Military Satellite Communications Fundamentals
Communication satellites can be passive or active. Passive satellites only reflect signals. The limited power transmitted from the ground (the uplink), the quality of the reflection, and the losses along the path both up and down can all weaken the returned signal (the downlink).
Active satellites, on the other hand, can repeat or process signals. After receiving uplinked radio signals, a repeater (or transponder) amplifies them, converts them to a different carrier frequency, and retransmits them. The resulting signal is much stronger than one that is passively reflected; however, because the transponder retransmits everything it receives, some of the retransmission will be uplink noise or interference. A processing satellite receives and demodulates the uplinked radio signal, potentially reformats the result and then remodulates the information onto a new, downlink radio signal. The process of demodulation removes most uplink noise and interference before transmission on the downlink, which further improves the signal quality.
Many useful orbits exist for communications satellites: for example, geosynchronous Earth orbit (GEO) at an altitude of 19,300 nautical miles, medium Earth orbit (MEO) from 500 to 12,000 nautical miles, and low Earth orbit (LEO) at approximately 200 nautical miles. When a satellite orbits at less than GEO altitude, its orbital period is less than Earth’s period of rotation, so it moves across the sky. The lower the altitude, the faster the satellite moves. Lower altitudes have advantages and disadvantages. For example, because the satellite is closer, less signal power is lost during transmission and achieving orbit is easier. On the other hand, terminals must be capable of tracking the moving satellite, more satellites must be orbited to obtain a given coverage of Earth, and the satellites can be more vulnerable to certain threats. Other specialized orbits include elliptical, which maximizes the time spent over a nonequatorial location such as the North Pole, and inclined, which indicates an orbit at an angle relative to the equator.
International treaties establish rules and regulations for using the electromagnetic spectrum with portions allocated to specific radio services and designations for space systems. U.S. law incorporates these rules and regulations. One of Aerospace’s roles is to support the U.S. government in the analysis and process for national and international frequency filing, coordination, licensing, and approval for every major military satellite communications system. The International Telecommunications Union oversees this process, which includes assignment of frequency and positions in space for each satellite. Four frequency bands are allocated in the United States for military space use, including UHF (225–400 MHz), X (7–8 GHz), Ka (30–31 GHz up, 20.2–21.2 GHz down) and EHF (43–45 GHz up, 20.2–21.2 GHz down). The S band (1.761–1.842 GHz and 2.20–2.29 GHz) is shared among all government users.
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