ISS Utilization: ASIM (Atmosphere-Space Interactions Monitor)
ASIM is an ESA science instrument assembly to be flown on the Columbus External Platform Facility (CEPF) of the ISS (International Space Station). The ASIM concept has been proposed by DNSC [Danish National Space Center, formerly DSRI (Danish Space Research Institute)], with the objective to observe TLEs (Transient Luminous Events) that occur in the Earth's upper atmosphere accompanied by thunderstorms in the lower atmosphere. These events are known as blue jets, sprites and elves, the phenomena were first observed in 1989. The ISS is considered a perfect platform from which to enhance our knowledge of them. 1) 2) 3) 4) 5) 6) 7)
The mission is realized through ESA. In January 2007, DNSC of Copenhagen, Denmark (formerly DSRI) became DTU-Space, an institute at the Technical University of Denmark. DTU-Space provides the scientific leadership, and the Danish company, Terma, the technical leadership. Other major partners include the University of Valencia in Spain, and the University of Bergen in Norway, who are both involved in the development of the instruments.
ASIM has a number of cameras, specially designed for the International Space Station, that will observe the Earth's atmosphere. ASIM will give new insights into climate processes that will improve climate models by quantifying the effect of electrical and chemical processes at the atmosphere/space boundary. DTU Space is responsible for delivering a package of instruments (2 cameras, 3 photometers and one X- and gamma-ray detector) which will be mounted on the International Space Station. 8) 9)
ASIM will study the Earth's atmosphere as one system, from the surface of the Earth to the edge of space. The atmosphere is the thin layer that covers the planet's crust, and protects life as we journey through space. ASIM will observe extreme thunderstorms, water vapor, clouds, aerosols and their interplay in the atmosphere.
Figure 1: The nature of electrical phenomena in the atmosphere with red sprites, blue jets and elves above thunderstorms (image credit: ESA)
The nature of the electrical phenomena (Figure 1) and their discharges are linked to violent storms in the tropics and inject water vapor, NOx and other greenhouse gases into the stratosphere where they become part of the climate moderators. ASIM will study these effects, as well as the electrical influence on the ionosphere and the atmospheric interactions with the particle radiation from the sun. Both of which also have a direct bearing on the Earth's climate.
ASIM, an approved ESA project, was selected in response to a call for flight opportunity issued by the Directorate of Human Spaceflight in December 2002 for external payloads to be flown to and operated onboard Columbus external platforms. The ASIM payload is planned to be taken to the ISS in the time frame 2014.
The ASIM project has been a Danish initiative, from the very start, headed in the initial phase by the Danish National Space Centre (DNSC) with participation of the universities of Valencia (Spain) and Bergen (Norway). As of 2007, the project is in Phase B planned to complete in 2009. The ASIM development is lead by TERMA, a high-tech company of Denmark. The consortium includes DNSC, Damec Research Aps, the University of Valencia, the University of Bergen, the University of Ferrara, and the University of Bologna. The production Phase C of ASIM was started in August 2010. 10) 11) 12)
The primary research objectives of the mission require the following measurements:
• Study the physics of TLEs (Transient Luminous Events). Optical detection of TLEs with high spatial- and time resolution in selected spectral bands - a comprehensive global survey
• Study the physics of TGFs (Terrestrial Gamma-ray Flashes) and their relationship with TLEs and thunderstorms. X-ray and γ-ray detection of TGFs with high time resolution and at photon energies reaching down to 10 keV
• Simultaneous optical detection of thunderstorm- and TLE activity with TGF activity. The optical instruments must view with the X- and γ-ray detector towards the nadir
• Study the coupling to the mesosphere, thermosphere and ionosphere of thunderstorms and TLEs
• Observations from space during a minimum of one year at all local times to observe seasonal and local time variations in thunderstorm-, TLE-, and TGF activity.
Secondary objectives based on observations:
- Spectroscopic studies of the aurora
- Studies of greenhouse gas concentrations above thunderstorms (NOx, O3)
- Studies of meteor ablation in the mesosphere and thermosphere.
