CHAPTER 5 SYSTEMS: COMMAND AND DATA HANDLING—THE BRAINS OF THE INTERNATIONAL SPACE STATION 104 Caution and Warning Figure 6. The C&W panel. Each button will illuminate red or yellow as appropriate to indicate the presence of an alarm. The crew will push the button to either manually initiate an alarm, or to silence the tones. A glass cover protects against crew members accidentally bumping one of the emergency alarms. Figure 7. The C&W Summary used by the crew and flight control team in mission control. Events that are actively in alarm are indicated with yellow or red, depending on the class. Less-critical advisories are shown in white. Text describing the alarm is shown in the middle, followed by the time the alarm annunciated (in Greenwich Mean Time). The bottom displays a log of events and history (e.g., in alarm, normal). One of the most critical functions of the C&DH system is that of C&W. Alarms, or C&Ws, come in four classes. The most dangerous problems for a crew are fire, rapid depression of air, and toxicity in the atmosphere (i.e., from a leak in the ammonia cooling system, failed environmental equipment, or a spilled experiment). These are class 1 alarms (emergencies). Class 2 alarms (warnings) indicate that the crew or ground needs to take immediate action to avoid injury or death of the crew or damage to the ISS. Emergencies and warnings are indicated by red on displays. Class 3 alarms (cautions) are indicated by yellow and do not require immediate response by the crew or ground however, if left uncorrected, such situations could develop into a warning-class event. The lowest level of alarms (advisories) indicate something is wrong that does not require immediate attention. These are more akin to the “check oil” light on a car. A special subset of advisories is the robotic advisories, which provide alerts for the robotic systems only. The number of alarms include approximately 80 emergencies, 800 warnings, 2300 cautions, and 6100 advisories. The majority of alarms indicate a failure of a redundant component, thereby posing no immediate threat. Failures are detected by an MDM in the chain and fed to the C&C MDM, which, in turn, determines the level of the alarm and routes it to audio speakers, light panels, and the PCS to alert the crew and ground. Most modules contain speaker systems for annunciating an audible alarm, much like a fire alarm in public buildings. Each class, except advisories, has a distinctive frequency to allow the crew to differentiate between alarms. Advisories do not produce an audible alarm. Distinctive frequencies are required because one anomaly (e.g., a fire) could produce multiple class failures and the crew needs to know quickly which is the most critical.