CHAPTER 13 SYSTEMS: COMMUNICATIONS AND TRACKING—THE VITAL LINK TO THE INTERNATIONAL SPACE STATION 226 the Ku-band return link rate as science experiments on the ISS require faster and faster return link rates. The ISS has two Ku-band systems only one system is used at a time. The Ku-band system in its current state consists of the Ku Communications Unit (located inside the ISS) and the Transmitter/Receiver/Controller and Space-to-Ground Antenna (both of which are mounted outside the ISS). See Figure 3. Figure 3. STS-132/ISS-ULF4 mission specialist Steve Bowen works to install the backup Space-to-Ground Antenna (the white radio dish near the top left of the figure) on the Z1 truss (located next to the original antenna, which is angled to the right). Astronauts Make the Call . . . From Space! The ability to make a telephone call from the ISS is extremely helpful to the crew members, psychologically, by allowing them to feel more connected to home. Calls may be personal—to a spouse or dear friend—or to the flight director to talk privately about an issue. However, this process is not as straightforward as making terrestrial phone calls. In particular, the crew can conduct a call only when the Ku-band satellite link is present. The calls must be initiated by the crew for privacy and security reasons. Receiving a phone call from space is a fun and unique experience. However, cases have occurred where recipients were incredulous about incoming calls from the ISS and would hang up on the caller. Also, as with phone calls on Earth, the wrong number can be dialed, which has happened on occasion. This system is also useful for the ground team. On more than one occasion, the crew was required to fix a problem with the S-band radio link. Because of the ground team’s inability to call the crew over the S-band radio link to request a procedure, the crew’s timeline was re-uplinked via the Ku-band system with a big banner-type message that instructed the crew to initiate an internet protocol call to Mission Control. The Ku-band system is used to transmit all video—internal and external—from the ISS. Live and recorded video are transmitted in high definition and standard definition. High definition has become the preferred format as the available bandwidth from the ISS has increased. Two-way video