Optical and X-ray measurements are used to study aurora, differential absorption of light emissions from lightning-illuminated thunderstorm clouds measured by photometers defines ozone column densities, NOx production in TLEs is to be monitored by photometer 5, and optical imaging, and photometers will be used to study meteor ablation.
The measurements include imaging in 4 auroral bands simultaneously, coupled with high time-resolution photometer observations and X-ray and γ-ray observations. The inclination of the ISS orbit at 51.6º brings the instruments over the auroral oval during periods of high solar (geomagnetic) activity when the auroral oval expands to lower latitudes. These are also periods with high auroral activity.
Figure 2: A bright red sprite appears above a lightning flash in a photo captured from the ISS (image credit: NASA, Universe Today)
Figure 3: Illustration of a TGF. Terrestrial gamma ray flashes are high energetic discharges in the Earth atmosphere, but the origin of events is unclear. This is why ASIM aims to study the correlation between the TGF with lightning and TLE (image credit: ASIM collaboration)
ASIM payload elements and accommodation:
The ASIM optical instruments make up the MMIA (Miniature Multispectral Imaging Array) consisting of 3 modules, each housing 2 video-rate cameras and two photometers. Two modules view in the ram-direction towards the limb and one module towards the nadir. The MXGS (Miniature X-ray and Gamma-ray Sensor) is pointed towards nadir.
In addition, there are the following subsystems:
• DHPU (Data Handling and Power Unit). The DHPU handles all electrical interfaces between ASIM and ISS. The DHPU receives two 120 V supplies, one for operational power and one for heaters. During the Dragon flight and robotic installation, a third 120 V heater supply is utilized. The DHPU converts the 120 V operational supply to 28 V instrument supplies. The 120 V heater supplies are distributed to two separate heater sets in the instruments and DHPU itself. Two sets are used since the necssary power for thermal conditioning is different in the Dragon and robotics phases compared to the mission life on Columbus.
- Apart from the 120 V supplies, the DHPU implements an ethernet connection for data link, MIL-BUS for monitored data and time synchronization with ISS, and finally a serial line which allows the ISS crew to patch the firmware of the DHPU.
Figure 4: Photo of the DHPU (image credit: ASIM collaboration)
• CEPA (Columbus External Payload Adapter). The CEPA is a standard structural item designed by Boeing for Columbus. It implements a standard interface to ISS called the FRAM (Flight Releasable Attachment Mechanism), which allows payloads and standard cargo like battery assemblies, to be attached on the ISS FRAM location like the Columbus External Payload Facility. The FRAM interface includes connectors which routes the electrical connections from the ISS through the FRAM system connectors on the top side of the CEPA. It is these connectors that are routed through the ASIM harness to DHPU and then distributed to the instruments.
- The CEPA provides the mechanical and electrical interface between the instrument and respectively the Columbus External Payload Facility (CEPF) and the carrier (LCC).
Figure 5: Photo of the CEPA (image credit: ASIM collaboration)
ASIM is designed to be accommodated on the starboard deck location of the CEPF (Columbus External Payload) platform (Figure 6).
Figure 6: Artist's view of the ASIM allocation at the Columbus External Platform Facility (image credit: ESA, DTU-Space)
The overall power consumption of ASIM is expected not to be 500 W (including 200W for thermal heaters). The mass of ASIM, including CEPA and the active FRAM (Flight Releasable Attachment Mechanism) is about 330 kg. The on-orbit lifetime is expected to be 2 years. ASIM has a downlink allocation of 200 kbit/s continuous data, which will be fully utilized since the instruments collect a wast amount of data, and low prioritized data cannot be fully downlinked.
Launch: A launch of the ASIM payload to the ISS is planned for the timeframe 2017 with the Falcon-9/Dragon vehicle of SpaceX. 14)
Orbit: Near-circular orbit of the ISS, altitude of ~400 km, inclination = 51.6º.
