Core Systems

The International Space Station (ISS) is orbiting 220 miles above Earth. It is made up of many parts that must work together to form one space station. These connected parts are called systems, or a set of connected things that form a complex whole. The ISS in particular has connected equipment and software that work together. These systems were assembled to form one amazing spacecraft. It is very important that the systems work together to provide the crew with a safe, comfortable, and livable environment in which crewmembers can perform scientific research.

For the crew to live and work successfully on the ISS, the core set of systems must function well together. These systems control the technology needed for a successful mission. The core systems include power, temperature control, life support, water recycling, structure safety, motion control, data collection, navigation, propulsion, and communications.

Learn more about the following ISS Core Systems:

Several educational lessons have been developed around the core systems. For more information visit the Educator section.


Command & Data System (CDS)

The Command and Data System (CDS) is responsible for the hardware and software that collects, stores and transfers data from systems and research cargo onboard the ISS. The Command and Data Handling (C&DH) system processes the data received. The system then sends commands to the correct equipment. The system also detects and reacts to Caution and Warning events.

The system has been functioning since the first module was placed on orbit. The system provides control of the ISS from the U.S., Russian, Canadian, European, and Japanese segments.

These are the resources that comprise the Command & Data Handling system:

  • 1553 Network - onboard network that sends and receives commands and data between International Partner research cargo systems.
  • Sensor - equipment or instrument that sends telemetry (information) readings to the crew and the ground. For example, a thermometer taking temperature readings.
  • Effectors - a mechanical device used to change the state of another system. For example, turn a switch on/off or open/close a valve.
  • Multiplexers/Demultiplexers (MDM) - a computer that receives telemetry (information) from sensors. The computer then sends commands to the effectors.
  • Bus - is like a telephone line. These wires allow communication between different computers and also from the crew through the portable computer system (PCS) (i.e. laptop).
  • Portable Computer System (PCS) - a laptop the crew uses to see the telemetry data, commanding, and caution and warning information onboard the ISS.
  • Caution & Warning - a warning on the PCS that signals there is a problem, or off-nominal situation. An off-nominal situation is a condition that is a threat to crew or vehicle health, or could become a threat.
Communications & Tracking System (CTS)

Communication is an integral part to operating the ISS in a safe, stable and reliable way. The Communications and Tracking System (CTS) is responsible for the dispersing of ISS systems data and scientific research cargo data. The system is also required for the mission control centers to send commands and files to the ISS systems. This network allows the ISS crews to talk to the ground control centers, visiting vehicles, and crew on spacewalks. The Communications & Tracking System also allows for the exchanging of vehicle and/or systems data between the ISS and visiting vehicles or crew on spacewalks (including video).

The Communications and Tracking System provides the following:

  • Transmission of ISS system and research cargo telemetry (information) from the ISS to flight control teams on the ground.
  • Distribution of ISS experiment data to the research community.
  • Commanding and control of the ISS by flight controllers.
  • Commanding and control of the visiting vehicles by ISS crewmembers or the flight control teams on the ground.
  • Distribution of visiting vehicle/spacewalker voice and systems data to the ISS and to the flight control team on the ground.
  • Two-way audio and video communication between the ISS and the flight control teams on the ground.
  • Two-way audio between crewmembers.
Electrical Power System (EPS)

Electrical power is one of the most important resources onboard the ISS. Electrical power is what keeps the space station and its crew alive. The ISS needs power for all functions onboard, such as command and control, communications, lighting, and life support. The ISS gets its power by converting sunlight to electricity using solar cells. The Russian Orbital Segment (ROS) and United States On-Orbit Segment (USOS) are responsible for providing electrical power for their own segments and share power as needed. The SPARTAN console is responsible for the operation of the Electrical Power System (EPS) that generates and stores power for the USOS. The EPS then converts and distributes power to users (i.e. equipment). The SPARTAN console is also responsible for monitoring system performance and protecting both the system and users from electrical hazards.

The EPS is an example of a distributed power system. It functions much the same way municipal electric utilities work here on earth. Higher voltage power is generated in one location (the eight Solar Arrays). This power is then distributed over distance to user locations. Power is then stepped down to a lower voltage level by a transformer. This makes it safer to be used by the consumer (astronauts, in this case).

The EPS is made up of the following:

  • Solar Array Wings - convert sunlight (solar power) into DC (direct current) power (electrical energy).
  • Photovoltaic Thermal Control System (PVTCS) - circulates cooling fluid to remove heat and maintain EPS battery temperature
  • Solar (Array) Alpha Rotary Joint - a joint which allows for rotation of either entire arm of solar array
  • Beta Gimbals - objects used to rotate the individual solar arrays so they face the Sun to provide maximum power
  • Main Bus Switching Units (MBSU) - route power to correct locations in the ISS
  • Direct Current Switching Unit (DCSU) - routes power from the solar arrays to the MBSUs
  • Remote Power Controllers (RPCs) - individual switches which control the flow of electric power to users (such as how a light switch controls a light bulb)
  • Electronics Control Unit (ECU) - controls pointing of solar arrays
Environmental Control and Life Support System Group (ECLSS)

On Earth, nature provides the air we breathe, the water we drink, and other conditions that make our environment livable. In space, these functions must be performed by artificial means. These systems are known as the Environmental Control and Life Support System (ECLSS).

