ASIM is a climate observatory designed for detecting transient luminous event and terrestrial gamma ray flashes. ASIM will be mounted externally on the European Space Laboratory on ISS, Columbus. The ASIM payload consists of four sub-systems, CEPA, DHPU, MXGS, and MMIA. The CEPA and DHPU form the structural and electrical platform servicing the MXGS and MMIA scientific instruments. The DHPU, MXGS, and MMIA are mounted on-top of the CEPA. The power and data connections from ISS are routed through the CEPA to the DHPU, which converts 120V ISS supply to 28V instrument supply and handles all data communication.
The ASIM mass is estimated at 330kg and a power consumption of 500W, including 200W for thermal heaters. ASIM has a downlink allocation of 200 kbps continuous data, which will be fully utilized since the instruments collect wast amount of data, and low prioritized data cannot be fully downlinked.
ASIM is controlled from a ground User Operator Center (USOC) connected to the ISS ground stations. When USOC sends a command to ASIM, it is routed to the Columbus Control Center (COL-CC), 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.
The ASIM MXGS instrument carries two set of detectors for TGF. The low energy detector senstive in the spectrcal band from 15 keV to 400 keV and the high energy detector 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.
ASIM uses optical observiations in carefully selected bands in order to filter out data with TLEs from the lightning data. Since downlink is limited, thse algorithms are implemented in the on-board software. The ASIM MMIA instrument is capable of observing 12 frames per second continuously in the 777.4 nm and 337 nm bands, both only 5 nm wide. Combined with 100 kHz photometer data from the same two bands in addition to a 180-230 nm band, data is filtered en realtime to optimize the available downlink capability allocated to ASIM on ISS.
The DHPU handles all electrical interfaces between ASIM and ISS. The DHPU receives two 120V supplies, one for operational power and one for heaters. During Dragon flight and robotic installation, a third 120V heater supply is utilized. The DHPU converts the 120V operational supply to 28V instrument supplies. The 120V 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 120V 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.
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 include connectors which routes the electrical connections from ISS through the FRAM system to 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.s