ASIM is transported to ISS in the external trunk of Dragon. Once Dragon is docked to ISS Node 2, the SSRMS (Space Station Remote Manipulator System) will install ASIM in its final location on Columbus. The SSRMS will utilize the SPDM (Special Purpose Dexterous Manipulator) to grab ASIM in the Dragon trunk for extraction. The SPDM can operate the FRAM (Flight Releasable Attachment Mechanism) to detach ASIM from the trunk. The same mechanism is installed on the SPDM iteself in case ASIM needs to be fixed to the SPDM in order receive power for thermal heaters during the seven hour transfer to Columbus. On the CEPF (Columbus External Payload Facility ), ASIM will be attached to the starboard facing deck location, named EPF SDX, which also suppports the FRAM.
ASIM instrument assemblies (MMIA, MXGS):
ASIM consists of two optical cameras, 3 photometers, and one large X- and Gamma ray detector. The instruments will be installed on the Columbus External Pallet to be mounted on the exterior of the Columbus module, housing ESA's laboratory on the ISS. 15) 16) 17) 18) 19) 20) 21) 22)
The optical assembly, referred to as MMIA (Modular Multispectral Imaging Array), comprises two optical narrow band cameras and three photometers with related optical and signal processing capabilities, including autonomous event detection algorithms to identify and prioritize events for download to Earth.
The MMIA instrument will be combined with the MXGS (Modular X- and Gamma-ray Sensor) into the Nadir Viewing Assembly looking directly down on top of thunderstorms according above the Earth.
MXGS is designed to detect radiation from TGF (Terrestrial Gamma Flashes) and from lightning induced electron precipitation. The detector is built around a BiGe (Bismuth Germanium) as well as a CZT (Cadmium Zinc Telluride) semiconductor detection plane of 32 cm x 32 cm with possible imaging capabilities.
Fast electronic circuitry used in the MXGS will provide time history and spectra over the course of the expected lifetime of 1-5 ms for each TGF. Also, a TGF burst trigger signal is passed to the adjacent MMIA module (and visa versa) for synchronization of the two types of observation.
X- and gamma-rays are strongly absorbed in the atmosphere. This is why the detector points directly downwards, such that a minimum of atmosphere is between the detector and the thunderstorms within its field of view. Most of the atmosphere is below the altitude where giant lightning and terrestrial gamma-ray flashes are generated. Therefore, space is particularly well suited to observe these phenomena in the band reaching from gamma-rays to UV, which is difficult to observe from the ground. ASIM is measuring in these bands (colors).
The ASIM mission will address a variety of important scientific and technological aspects which will include:
• Understanding of the processes involved in thunderstorm initiated electrical discharges
• Understand their impact on atmospheric processes and possible links to climate determining factors
• Development of new technologies with spin-off into terrestrial applications for advanced process control and optical instrumentation
• Demonstration of the fruitful utilization of the collaborative investments in the International Space Station.
In view of the unique observation point, the advanced instrumentation set, and the long duration of the mission, it is expected that ASIM will produce scientific data of high quality which will give an unprecedented contribution to the understanding of interaction mechanisms between the atmosphere and space.
Figure 7: Layout of the ASIM instrument assembly (image credit: ASIM collaboration)
MMIA (Modular Multispectral Imaging Array):
ASIM uses optical observiations in carefully selected bands in order to filter out data with TLEs from the lightning data. Since the data downlink is limited, the algorithms are implemented in the on-board software. The ASIM MMIA instrument is capable of observing 12 frames/s continuously in the 777.4 nm and 337 nm bands, both only 5 nm wide. Combined with the 100 kHz photometer data from the same two bands in addition to a 180-230 nm band, data is filtered in realtime to optimize the available downlink capability allocated to ASIM on ISS.
Figure 8: Illustration of the MMIA assembly (image credit: ASIM collaboration)
The optical instruments are grouped into two) groups, each composed of two optical narrow band cameras and 2 photometers with related optical and signal processing capabilities including autonomous event detection algorithms to identify and prioritize events for download.
• 4 cameras and 4 photometers look forward towards the limb
• 2 cameras and 2 photometers look downwards towards the nadir
The cameras and photometers are equipped with baffles for stray light protection. The camera sampling is 12 bit 1024 x 1024 pixel frames at a maximum of 25 Hz.
