Onboard systems

Life supportedit

The critical systems are the atmosphere control system, the water supply system, the food supply facilities, the sanitation and hygiene equipment, and fire detection and suppression equipment. The Russian Orbital Segment's life support systems are contained in the Zvezda service module. Some of these systems are supplemented by equipment in the USOS. The Nauka laboratory has a complete set of life support systems.

Atmospheric control systemsedit

The atmosphere on board the ISS is similar to the Earth's. Normal air pressure on the ISS is 101.3 kPa (14.69 psi); the same as at sea level on Earth. An Earth-like atmosphere offers benefits for crew comfort, and is much safer than a pure oxygen atmosphere, because of the increased risk of a fire such as that responsible for the deaths of the Apollo 1 crew. Earth-like atmospheric conditions have been maintained on all Russian and Soviet spacecraft.

The Elektron system aboard Zvezda and a similar system in Destiny generate oxygen aboard the station. The crew has a backup option in the form of bottled oxygen and Solid Fuel Oxygen Generation (SFOG) canisters, a chemical oxygen generator system. Carbon dioxide is removed from the air by the Vozdukh system in Zvezda. Other by-products of human metabolism, such as methane from the intestines and ammonia from sweat, are removed by activated charcoal filters.

Part of the ROS atmosphere control system is the oxygen supply. Triple-redundancy is provided by the Elektron unit, solid fuel generators, and stored oxygen. The primary supply of oxygen is the Elektron unit which produces O
2 and H
2 by electrolysis of water and vents H2 overboard. The 1 kW (1.3 hp) system uses approximately one litre of water per crew member per day. This water is either brought from Earth or recycled from other systems. Mir was the first spacecraft to use recycled water for oxygen production. The secondary oxygen supply is provided by burning O
2-producing Vika cartridges (see also ISS ECLSS). Each 'candle' takes 5–20 minutes to decompose at 450–500 °C (842–932 °F), producing 600 litres (130 imp gal; 160 US gal) of O
2. This unit is manually operated.

The US Orbital Segment has redundant supplies of oxygen, from a pressurised storage tank on the Quest airlock module delivered in 2001, supplemented ten years later by ESA-built Advanced Closed-Loop System (ACLS) in the Tranquility module (Node 3), which produces O
2 by electrolysis. Hydrogen produced is combined with carbon dioxide from the cabin atmosphere and converted to water and methane.

Power and thermal controledit

Double-sided solar arrays provide electrical power to the ISS. These bifacial cells collect direct sunlight on one side and light reflected off from the Earth on the other, and are more efficient and operate at a lower temperature than single-sided cells commonly used on Earth.

The Russian segment of the station, like most spacecraft, uses 28 V low voltage DC from four rotating solar arrays mounted on Zarya and Zvezda. The USOS uses 130–180 V DC from the USOS PV array, power is stabilised and distributed at 160 V DC and converted to the user-required 124 V DC. The higher distribution voltage allows smaller, lighter conductors, at the expense of crew safety. The two station segments share power with converters.

The USOS solar arrays are arranged as four wing pairs, for a total production of 75 to 90 kilowatts. These arrays normally track the sun to maximise power generation. Each array is about 375 m2 (4,036 sq ft) in area and 58 m (190 ft) long. In the complete configuration, the solar arrays track the sun by rotating the alpha gimbal once per orbit; the beta gimbal follows slower changes in the angle of the sun to the orbital plane. The Night Glider mode aligns the solar arrays parallel to the ground at night to reduce the significant aerodynamic drag at the station's relatively low orbital altitude.

The station originally used rechargeable nickel–hydrogen batteries (NiH
2) for continuous power during the 35 minutes of every 90-minute orbit that it is eclipsed by the Earth. The batteries are recharged on the day side of the orbit. They had a 6.5-year lifetime (over 37,000 charge/discharge cycles) and were regularly replaced over the anticipated 20-year life of the station. Starting in 2016, the nickel–hydrogen batteries were replaced by lithium-ion batteries, which are expected to last until the end of the ISS program.

The station's large solar panels generate a high potential voltage difference between the station and the ionosphere. This could cause arcing through insulating surfaces and sputtering of conductive surfaces as ions are accelerated by the spacecraft plasma sheath. To mitigate this, plasma contactor units (PCU)s create current paths between the station and the ambient plasma field.

