MATS (Mesospheric Airglow/Aerosol Tomography and Spectroscopy) Mission
MATS is a microsatellite mission of Sweden, funded by SNSB (Swedish National Space Board), with the goal to conduct optical studies of mesospheric gases for two years. Spaceborne limb imaging in combination with tomographic retrieval opens exciting new ways of probing horizontal and vertical structures in the atmosphere. SNSB approved the project following a detailed project study by OHB Sweden, Stockholm University's Department of Meteorology (MISU), Omnisys Instruments and ÅAC Microtec. 1) 2) 3)
Background: The Earth's MLT (Mesosphere and Lower Thermosphere), in the atmospheric altitude range 50-130 km, are an important transition region with strong dynamical links to above and below. Despite this fact, this region has frequently been treated as a dividing line for atmospheric research. Traditional circulation models for the lower and middle atmosphere have the MLT region as their upper boundary, while thermospheric and ionospheric models have this region as their lower boundary. Only recently a broad interest has developed in "whole atmosphere" approaches that try to overcome this division. 4)
As for the middle atmosphere (below ~100 km), wave dynamics ranging from gravity waves to planetary waves provides strong coupling mechanisms between atmospheric regions. This is most evident in the MLT where upward propagating disturbances reach their maximum amplitudes and break, thus depositing momentum and energy onto the large-scale flow, and driving a global mesospheric circulation that is far from radiative equilibrium. This results in a strong coupling of MLT conditions to processes in the lower atmosphere. Prominent examples are recent discoveries of global coupling processes that govern the cold summer mesopause region. Proposed mechanisms for these teleconnections are based on the effect of lower atmospheric circulation on gravity wave filtering and, thus, on the dynamical forcing of the mesospheric circulation.
While the basic nature of the wave-driven circulation of the middle atmosphere is today understood, important mechanisms and interactions remain to be specified. The multiple roles of waves concerning local turbulent heating, vertical mixing and large-scale momentum deposition need quantification. Wave-wave interactions between different scales remain unclear, including the role of gravity waves in the in-situ generation of planetary waves in the MLT. In terms of climate trends, the MLT may be the atmospheric region where anthropogenic climate change produces its largest effect. In order to critically assess the significance of climatological trends observed in long-term datasets, it is necessary to better quantify the natural variability of the MLT. Again, wave activity is a major issue.
As for the upper atmosphere (above ~100 km), the MLT region is an important driver in terms of wave input. Over the last decade, a broad research field has emerged with focus on this upward coupling. Resulting model developments are trying to overcome the division of the atmosphere into separate altitude regions and separate communities. Complementary to these model efforts, there has been growing observational evidence of thermospheric and ionospheric responses to dynamical processes in the lower and middle atmosphere. The role of tidal wave influences on the formation of ionospheric morphology has long been recognized.
Newer fields of research concern effects of planetary waves and gravity waves. Recent model studies show that gravity waves may create large fluctuations in the 100 to 300 km region over spatial scales of tens to several hundred kilometers with temperature variations of 50 K or more and density variations as large as 10-25%. Additionally, the dissipation of gravity waves creates localized regions of heating and cooling over scales of several hundred kilometers. Before the gravity waves reach these altitudes, filtering takes place by the circulation systems at lower altitudes, thus creating fingerprints of the middle atmosphere in the thermosphere and ionosphere.
A recent review of wave effects on the thermosphere and ionosphere has been provided by Akmaev. 5) The author estimates that more than half of regular daily and seasonal variability in the thermosphere and ionosphere is forced from below. Akmaev strongly emphasizes the need for more data in the MLT region for validation of models under development. As compared to the data-rich lower atmosphere and upper thermosphere, global data on the dynamics and variability of the MLT transition region are described as limiting factors for future scientific progress.
Various satellite missions perform today global observations of MLT structure and variability that have been analyzed in terms of wave activity on various scales. On the TIMED satellite (launched 2001), infrared limb emission measurements by SABER provide species and temperature distributions that have revealed middle atmosphere gravity waves and planetary waves. The TIDI instrument has provided limited gravity wave data in terms of airglow Doppler wind measurements. On ENVISAT (active in 2002-2012), infrared limb emission measurements by MIPAS cover the MLT and can provide large-scale structures that have been traced into the thermosphere. Measurements by the SCIAMACHY instrument have been analyzed in terms of MLT planetary wave structures in noctilucent clouds. On the AURA satellite (launched 2004), measurements by the Microwave Limb Sounder (MLS) have provided large-scale wave activity based on composition, temperature and Doppler wind analysis. On the AIM satellite (launched 2007), the CIPS instrument provides nadir imaging of noctilucent clouds (NLC) in the ultraviolet, which results in detailed horizontal maps of gravity wave structures. Also on Odin (launched 2001), MLT gravity wave structures have been analyzed from LC observations, based on recent tomography developments. Planetary waves have been retrieved from Odin's SMR instrument. It is important to note that all of the above satellites only address a limited spatial range of the MLT gravity wave spectrum.
