BASIC SATCOM SYSTEM

A satellite communications system relays radio transmissions between Earth terminals. There are two types of communications satellites: active and passive. An active satellite acts as a repeater. It amplifies signals received and then retransmits them back to Earth. This increases the signal strength at the receiving terminal compared to that available from a passive satellite. A passive satellite, on the other hand, merely reflects radio signals back to Earth.

A typical operational link involves an active satellite and two Earth terminals. One terminal transmits to the satellite on the uplink frequency. The satellite amplifies the signal, translates it to the downlink frequency, and then transmits it back to Earth, where the signal is picked up by the receiving terminal. Figure 2-7 illustrates the basic concept of satellite communications with several different Earth terminals.

The basic design of a satellite communications system depends a great deal on the parameters of the satellite orbit. Generally, an orbit is either elliptical or circular. Its inclination is referred to as inclined, polar, or equatorial. A special type of orbit is a synchronous orbit in which the period of the orbit is the same as that of the Earth's.

Two basic components make up a satellite communications system. The first is an installed communications receiver and transmitter. The second is two Earth terminals equipped to transmit and receive signals from the satellite. The design of the overall system determines the complexity of the components and the manner in which the system operates.

The U.S. Navy UHF/SHF/EHF combined communications solution allows each system to provide unique contributions to the overall naval communications needs.

The SHF spectrum is a highly desirable SATCOM medium because it possesses characteristics absent in lower frequency bands: wide operating bandwidth, narrow uplink beamwidth, low susceptibility to scintillation, anti-jam (AJ), and high data rates. Recognizing these characteristics, the U.S. Navy developed and installed shipboard SHF terminals. These attributes are discussed in the following paragraphs.

Figure 2-7.-Satellite communications systems.

Wide operating bandwidth permits high information transfer rates and facilitates spread spectrum modulation techniques. Spread spectrum modulation is a particularly valuable technique for lessening the effects of enemy jamming. Although wide bandwidth permits both high information transfer rates and AJ capabilities when using the OM-55(V)/USC modem, it may not permit both simultaneously in the presence of jamming. Therefore, high information transfer rates will be significantly reduced when jamming is encountered, permitting only certain predetermined critical circuits to be maintained.

Narrow uplink transmission beamwidth provides a low probability of intercept (LPI) capability. An uplink LPI capability reduces the threat of detection and subsequent location, but does not in and of itself deny enemy exploitation of those communications if detection is achieved. SHF frequencies are rarely affected by naturally occurring scintillation, making

SHF SATCOM a particularly reliable form of communications.

A characteristic of SHF, favorable to flagships, is the ability to communicate critical C4I for the user information in the presence of enemy jamming and with due regard for enemy detection capabilities. SURTASS Military Sealift Command Auxiliary General Ocean Surveillance (T-AGOS) ships were initially equipped with SHF SATCOM, taking advantage of the high information transfer rate capability and LPI characteristics. Because of larger available bandwidths, inherent jam-resistance, and increasing demands on limited tactical UHF SATCOM resources, additional applications for DSCS SHF SATCOM afloat are continually being investigated for the Fleet.

The radio group consists of a high power amplifier (HPA) or medium power amplifier (MPA), low noise amplifier (LNA), up-converter, down-converter, and frequency standard. For transmit operations, the up-converter translates the modem's 70 or 700 for antenna control and the down-converter for translation to 70 or 700 MHz IF. This signal is then sent to the modem for conversion to digital data. System frequency stability is provided by a cesium or rubidium standard.