Figure 2-7.—Satellite communications systems

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 2-8