refine the requirement on spectral resolution.
ACKNOWLEDGEMENTS
Funding for this research was supplied by the EPSRC
iCASE studentship no. 19000187 supported by Thales
Alenia Space UK, or TAS UK, as well as by ESA contract
4000128582/19/uk/AB EUROSIM CBRN.
The authors would also like to acknowledge the
contributions of Dr David Moore and Dr Jeremy Harrison
from the University of Leicester for their guidance
concerning IASI. Additionally, the authors would like to
acknowledge Dr Damien Weidmann from RAL Space for
providing information on the LHR, and Cristina Ruiz
Villena from the University of Leicester along with Mark
Stinchcombe from TAS UK for providing information
concerning HAPI.
11
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BIOGRAPHY
Gary Sutlieff is a PhD student at
the University of Bristol, where he
is investigating the applications of
satellite technology to CBRNe
(Chemical, Biological,
Radiological, Nuclear, explosive)
detection. He received his MPhys
degree in Physics with Space
Science and Technology in 2019.
His current research is focused on
the detection of chemical weapons
from space, using modelling software such as RFM and
HYSPLIT to simulate observations of agent releases.
13
Lucy Berthoud is a Professor
of Space Systems Engineering
with the Department of
Aerospace Engineering at the
University of Bristol. She also
works for the European
spacecraft manufacturer
Thales Alenia Space part-time.
She has a background in
advanced mission concepts and systems engineering for
spacecraft. Her research expertise includes CBRN
monitoring from space, model-based systems engineering,
Mars Sample Return and IR CubeSats.
Andrei Sarua obtained his PhD
degree (Dr rer nat) in 2001 from
Technical University in
Freiberg, Germany. In 2007 he
was awarded a Great Western
Research Fellowship, and in
2010 he became a Senior
Research Fellow/Lecturer in the
School of Physics at University
of Bristol. He has more than 20
years of research track record in the field of wide-band
gap materials, device physics as well as optical
spectroscopy. Together with Prof M Kuball he has co-
founded the Center of Device Thermography and
Reliability in Bristol - the world leading group on
reliability of GaN based electronic devices, supported by
numerous links with academic and industrial
collaborations. His main research interests are in the area
of functional nanostructures, semiconductor devices and
reliability, bio-chemical sensors, as well as optical
spectroscopy and thermal imaging. He is an author on
more than 60 publications in peer reviewed journals, 25
regular and several invited conference contributions.
