CHAPTER 13 SYSTEMS: COMMUNICATIONS AND TRACKING—THE VITAL LINK TO THE INTERNATIONAL SPACE STATION 228 biomedical data from the astronauts are transmitted to the ISS on the UHF system and then sent to the ground via the S-band system. The UHF system is only meant to be used around the vicinity of the ISS. The system cannot communicate directly with UHF stations on the ground because of the way the data are incorporated into the radio signal. Wireless External Transceiver Assembly Another radio frequency communication system— the Wireless External Transceiver Assembly—is associated with US spacewalks (Figure 5). The more common name for this system is the helmet cam video. Through a combination of a UHF link and an S-band link, the cameras that are mounted to the spacewalkers’ helmets provide real-time video back to the ISS. This video can be transmitted to the ground via the Ku-band system. The UHF link is used to issue commands to the three cameras that are mounted on the helmet. The S-band link is used to transmit the live video from one camera at a time to antennae mounted on the ISS. Figure 5. Mission Specialists Michael E. Lopez-Alegria (left—red stripes) and John B. Herrington (right—no stripes) perform Starboard Keel Pin operations on the P1 truss during STS-113/ISS-11A. The Wireless External Transceiver Assembly antenna is in view in the upper left (red arrow), above the Crew and Equipment Translation Cart. Ship-to-Ship Communication Systems All spacecraft that approach the space station communicate directly with the ISS, once in range. Communications between spacecraft is often called “ship-to-ship.” The type of communication system, the capabilities, and the usable range vary from one spacecraft to another even though they are typically in the S-band portion of the spectrum. The Space Shuttle communicated with the space station via a UHF communication system that supported limited commanding, limited telemetry exchange, and two-way voice between the commanders of the two vehicles. The uncrewed cargo vehicles that resupply the space station (see Chapter 14) also communicate directly with the ISS, when in range. Incoming vehicles receive navigation information from the ISS that may be used to perform a rendezvous using Global Positioning Satellite data for each vehicle. Uncrewed cargo vehicles also accept crew-issued commands over the ship-to-ship communication system (e.g., abort and retreat) and provide telemetry on their key systems back to the ISS for the crew to monitor. These RF ship-to-ship communication systems are typically active only during the approach of the visiting vehicle. After a successful capture or docking, the system is deactivated. The operation of the system is the joint responsibility of Mission Control and the control center associated with the visiting vehicle (see Introduction). Baseband Communication Systems Audio An audio system on board the ISS allows crew members to talk to each other across modules and talk to the control centers on the ground. A key component of the audio system is the Audio Terminal Units (ATUs), shown in Figure 6. The ATU is essentially a complicated intercom system. The microphone converts speech-to-digital signals that the audio hardware then routes to any other communication panel on the ISS. A speaker reverses the digital-to-speech conversion.