223 SYSTEMS: COMMUNICATIONS AND TRACKING—THE VITAL LINK TO THE INTERNATIONAL SPACE STATION CHAPTER 13 Radio Frequency Communication Systems Conventional communication systems dating back to the first spacecraft—Sputnik, launched by the Russians in 1957—use RF links. Radio waves of varying frequencies are the same as those used by car radios, televisions, and cell phones. They are generally ideal for communicating with spacecraft because they tend to use minimal power, and the longer wavelengths can easily penetrate the Earth’s atmosphere. But a trade-off exists between frequency and the amount of information the signal can contain. Higher-frequency radio waves can carry more information but are more susceptible to signal degradation as the waves travel through the atmosphere. For example, satellite TV systems operate in the Ku-band the radio waves are susceptible to heavy rain showers when the signal might be lost temporarily. The ISS uses different systems in different bands, depending on the specific need. Figure 1 depicts the electromagnets spectrum with the key bands used by the ISS. Maritime Navigation Signals 3 KHz 1 L S C K Ku X Ka 2 4 8 12 18 26 GHz40 30 KHz 300 KHz 3 MHz 30 MHz 300 MHz 3 GHz 1-40 GHz 30 GHz 300 GHz Navigation Aids (e.g. loran-C) AM Maritime Radio Shortwave Radio, Radiotelephohony VHF TV, FM Radio, Navigation Aids UHF TV, Cell Phones, GPS Satellite/ Microwave Telecommunications Radio Astronomy, Radar Landing Systems Very Low Frequency Low Frequency Medium Frequency High Frequency Very High Frequency Ultra High Frequency Super High Frequency Extra High Frequency X-ray and Visible Light Increasing Wavelength Satellite Frequency Increasing Frequency Figure 1. The RF spectrum used by ISS communication systems. By convention, the electromagnetic spectrum has been divided into specific bands. Several bands were delineated by their frequency range—high, very high, ultra, etc.—but several bands were also classified into smaller subdivisions and indicated by letters of the alphabet. The main communication path relies on the NASA Tracking and Data Relay Satellite (TDRS) system, whereas the VHF link provides a direct radio transmission between a ground station and the ISS. The TDRS system consists of a number of satellites in geosynchronous orbit about the Earth that provide a relay link for the S- and Ku-bands between ground stations and the space station. With the satellites being distributed around the planet, the ISS can be in continuous communication with Mission Control. S-band The term S-band refers a specific range of radio frequencies at which the communication systems operate. The S-band range is a subset of the super-high-frequency range specifically, 2 to 4 gigahertz. This type of RF communication system is the most common on the ISS. The S-band communication system is comprised of three boxes referred to as Orbital Replacement Units: the Baseband Signal Processor, the Transponder, and the Radio Frequency Group (RFG). See Figure 2. The Orbital Replacement Units are mounted outside the ISS. One set is mounted on the P1 truss segment and the other set is mounted on the S1 truss segment. Only one set is used at a given time, and the two sets are alternated.