CHAPTER 13 SYSTEMS: COMMUNICATIONS AND TRACKING—THE VITAL LINK TO THE INTERNATIONAL SPACE STATION 224 Keeping the Lines of Communication Open The official name of the ISS S-band communication system is the Assembly Contingency Subsystem. Originally, the S-band system was intended to be used only during the ISS assembly sequence and, after that, only in contingency cases. The Ku-band system was intended to be the primary communication link for the ISS. To defer costs, the build-up of the Ku-band system was extended over more years than originally planned. The S-band system continues to operate as the prime communication system for the ISS. The Ku-band system is used primarily to support ISS utilization. The S-band system is used to send commands from control centers around the world to the ISS. Because of this, the system is designated with the highest level of criticality. The second S-band system is activated and configured as a “hot” backup for critical operations where an unexpected failure of the S-band system would threaten the safety of the ISS crew members, spacecraft, or mission objectives. Upon a failure of the primary system, the communication link is recovered in a minimal amount of time (~40 seconds). The S-band system is also the primary means of sending telemetry data from the ISS to the ground. The telemetry data are sent to the ground in real time to support operations. These data can be stored on virtual recorders on board the ISS and sent down later. Another critical function of the S-band system is to provide two-way voice communication with the ISS crew members and the control centers around the world. The S-band system has two separate voice channels referred to in operations as Space-to- Ground (S/G) 1 and S/G 2. Although interchangeable, the channels are allocated specific uses that are defined in operations policies. Software files that can be used by the on-board computers and records containing data about the ISS can be exchanged between the vehicle and the ground using the S-band system. Almost 300,000 individual pieces of data (e.g., temperature, energy level of a battery) have been designated for possible radioing to the ground. However, this represents only a subset of the data available on the ISS. About 10% of those values can be sent down via S-band every 10 seconds. Sometimes, the flight controllers need to look at these additional data. Therefore, bulk quantities of unprocessed data can be retrieved directly from memory in the on-board computers. This data retrieval is useful in troubleshooting a malfunctioning piece of equipment or misbehaving computer software without exhausting the available bandwidth all the time. The S-band system is capable of being operated in two modes: a high-data-rate (HDR) mode and a low-data-rate (LDR) mode. The HDR mode consists of a forward link (i.e., transmission to the ISS) of 72 kilobits per second (kbps) and a return link (i.e., transmission from the ISS) of 192 kbps, and this mode simultaneously supports a pair of two-way voice channels (S/G 1 and S/G 2). The term “high” is relative as, these days, most people carry RF communication devices that communicate at speeds significantly faster. For example, depending on the city, carrier, and plan, one can expect smartphone download speeds of at least approximately 3 to 10 megabits per second (mbps)—about 15 to 52 times faster than the ISS S-band system. The LDR mode consists of a forward link of 18 kbps and a return link of 24 kbps, and supports one two-way voice channel (S/G 1). This is slower than an old dial-up modem, which had speeds of 56 kbps. Originally, link speeds were 6 kbps and 18 kbps, respectively, with no voice channels. Later, the S-band system was upgraded, primarily to add one two-way voice channel to the LDR mode. HDR is the primary operation mode. This mode requires the use of the high-gain antenna (HGA) located in the RF group. The HGA is a steerable antenna and needs to be pointed precisely at the TDRS being used to relay the signal from the ISS to the ground. The data used to point the HGA originate in the Guidance, Navigation, and Control (GNC) Multiplexer/DeMultiplexer (MDM) and are sent to the Command and Control (C&C) MDM where they are converted to commands for the RFG (see Chapters 5 and 7). The commands are automatically sent to the RFG once per second to keep the HGA pointed at the correct satellite as the ISS orbits the Earth. If computers are not able to point the HGA due to a failure of its gimbal motors or a failure preventing the pointing data to be generated or sent to the RFG, then the flight control team will use the LDR mode. The