229 SYSTEMS: COMMUNICATIONS AND TRACKING—THE VITAL LINK TO THE INTERNATIONAL SPACE STATION CHAPTER 13 Fourteen ATUs are located throughout the United States On-orbit Segment (USOS) (Figure 7). The audio system also connects to the S-band and Ku-band systems to give the crew members access to the space-to- ground voice loops. Three alarm tones (emergency, warning, and caution) signal that something serious has occurred and the crew members are to respond (see Chapter 5). These alarm tones are also annunciated through the audio system. Figure 6. Three ATUs (red arrow) are located at the top, above the spacesuit, in the airlock. During preparation for a spacewalk, the suited crew members can plug into these units to communicate with each other and the rest of the crew or to the ground without using the battery-powered UHF system in the suit. Or, a crew member who is assisting the spacewalkers can use the ATU as a normal intercom. Node 3 Aft Lab Aft Lab Forward Airlock Forward Node 2 Port Node 2 Starboard Columbus Aft Columbus Starboard JPM Aft JPM Forward Node 3 Starboard Cupola Figure 7. Location of the ATUs throughout the USOS. In addition to providing communication between crew and ground, the ATUs provide caution and warning alarms. The audio system is a Time Division Multiple Access digital system. As many as five conference calls can be supported simultaneously with up to 12 ATUs connected to each conference. An ATU can be, and usually is, connected to more than one conference at a time. The astronaut is able to talk to anyone on the ISS or the ground that is tied into that conference by simply pushing a button on the ATU for the appropriate conference call. In a typical operational configuration, the first conference, known as Public Call 1, will include the Russian Segment and is used to talk to all crew members throughout the ISS during an emergency. The Russian Mission Control Center uses Public Call 1 for daily operations with the cosmonauts. The Americans, Europeans, and Japanese use two other conferences, known as Public Calls 2 and 3, for day-to-day systems operations and for payload (science) operations. The fourth conference, known as Public Call 4, usually includes only the USOS modules and is used by American, European, and Japanese control centers for private communications (e.g., when a crew member is conducting a planned, weekly conference with a flight surgeon or family member).