A summary of the current capabilities for global measurement of gravity waves in the MLT was given by Preusse et at al., 2008. 6) Figure 1 shows this summary together with the mapping capabilities of MATS. As can be seen, the MATS mission fills the current gap (given by the red area).
Figure 1: The red area represents the gap in the instrument coverage of gravity waves in the MLT region. The MATS mission will close this gap (given by the shaded area), image credit: MISU
Figure 2: Illustration of the atnospheric layers (image credit: MISU
The MATS satellite is the first satellite based on the InnoSat platform will also serve as a pilot mission in an intended program of small, low-cost research satellites funded by SNSB (Swedish National Space Board). The 50 kg microsatellite is planned to be launched in 2019 as an auxiliary payload into a 600 km dawn/dusk circular sun-synchronous orbit. There are two science instruments on board: the limb imager and the nadir imager (Ref. 4). 7)
Figure 3: MATS satellite -preliminary design of the InnoSat service module and the science instruments (image credit: OHB Sweden, Omnisys)
The platform is based on the InnoSat spacecraft bus with only minor mission specific adaptations. This includes the accommodation of the limb imager's baffle system and complementing the AOCS sensor suite with fiber-optic gyros. The latter are needed for fulfilling the pointing stability requirements stemming from the limb imaging requirements. Also, the star tracker sensor will be integrated on to the limb imager's optical bench for minimizing the absolute pointing error of the limb imager.
The platform equipment is accommodated into one single module, called the Service Module, with the launch vehicle adapter on one side and a payload interface on the other side. One single, body-mounted solar panel with high-efficiency photovoltaic cells provides up to 174 W of power. The sizes of the solar panel as well as of the scientific payload are strictly constrained by the allowable volume for a piggyback launch.
The bus has been designed to utilize the most of the available launcher volume available for a piggyback launch, to be optimized for a sun-synchronous orbit and to provide maximum possibly accommodation volume for the payload.. The size of the service module is kept to a minimum thanks to COTS electronics and a consistent single string approach. This allows the payload and solar array to utilize most of the allowable launcher volume. Smart reconfiguration of these three modules will allow both earth-facing and space-facing payloads.
Figure 4: Illustration of the standard InnoSat platform (image credit: OHB Sweden)
The satellite is three-axis stabilized using star tracker and reaction wheels. Magnetic torque rods together with a magnetometer are used for wheel momentum control and for safe mode operations. GPS is used for orbit determination time correlation. The AOCS design relies heavily on heritage from the PRISMA formation flying satellites by OHB Sweden. The Data Handling Subsystem is based on a new generation of radiation tolerant products of ÅAC Microtec including a 50 MHz OpenRISC processor, a SpaceWire communication infrastructure and a CCSDS TM/TC unit.
Table 1: InnoSat key performance factors
Possible extensions to the standard configuration includes the high power option but also a propulsion option using the Swedish green propellant (HPGP) flown on PRISMA. This system will provide approximately 120 m/s ΔV (constrained by the maximum tank size with the given accommodation volume).
Launch: A launch of the MATS microsatellite as a secondary payload is scheduled for 2019.
Orbit: Sun-synchronous dawn-dusk orbit, altitude of ~ 600 km.
Sensor complement: (Limb Imager, Nadir Imager)
In order to capture 3D wave structures in the MLT, MATS is designed to measure two phenomena occurring in the MLT region. NLCs (Noctilucent Clouds) and atmospheric airglow from the Oxygen A-band. This will be achieved by imaging the limb of the atmosphere at six different wavelengths, two in UV (between 270-300 nm) and four in IR (760-780 nm). The two UV channels will give information on small scale structures using solar light scattered of NLCs, while the IR channels provide larger scale structures, as well as atmospheric temperatures by measuring the emission from photochemically exited oxygen molecules.
