93 SYSTEMS: COMMAND AND DATA HANDLING—THE BRAINS OF THE INTERNATIONAL SPACE STATION CHAPTER 5 “command” while moving, or handling, information around the vehicle. In essence, C&DH would be familiar to the average person as the computer network. A command is a set of instructions telling a computer to perform an action. For example, the print command is common among computer users on Earth. Spacecraft employ a variety of C&DH systems, depending on the functional need of the vehicle and its architecture. Complicating matters further, the ISS C&DH system—the largest such system ever operated in space—is in fact an amalgam of computer networks developed in multiple countries. Each segment, Russian and United States On- orbit Segment (USOS), has its own architecture bridged by Node 1, aptly named Unity. The USOS is split into the American, European, and Japanese modules, each with its own computer network. As part of its function, the C&DH system will detect failures—whether they involve a piece of hardware such as a valve or are in the computer system itself—and alert the crew and ground via alarm. Astronauts use a laptop called the Portable Computer System (PCS) to interface with the C&DH system. Using this laptop, they can run procedures that operate the vast majority of ISS systems, although the ground tries to perform most procedures to free up the crew for research. Due to its important role in operating the spacecraft, the C&DH system (including the PCS) is classified as critical and therefore requires robust hardware or redundant components to ensure proper operation as well as exhaustive software testing. Astronauts also have a separate laptop called the Station Support Computer (SSC) upon which they can view the timeline and read procedures. SSCs are not linked to the C&DH system because the SSCs provide only a support role (i.e., perform no critical function that would impact operating the ISS if an SSC failed), they are not considered critical. The SSC laptops connect to a Local Area Network (LAN) that would be familiar to anyone using laptops on a network. Payloads are generally controlled from the SSC. Through the SSC, astronauts can read email, access the internet, and use an Internet Protocol (IP) phone. Initially, the Onboard Data Interfaces and Network (ODIN) officer operated the ISS C&DH system. Later, the ODIN function merged with the Communications and Tracking Officer function to form the Communications Radio Frequency Onboard Network Utilization Specialist (CRONUS) position. ODIN was supported in the back room by a resource avionics engineer. Although the SSCs are computers, they are not part of the C&DH system. Rather, the SSCs are handled by the PLug- in-plan and UTilization Officer flight control position (see Introduction)— also known as PLUTO—and are not discussed here. Overview The USOS C&DH system consists primarily of 46 nearly identical computers networked into a top- down tiered structure, as illustrated in Figure 1. At the top of the pyramid is the Command and Control System (CCS), a triply redundant set of computers located in the Laboratory Module that act as the brains of the USOS. Only one computer controls the system at a given time if that computer fails, the second or third will take over operations. Due to the critical role of the CCS, three computers are required to ensure that multiple failures would not disrupt the control of the vehicle. The ground directly interfaces with the CCS via uplinked commands through the Communication and Tracking (C&T) system (see Chapter 13), whereas the crew can interact using a PCS. The CCS interfaces directly with the top-level computer on the Russian Segment known as the Service Module Central Computer (SMCC), as well as the computers in the European and Japanese modules. The local tier (Tier 2) is located below the control tier. Computers at the local level control most spacecraft functions as well as the partner modules. Computers inside the ISS control such functions as the regenerative life support systems, ventilation, and temperature control while those on the exterior control heat rejection and the giant solar arrays. Another Tier 2 function is that of Guidance, Navigation, and Control (GNC), which drives the Control Moment Gyros while calculating the trajectory of the ISS using Global Positioning Satellite (GPS) sensors. Although still important, the Tier 2 computers are not as critical as the CCS and, therefore, have only a single backup in case of failure. At the bottom of the triangle is the user tier, indicated as Tier 3. This tier is responsible for control of all sensors and end effectors that are wired to computer cards within the