There are many things to marvel at when you think about the International Space Station (ISS). It has been in continuous operation since being put into orbit in 1998. It brought several countries together into a unified commitment to space exploration. The conditions within the station remain stable, despite being deployed in the harshest environment known to man.
And, it’s probably the most complicated machine humankind has ever built, especially given that, only after they were launched, the individual modules were then put together in space itself, never having been integrated with each other before on earth. This is the same with the new systems and platforms that are sent to the ISS to add functionality, integrate technology updates and add new capabilities, since the ISS can’t exactly be taken out of orbit.
Maintaining Communication Connections
In June of 2018, a new mission made its way to the ISS. As part of that supply mission, an Air Transport Rack (ATR) using the OpenVPX architecture was loaded in the communications module. The OpenVPX system, designed and manufactured by Elma Electronic, was installed as part of the "Broadband Communication System User Terminal" in the ISS communications service module. (Figure 1)
ATR is been a proven form factor, and a first choice, for rugged computer systems in the aerospace industry for many years. In this industry in particular, maneuvers of the aircraft create enormous forces that a "simple" computer case just would not survive. An ATR has particular resistance to high levels of shock and vibration, and therefore, does not bend as much as other enclosures. It can also survive stronger impacts, such as a particularly hard landing.
For this implementation, the ATR forms the housing, which then encases a computer unit using OpenVPX architecture. When the system went into service in space, it enabled a "near-permanent" connection of the ISS to ground control.
Eliminating Gaps in Data Transmission
Due to the high speed and steadily increasing data volumes to and from the ISS, the connection must be re-routed almost permanently, depending on which satellites offer the best transmission route at the respective time. Communication satellites—in this case, via the GEO-satellite network "Luch" used as relays—are the solution. Originally developed for communication between the Mir space station and the Buran space glider, these GEO relay satellites are a series of communications satellites that transmit signals between spacecraft (such as the ISS) and Earth.
"Because the ISS only takes about 90 minutes to orbit around the world, the space station would cross over an area of the earth not covered by ground reception systems, and the connection would break off each time,” explains Vitali Siris, of Elma Electronic GmbH, who oversaw development of this OpenVPX-based system. “We established a more reliable communications infrastructure using a system based on a proven open standard architecture in a rugged housing.” (Figure 2)
The OpenVPX-based system takes on the task of routing the signals, enabling data throughputs with an uplink speed of up to 100 Mbps and a download speed of 6 to 8 Mbps. The system represents the hardware part of a Multiplexer / Demultiplexer Modem (MDM) unit. The MDM is responsible for capturing, storing and transmitting the results of onboard experiments.
Among other things, the unit includes a modem, DVB-S2 (for satellite signals), PAL (phase-alternating line, so an analog video signal) and HD video inputs and outputs. The MDM’s serial interface cards comply with MIL-STD-810F, thus guaranteeing the needed resistance to shock and vibration.
"Resistance to environmental impacts is necessary," continues Siris. "While the technology inside the ISS itself benefits from constant climatic conditions of 20 to 22°C and similarly, a constant humidity, the electronics must be able to withstand forces of up to forty times the gravitational acceleration at rocket launch."
Handling cooling and other design elements
The subject of cooling is always a particularly sensitive issue in embedded systems that operate in harsh and remote environments, of which space is the epitome. The system for the ISS is cooled using the forced air-conduction-cooled method. The processor card, graphics card or memory card are connected to the side panel via a special mechanism and the side walls of the ATR are equipped with fins that face outward, drawing heat out as the air is forced across the system.
Although the ruggedized cards are actively cooled (passive cooling is cooling without a fan, also known as conduction), the fans work just outside the housing. This practical solution also protects the interior of the system against dust and eliminates a potential source of damage during transport. At the same time, the cooling variant saves space without interfering with the multi-functionality of the system. (Learn more about Elma’s competencies in space electronics)
Moving deeper into communications
As the networking density and functionality of the ISS increase, modern methods to handle the growing system requirements will continue to evolve. Building on the success of the OpenVPX-based ATR system, plans are in the works to develop another, similar system, for a future module.
