Tropospheric scatter and Line of Sight Communications
US Army TRC-170 Tropo Scatter Microwave System
Also known as troposcatter, or simply tropo communications, such systems bounce signals off particles in the troposphere (the atmospheric layer closest to earth). Troposcatter systems were developed before communication satellite systems became available to provide long-distance links. The quality of early troposcatter communications often varied from hour to hour. As troposcatter signal strengths were normally low, they required high-power, high-gain antennas and sensitive receivers. However, their advantages—a cost-effective communications system for data and voice over medium and long distances—long outweighed its disadvantages.
In the 1950s and early 1960s, troposcatter systems were used for communications systems operating over longer distances. The Pinetree Line, which became operational in 1952, was the first of three communication lines supporting air defense radar sites constructed across Canada. Tropo links connected the forty-four Canadian and six U.S. Air Force manned radar sites. The WHITE ALICE Communications System was built to connect the various aircraft control and warning radars located in Alaska. WHITE ALICE attained full operational capability on 26 March 1958. The system would eventually be expanded to connect the Distant Early Warning Line and Ballistic Missile Early Warning System line of Canadian radar stations. The systems provided communications with a reliability of more than 99 percent in the harsh Arctic conditions.
The North Atlantic Treaty Organization used troposcatter for its ACE High system as early as 1956. The ACE High system consisted of forty-nine troposcatter links and forty line-of-sight microwave links extending from Norway to Turkey. In the mid- 1980s, troposcatter systems were used to extend the long-haul capability of the Joint Tri-Service Tactical Communications Program network. While advances in satellite communications in the 1980s and 1990s seemed to portend the end of troposcatter systems, ever-increasing demand for more communications capacity exceeded the capabilities of the satellite systems. Consequently, tropospheric scatter systems continued to be used for tactical communications systems.
During Operation Desert Storm in 1991, troposcatter systems provided the long-haul communications backbone for one of the longest tactical communications networks ever established. During the 1990s, new troposcatter systems were fielded by the U.S. Army, Air Force, and Marine Corps. During the Iraq War (begun in 2003), troposcatter systems again provided the essential long-haul communications.
Troposcatter systems offer reliable, cost-effective, and high-capacity (up to 4 mps) long-haul communications without relying on availability of satellite channels. The systems are utilized not only by a number of the world’s armed forces, including China and India, but also by commercial entities that require reliable long-haul communications.