The station's systems and experiments consume a large amount of electrical power, almost all of which is converted to heat. To keep the internal temperature within workable limits, a passive thermal control system (PTCS) is made of external surface materials, insulation such as MLI, and heat pipes. If the PTCS cannot keep up with the heat load, an External Active Thermal Control System (EATCS) maintains the temperature. The EATCS consists of an internal, non-toxic, water coolant loop used to cool and dehumidify the atmosphere, which transfers collected heat into an external liquid ammonia loop. From the heat exchangers, ammonia is pumped into external radiators that emit heat as infrared radiation, then back to the station. The EATCS provides cooling for all the US pressurised modules, including Kibō and Columbus, as well as the main power distribution electronics of the S0, S1 and P1 trusses. It can reject up to 70 kW. This is much more than the 14 kW of the Early External Active Thermal Control System (EEATCS) via the Early Ammonia Servicer (EAS), which was launched on STS-105 and installed onto the P6 Truss.

Communications and computersedit

Radio communications provide telemetry and scientific data links between the station and mission control centres. Radio links are also used during rendezvous and docking procedures and for audio and video communication between crew members, flight controllers and family members. As a result, the ISS is equipped with internal and external communication systems used for different purposes.

The Russian Orbital Segment communicates directly with the ground via the Lira antenna mounted to Zvezda. The Lira antenna also has the capability to use the Luch data relay satellite system. This system fell into disrepair during the 1990s, and so was not used during the early years of the ISS, although two new Luch satellites—Luch-5A and Luch-5B—were launched in 2011 and 2012 respectively to restore the operational capability of the system. Another Russian communications system is the Voskhod-M, which enables internal telephone communications between Zvezda, Zarya, Pirs, Poisk, and the USOS and provides a VHF radio link to ground control centres via antennas on Zvezda's exterior.

The US Orbital Segment (USOS) makes use of two separate radio links mounted in the Z1 truss structure: the S band (audio) and K_u band (audio, video and data) systems. These transmissions are routed via the United States Tracking and Data Relay Satellite System (TDRSS) in geostationary orbit, allowing for almost continuous real-time communications with Christopher C. Kraft Jr. Mission Control Center (MCC-H) in Houston. Data channels for the Canadarm2, European Columbus laboratory and Japanese Kibō modules were originally also routed via the S band and K_u band systems, with the European Data Relay System and a similar Japanese system intended to eventually complement the TDRSS in this role. Communications between modules are carried on an internal wireless network.

UHF radio is used by astronauts and cosmonauts conducting EVAs and other spacecraft that dock to or undock from the station. Automated spacecraft are fitted with their own communications equipment; the ATV uses a laser attached to the spacecraft and the Proximity Communications Equipment attached to Zvezda to accurately dock with the station.

The ISS is equipped with about 100 IBM/Lenovo ThinkPad and HP ZBook 15 laptop computers. The laptops have run Windows 95, Windows 2000, Windows XP, Windows 7, Windows 10 and Linux operating systems. Each computer is a commercial off-the-shelf purchase which is then modified for safety and operation including updates to connectors, cooling and power to accommodate the station's 28V DC power system and weightless environment. Heat generated by the laptops does not rise but stagnates around the laptop, so additional forced ventilation is required. Laptops aboard the ISS are connected to the station's wireless LAN via Wi-Fi and ethernet, which connects to the ground via K_u band. While originally this provided speeds of 10 Mbit/s download and 3 Mbit/s upload from the station, NASA upgraded the system in late August 2019 and increased the speeds to 600 Mbit/s. Laptop hard drives occasionally fail and must be replaced. Other computer hardware failures include instances in 2001, 2007 and 2017; some of these failures have required EVAs to replace computer modules in externally mounted devices.

The operating system used for key station functions is the Debian Linux distribution. The migration from Microsoft Windows was made in May 2013 for reasons of reliability, stability and flexibility.

In 2017, an SG100 Cloud Computer was launched to the ISS as part of OA-7 mission. It was manufactured by NCSIST of Taiwan and designed in collaboration with Academia Sinica, and National Central University under contract for NASA.