ECLSS on the ISS provides the following functions:

  • Maintains a pressurized, breathable atmosphere
  • Controls temperature and humidity
  • Provides and recycles drinking water
  • Responds to ISS environmental emergencies (such as fire outbreak)


Motion Control System (MCS)

The International Space Station (ISS) is a large, free-flying vehicle. The attitude (or orientation) of the ISS in reference to Earth and the Sun must be controlled in order to maintain power, proper temperatures, microgravity levels, and communication. The Motion Control System is responsible for maintaining the correct attitude and orbit of the ISS. The system is composed of the U.S. and Russian Guidance, Navigation, and Control (GNC) Subsystems.

The primary functions for the system are:

  • State Determination - answers the question, "Where am I?" It uses Global Positioning System (GPS) satellites to determine where the ISS is in space.
  • Attitude Determination - answers the question, "How am I oriented?"
  • Pointing & Support (P&S) - answers the question, "Where is everything else?" (communication satellites, Sun, etc). This information is used to calculate which angles to position the communication antenna, solar arrays, radiator panels, etc.
  • Attitude Control - four large gyroscopes are used to maintain, and often change, the orientation of the ISS in its orbit. Propulsive thrusters are also used at times for this purpose.
  • Translational Control (reboost) - the ISS maintains its altitude by performing maneuvers, or reboosts, monthly to offset orbital decay. It uses rocket thrusters to perform these reboosts.
  • Docking - helps position the ISS so a visiting vehicle can connect.


Structures & Mechanisms System (SMS)

The Structures and Mechanisms System (SMS) is responsible for the parts that hold the ISS together. Without these structures and mechanisms (S&M), the ISS would just be randomly floating space junk. Structures and mechanisms are made up of the following:


  • Pressurized Elements - habitable modules where the crew lives. These are the cylindrical, soda can shaped parts of the ISS.
  • Truss Structures - are the "backbone" of the entire ISS. They provide places for solar arrays, thermal control radiators, and external payloads to attach. They contain electrical and cooling utility lines, as well as the mobile transporter rails.


  • Androgynous Peripheral Attach System (APAS) - used for docking with the Space Shuttle.
  • Probe/Drogue - used to dock Progress and Soyuz Vehicles (visiting vehicles), connects modules.
  • Common Berthing Mechanism (CBM) - Connects U.S. modules together.
  • Rocketdyne Truss Attachment System (RTAS) - connects integrated truss segments together.
  • Common Attach System (CAS) - Connects exposed research equipment and logistics carriers to the integrated truss segments.


Thermal Control System (TCS)

The Thermal Control System (TCS) keeps the space station's equipment and research cargo at their required temperatures. A large amount of heat is generated onboard the ISS by the crew, equipment, and even the heat from the sun. In addition, certain surfaces of the ISS must be kept at proper temperatures to prevent equipment from becoming too hot or too cold. Moisture from the air inside of the ISS needs be removed to prevent condensation (water droplets) from forming that can cause damage to equipment. The TCS is divided into two subsystems known as the Passive Thermal Control System (PTCS) and the Active Thermal Control System (ATCS).

The PTCS uses insulation, special paints, heaters, and heat pipes in the areas of the ISS where temperature cannot be easily controlled by water or ammonia cooling systems. The ATCS is used when the amount of heat that needs to be removed is more than the PTCS can handle. The ATCS pumps fluids in closed-loop system to collect, transport, and release heat from the space station, in the same way that an air conditioner pumps cool fluid into a home and blows air across it to cool the air.

There are two ATCS systems: internal and external. The internal ATCS system controls temperatures and moisture levels inside the habitable modules. The internal ATCS circulates water to transport heat and cool equipment. The internal ATCS removes heat and humidity (water) from the cabin air. This air is then used to cool the crew and electrical equipment. The water removed is circulated back to the life support system for processing. The internal ATCS is the responsibility of the ETHOS officer.

The external ATCS system controls the temperatures outside of the habitable modules. The external ATCS circulates a refrigerant (similar to a home air conditioning unit) to transport heat and cool equipment. In the case of the ISS, the refrigerant is ammonia. However, unlike a home air conditioner, there is no air around the ISS for the heat to be released into. Instead, the ISS must utilize radiators to remove excess heat from the external ATCS.