227 SYSTEMS: COMMUNICATIONS AND TRACKING—THE VITAL LINK TO THE INTERNATIONAL SPACE STATION CHAPTER 13 conferencing is supported and used routinely. The Ku-band system was upgraded to support a pair of two- way voice communication channels. Operationally, these channels are called S/G 3 and S/G 4. They augment the S/G 1 and S/G 2 channels in the S-band system. The ISS crew members can use the Ku-band system for email, internet access, and phone calls (i.e., internet protocol calls) to any number on Earth. The biggest use of the Ku-band system are the payloads that conduct scientific experiments on board the ISS. All the scientific data are transmitted to the ground by the Ku-band system. The forward link of the Ku- band system is used to transmit operational planning data, control experiments, and to remote log-in to the ISS Local Area Network. The remote log-in process is needed to conduct operations (e.g., controlling Robonaut) and troubleshoot equipment, and to support email, two-way video conferencing, and telephone calls. The Ku-band system is the communication workhorse of the ISS. Every nation involved in the ISS depends on the system, mainly for their scientific research. The Ku-band system is highly directional. It must be pointed at the TDRS satellite within 0.5 degrees to transmit the high data rates. The GNC MDM and C&C MDM produce the pointing angles for the Space-to- Ground Antenna gimbals at a rate of once per second. The antenna then uses these angles to initiate a search of the forward link signal that is being relayed from the ground by the TDRS satellite. Once locked onto the signal, the antenna uses that signal to automatically track the TDRS satellite within the required accuracy. Figure 4. Mission Specialist Dave Wolf during STS-127/ISS-2 J/A on the P1 truss directly behind the UHF antennae. Ultra-High Frequency The ISS is equipped with a bidirectional UHF communication system, as seen in Figure 4. The UHF systems provides two-way voice communication with the spacewalking astronauts and the ISS. A two-way voice link between the spacewalking astronauts and the flight control team on the ground is established when the audio portion of the UHF system is connected to the S-band system. Data from the astronauts’ suit and