To perform the limb imaging, an off-axis 3 mirror telescope based on free-form mirrors is used. The limb of the atmosphere is imaged onto the 6 CCD channels, where wavelength separation is achieved using a combination of dichroic beam splitters, and narrowband filters.
The image detection is based on advanced CCD sensors with readout electronics that allows for flexible pixel binning and image processing. A critical challenge for the limb instruments is stray light. Sources of stray light are both direct sunlight and the bright lower atmosphere that is less than 1º from the nominal field of view. Telescope optics and interior instrument setup are carefully designed to suppress stray light effects. Of particular importance is the layout of the limb baffle system that is optimized by making use of the entire available length of the InnoSat platform.
Tomographic and spectroscopic retrievals are based on a co-analysis of limb data from the six spectral channels and from different locations along the orbit. This requires sequences of limb images together with appropriate information about pointing and geolocation. Thus the need for image co-analysis defines the requirements on fields of view, instrument alignment, accuracy of pointing reconstruction, and image quality.
Figure 5: Schematic view of the Limb Imager (image credit: Ominisys)
In addition to the limb imager, a nadir imager takes pictures of atmospheric band emissions from below the satellite. This provides complementary information on smaller spatial scales, albeit restricted to a single spectral channel. Because of the nadir instruments susceptibility to the background light of the lower atmosphere, the nadir imaging will be restricted to the night-side of the terminator. The nadir instrument is basically a wide angle camera. Data are taken one row at a time, covering a horizontal distance of about 300 km swath width. In contrast to the limb instrument, simpler lens optics is used to image nadir structures on a CCD sensor. As for the limb imager, it is as well a critical design driver to baffle the nadir imager from direct sunlight.
Figure 6: Simulated results (image credit: MISU)
In summary, the MATS instrumentation will be able to provide the first global map of gravity wave structures with horizontal wavelengths < 100 km, and hence close an important gap in the current observational capabilities.
Closing this gap will allow for an increased understanding of wave activity in the middle atmosphere, with important implications for the modeling of this region in weather, climate and whole atmosphere models. Hence, with a rather small instrument, a large impact can be made on the science of the middle atmosphere.
1) Peter B. de Selding, "Sweden Strokes Check for 50-Kilogram Science Satellite," Space News, Oct. 23, 2014, URL: http://www.spacenews.com/article/civil-space/42287sweden-strokes-check-for-50-kilogram-science-satellite
2) Jörg Gumbel and the MATS Science Team, "Mesospheric Airglow/Aerosol Tomography and Spectroscopy (MATS): a satellite mission on 3D structures and waves," 41st Annual European Meeting on Atmospheric Studies by Optical Methods Stockholm, Sweden, August 17-21, 2014
3) "OHB Sweden and ÅAC Microtec to develop the InnoSat platform and implement its first mission named MATS," ÅAC Microtec, Press Release, Feb. 19, 2015, URL: http://www.mynewsdesk.com/se/aaac-microtec-ab/pressreleases/ohb-sweden-and-aaac-microtec-to-develop-the-innosat-platform-and-implement-its-first-mission-named-mats-1119420
4) N. Larsson, R. Lilja, J. Gumbel, O. M. Christensen, M. Örth, "The MATS Microsatellite Mission -Tomographic Perspective on the Mesosphere," Proceedings of the 4S (Small Satellites, System & Services) Symposium, Valletta, Malta, May 30-June 3, 2016, URL: http://congrexprojects.com/docs/default-source/16a02_docs/4s2016_final_proceedings.zip?sfvrsn=2
5) R. A. Akmaev, "Whole atmosphere modeling: Connecting terrestrial and space weather," Reviews of Geophysics, Vol. 49, Issue 4, December 2011, DOI: 10.1029/2011RG000364
6) Peter Preusse, Stephen D. Eckermann, Manfred Ern, "Transparency of the atmosphere to short horizontal wavelength gravity waves," Journal of Geophysical Research, Volume 113, Issue D24, 27 December 2008, URL: http://onlinelibrary.wiley.com/doi/10.1029/2007JD009682/full
7) N. Larsson, R. Lilja, M. Örth, S. Söderholm, J. Köhler , R. Lindberg, J. Gumbel, "InnoSat and MATS – An Ingenious Spacecraft Platform applied to Mesospheric Tomography and Spectroscopy, URL: https://www.dlr.de/iaa.symp/Portaldata/49/Resources/dokumente/archiv10/pdf/1002.pdf
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).