EMIRATES MARS MISSION 2020: EMIRATES EXPLORATION IMAGER
(EXI) STATUS UPDATE
M. J. Wolff, Space Science Institute, Boulder, CO, USA (mjwolff@spacescience.org), M. Al Shamsi, Mohammed
Bin Rashid Space Centre, Dubai, UAE, C. Jeppesen, A. R. Jones, Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, USA, M.M. Osterloo, R. Shuping, Space Science Institute, Boulder, CO, USA, C. Edwards, Northern Arizona University, J. Espejo, C. Fisher, J. Knavel, E. Pilinski, Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, USA, A. Fernando, Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, USA; Space and Planetary
Science Center, Khalifa University, Abu Dhabi, UAE.

Introduction: The Emirates eXploration Imager
(EXI) is a camera system onboard the Emirates Mars
Mission (EMM), which went into orbit around Mars
on February 9, 2021. The goal of EMM is to explore
the dynamics of the Martian atmosphere across diurnal and seasonal timescales (e.g., Amiri et al., 2022).
A particular focus of the mission is the circulation of
the lower atmosphere and the connections to the upward transport of energy of the escaping atmospheric
particles from the upper atmosphere. EXI is one of
three complementary scientific instruments chosen to
accomplish this goal (e.g., Almatroushi et al., 2021).
The instruments and their specific goals have been described in detail elsewhere (Emirates Mars InfraRed
Spectrometer (EMIRS) - Edwards et al, 2021; Emirates Mars Ultraviolet Spectrometer (EMUS) –
Holsclaw et al, 2021; EXI - Jones et al., 2021). In this
presentation, we will provide a very brief overview of
several aspects of EXI. However, the focus of our
presentation (but not necessarily in this abstract) will
be a series of on-orbit status updates.
Brief Instrument Description: As mentioned
above, EXI is described in detail by Jones et al.
(2021). EXI is a multi-band, camera capable of taking
12- megapixel images. Given the science orbit of
EMM, this translates to a spatial resolution of 2-4 km
per un-binned pixel. It employs a filter-wheel mechanism consisting of 6 discrete bandpass filters that
sample the optical spectral region. This are listed in
Table 1.
Table 1: EXI Bandpass Description
Channel
FWHM (nm)
l0 (nm)
f220
250
107
f260
264
48.3
f320
321
24.1
f437
437
14.2
f546
546
15.0
f635
637
14.5

Radiometric fidelity is optimized while simplifying
the optical design by separating the ultraviolet (UV)
and visible (VIS) optical paths. The filters are naturally associated with a particular channel: UV(f220,
f260, f320) and VIS (f437, f56, f635). An image
cross section shown in Figure 1.

Figure 1 - Instrument cross section (after Figure 2
of Jones et al., 2021).
The instrument was developed jointly by the Mohammed Bin Rashid Space Centre (MBRSC), Dubai,
UAE, and the Laboratory for Atmospheric and Space
Physics (LASP) at the University of Colorado, Boulder, USA. The instrument underwent an extensive
ground-calibration effort, and included the following
tests, Field of View Mapping (FOV), Focus/distortion
mapping, Flat field/Broadband radiometry, Bandpass
shape/Absolute radiance mapping. The calibration activities and their results were described by Jones et al.
(2021).
In-flight Updates: During cruise and orbital operations, observations were obtained that allowed us
to improve/update certain aspects of the instrument
calibration with respect to the pre-flight values. These
will be provided as part of our presentation, but we
provide some current information in this abstract:
Radiometric Performance. EXI regularly observes two UV standard stars: a-Lyrae (Vega) and
h-Ursae Majoris (h-UMa). These stars are part of a
standard star program carried out by the Hubble Space
Telescope; and have well characterized spectra
(Bohln
et
al.,
2001;
2019;

https://www.eso.org/sci/observing/tools/standards/spectra/hststandards.html). Using a combination
of stellar photometry and the EXI instrument performance (i.e., bandpasses and spectral response), the
conversions from Data Number to irradiance can be
derived. Although still updated/finalized at the time
of this abstract, the radiometric accuracy/precision of
each band is summarized in Table 2. Updates will be
providing during our presentation.
Table 2: Radiometric Accuracy of EXI
Channel
precision
Notes
F220
< 5%
Bandpass
much
wider
than designed;
see Table 1
F260
< 10-20%
This is due to
out-of-band
visible light;
correction being developed
F320
< 5%
F437
-Awaiting additional more hUma observations, expected
to be similar to
f635
F546
-See f437
F635
< 5%
Camera Model and Pointing Offsets. The use of
EXI data for remote sensing assumes the ability to
map a pixel location to a physical coordinate system
and associated directional cosines with a reference
surface. However, a starting point is an undistorted
detector frame. The camera model of EXI removes
this distortion, introduced by the lens system. In addition, the orientation of the EXI optical axis with respect to the spacecraft reference frame(s) is necessary.
Although it appears that this knowledge is accurate to
a few pixels in each band, we are still actively attempting to quantify and improve on this result, our
goal is to be provide registration to within a single
pixel. The status of this effort will be presented.
eXi Observation Sets (XOS). There are two
“workhorse” sets of observations for EXI, the socalled XOS-1 and XOS-2.
XOS-1 consists of 5 filters (f220 is not a part of
this XOS because of the wide bandpass, i.e., it is not
useful for the intended purpose of the dust column retrieval). In order to maximize the number of observations with respect to available data volume, on-chip
binning is used: three bands employ 2×2 binning
model (f260, f320, f635) and the other two use 4×4
(f437, f536).
XOS-2 includes all six filters but is heavily
summed: 16×16. Because of the small data volume,