Table 1: Parameters of the MMIA optical instruments
The photometers are used to measure rapid time variations, which cannot be done by the imaging cameras. They view the exact same region but measure only the total photon flux from the region - but with high time resolution. The photometer FOVs are identical to those of the cameras: 20º x 20º (limb or ram direction) and 80º x 80º (nadir direction).
Table 2: Optical parameters of the MMIA instruments
Legend: *extension under consideration – will allow also day time observations of lightning
Figure 9: Block diagram of the MMIA photometers (TNO Science and Industry)
Figure 10: View of the MMIA limb assembly with 4 cameras and 4 photometers (image credit: DNSC)
Data handling subsystem: Time synchronizing of instrument measurements with a relative time accuracy < 10 µs and with an accuracy of 100 µs compared to GPS/UTC.
Figure 11: Overview of system data and signal interfaces (image credit: DNSC)
Figure 12: The nadir-viewing assembly (MMIA) of 2 cameras + 2 photometers + MXGS, (image credit: DNSC)
MXGS (Modular X-ray and Gamma-ray Sensor):
The MXGS instrument carries two set of detectors for TGFs (Terrestrial Gamma-ray Flashes). The low energy detector is senstive in the spectral band from 15 keV to 400 keV and the high energy detector is sensitive from 200 keV to 40 MeV. The low energy detector is pixellated in 128 by 128 channels, which, in combination with a high mass density coded mask in front of the detector, allows advanced post-processing algorithms to pin point the direction to the TGF source. Overlaying the TGF direction with the optical imaging by the MMIA instrument, the correllation with lightning and TLE is possible. 23) 24) 25) 26) 27) 28) 29) 30) 31)
Table 3: Technical parameters of MXGS 32)
The MXGS detector plane consists of a 1024 cm2 array of CZT detector crystals. It is protected against the background radiation by a passive graded shield surrounding the detector housing. A hopper shaped collimator defines the 80º x 80º field of view for MXGS and shields the detector plane against the Cosmic X-ray Background. The DFEE (Detector Front End Electronics) is mounted in the housing below the detectors. The electronics contains also the HVPS (High Voltage Power Supply) and LVPS (Low Voltage Power Supply) as well as the DPU (Data Processing Unit).
The DFEE design consists of 4 DAUs (Detector Assembly Units), and each DAU consists of 16 DM (Detector Modules) and one DAB (Detector Assembly Board). The DAB holds the read-out electronics and the RCU (Readout Control Unit). The RCUs interface to the DPU. The purpose of the DAU is to read out the events and transfer the data to the DPU.
The DM consists of two separable units, a CZT sensor and an ASIC. The sensor comprises four 20 mm x 20 mm x 5 mm CZT detectors tiled together on a PCB. Each detector is pixelated into 64 pixels (2.5 mm pixel pitch), making a 16 x 16 pixel array in total. The detector unit is stacked onto the ASIC unit via three connectors (Figure 15).
The MXGS uses fast ASICs to provide the time history and spectra over the course of the expected TGFs lifetime of 1-5 ms and a TGF burst trigger signal is passed to the companion MMIA module (and visa versa). The observation plane is protected from background radiation by a passive shield and the field of view is defined by a hopper shaped collimator.
Figure 13: Illustration of the MXGS instrument (image credit: University of Bergen)
Figure 14: Schematic view of the MXGS instrument elements (image credit: MAPRAD)
Figure 15: Photo of the detector module (image credit: DNSC, University of Bergen)
Figure 16: Illustration of the MXGS instrument (image credit: ASIM collaboration)
ASIM is controlled from a ground USOC (User Operator Center) connected to the ISS ground stations. When USOC sends a command to ASIM, it is routed to theCOL-CC ( Columbus Control Center), then to the ISS ground station in Houston responsbile for uplinking the command to ISS, which eventually reaches ASIM through Columbus. The DHPU will process all commands and it also supports a command schedule for autonomous observation timelines. The ASIM science and housekeeping data follow the same route in reverse back to the USOC.
Figure 17: Overview of the ground segment elements for ASIM operations (image credit: ESA)
Ground observations are important parts of the ASIM and TARANIS missions (observe from space what is best observed from space and from ground what is best observed from ground).
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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 (email@example.com).