225 SYSTEMS: COMMUNICATIONS AND TRACKING—THE VITAL LINK TO THE INTERNATIONAL SPACE STATION CHAPTER 13 LDR mode uses a hemispherical antenna. This antenna is not steerable and therefore does not depend on the GNC or C&C MDMs. However, its field of view is limited therefore, the amount of time this antenna is in a line of sight of a TDRS satellite is also limited. For this reason, and because of the slower data rates, the LDR mode is used for contingency purposes only. A total of four operating modes are possible with HDR and LDR modes on each of the two strings of S-band. The modes can be changed manually by the flight control team on the ground or by crew members on the ISS. Generally, only one mode is used: HDR on one of the strings of S-band. The other modes and string act as a backup configuration. Fault detection, isolation, and recovery software, which can detect whether a component fails and then reconfigure to a backup unit, can also change the mode. Figure 2. The S-band RFG on the S1 truss segment, with the HGA and LGA highlighted. Ku-band The Ku-band system is officially known as the Space-To-Ground Subsystem and operates at a forward link frequency of 13.7 MHz and a return link frequency of 15.0 MHz. This system was designed for and intended to be the prime communication system for the ISS precursor: Space Station Freedom. The Ku-band system was redesigned when the plans for Space Station Freedom changed to the ISS (see Introduction). The redesign was needed to accommodate the higher radiation environment encountered at the orbit in which the ISS would fly (see Introduction). In the beginning of ISS operations, the forward link operated at 3 mbps and the return link operated at 50 mbps. Although these data transfer rates are significantly higher than the S-band capacity, there was a growing need to have more bandwidth, especially to accommodate some of the planned payloads on the ISS. Upgrades that were made to ground systems allowed the downlink rate to increase to 150 mbps and the uplink rate to 6 mbps. In 2007, NASA completed a major upgrade of the Ku-band system that boosted the forward link rate to 25 mbps and the return link rate to 300 mbps. NASA engineers are exploring options to further increase