it was decided to keep the f220 image to allow for its
potential use in monitoring in the degradation in the

Figure 2: Schematic representation
of the use of XOS-1 and XOS-2
with respect to a standard EMIRS
observation.
UV throughput. The XOS-2 is used after an EMIRS
observation to capture any pixels that were observed
by EMIRS but not present in the XOS-1. This order
of observations can be seen in the schematic in Figure
2.
Additional XOS have been defined since launch
as well as some no longer being used. A list of XOS
potentially relevant to the community will be summarized in our presentation.

Figure 3 - Color image created through an automated processing of a the RGB channels of a
Level 2B file for an observation in early December 2021.
Data Levels. There have been some changes to the
design of the various levels of data products since
launch Jones et al. (2021) versus those provided by
the Science Data Center (see URL in references). In
addition, a particular product is still being developed

that is map-project data cube (the so-called Level
2B), which allows for easy generation of registered,
multi-band color (e.g., RGB color) such as seen in
Figure 3. This removes the effect of spacecraft motion, planetary rotation, and offsets between the UV
and VIS detector FOVs. Any of the channels can be
wrapped around a globe again to provide an image
without the camera distortion discussed above. See
Figure 4.

Amiri HES, Brain D, Sharaf O, Withnell P, McGrath M, Alloghani M, et al. The Emirates Mars Mission. Space
Science Reviews. 2022;218:4.
Bohlin RC, Deustua SE, de Rosa G. Hubble Space Telescope
Flux Calibration. I. STIS and CALSPEC. The Astronomical Journal. 2019;158:211.
Bohlin RC, Dickinson ME, Calzetti D. Spectrophotometric
Standards from the Far-Ultraviolet to the Near-Infrared: STIS and NICMOS Fluxes. The Astronomical
Journal. 2001;122:2118-28.
Edwards CS, Christensen PR, Mehall GL, Anwar S, Tunaiji
EA, Badri K, et al. The Emirates Mars Mission
(EMM) Emirates Mars InfraRed Spectrometer
(EMIRS) Instrument. Space Science Reviews.
2021;217:77.
Emirates Mars Mission Science Data Center,
(https://sdc.emiratesmarsmission.ae
EXI Data Product Guide, 2022, https://sdc.emiratesmarsmission.ae/documentation

Figure 4: The Level 2B-based product shown in
Figure 3 wrapped around a globe to provide the
original observational perspective, but without
the effects of camera distortion.

Holsclaw GM, Deighan J, Almatroushi H, Chaffin M, Correira J, Evans JS, et al. The Emirates Mars Ultraviolet Spectrometer (EMUS) for the EMM Mission.
Space Science Reviews. 2021;217:79.

Also being finalized is the so-called Level 3 water
ice cloud optical product (which is the focus of the
analyses presented by Wolff et al. elsewhere at this
conference). These two formats, as well as those
discussed in the current EXI Data Guide (2022),
will be summarized in our presentation.

Jones AR, Wolff M, Alshamsi M, Osterloo M, Bay P, Brennan N, et al. The Emirates Exploration Imager (EXI)
Instrument on the Emirates Mars Mission (EMM)
Hope Mission. Space Science Reviews.
2021;217:81.

EXI in the Science Data Center (SDC): The
EXI team is developing the concept of calibration
memos and instrument science reports which will be
distributed through the Science Data Center Archive
(see URL in references). These are meant to address
topics that are beyond the scope of the EXI Data Product Guide. Examples include the analyses and details
behind such things as the Top-Of-the-Atmosphere Solar flux values and the radiometric conversion coefficients found in the data product headers. These efforts will be summarized in our presentation.
Bibliography:
Almatroushi H, AlMazmi H, AlMheiri N, AlShamsi M, AlTunaiji E, Badri K, et al. Emirates Mars Mission
Characterization of Mars Atmosphere Dynamics and
Processes. Space Science Reviews. 2021;217:89.