Troposcatter communications systems emerged during the 1950s, during the period of intensive strategic competition between the NATO nations and the Warsaw Pact, prior to the advent of satellite communications. Such systems were extensively deployed by the US and the Soviets, and to a lesser extent by their respective allies, to provide C3 channels typically in sparsely populated areas. The Soviets deployed an extensive network of troposcatter relays through northern Siberia and the Far East, while the US deployed an extensive network along the DEW lines, and through Alaska and the Aleutians. These fixed networks were later supplemented by mobile tactical systems, intended to provide digital trunk communications for manoeuvre land force elements.

In technological terms, troposcatter communications are an offshoot of early radar technology, and such systems were constructed using elements of the same technology base, but also exploiting initially technology used in period analogue telephony, and later digital telephony.

The physics underpinning troposcatter communications are most interesting, and remain the subject of academic research despite the decline of troposcatter networks, mostly replaced by satellite communications links.

In a conventional microwave relay communications network, a transmitter must have a direct line of sight to a receiver. Modulated radio-frequency power emitted by the transmitting antenna propagates, with diminishing amplitude following the inverse square law Friis equation, until it impinges on the receiving antenna, where it produces an electrical signal which is amplified and demodulated by the receiver equipment. While refraction due to the gradient in atmospheric refractivity (due to density lapse rate with increasing altitude) can “bend the beam”  over the horizon slightly, typically this cannot increase range beyond the line of sight (BLOS) to any significant extent.

In a troposcatter system, the beam is bounced off the upper troposphere, providing a true BLOS point to point communications capability. These systems rely on the  irregularity of the refractivity gradient at such altitudes, resulting in impinging microwave power being scattered forward in an irregular fashion.

In terms of achievable range performance, smaller troposcatter systems are able to repeatably achieve 100 - 150 km ranges between a pair of stations. Larger systems, with 10+ metre antenna diameters and kiloWatt class transmit powers levels, have been reported with ranges of up to 400 km between a pair of stations. It is this range performance which has underpinned the popularity of troposcatter technology for use in undeveloped or underdeveloped regions, as it permits operation of a microwave channel in terrain where the cost of both deploying and maintaining a conventional microwave relay would be prohibitive. In land warfare contingencies, troposcatter stations deployed by an advancing manoeuvre force permit the maintenance of a chain of relays back to the initial staging area, providing voice and data connectivity, with no dependency on airborne or satellite relays.

Achievable channel capacity and thus data rates for troposcatter systems are quite poor in comparison with direct line of sight (LOS) microwave systems operating in the same bands, such as links between aircraft and ground stations. This is for two basic reasons. The first is because the forward scattered power levels are relatively low, compared to inverse square law power levels across the same pathlength in a direct LOS link. The second reason is a byproduct of the irregularities in the scattering mechanisms, and variations in pathlength arising from the propagation path, both resulting in a strongly dispersive propagation medium. As with all dispersive media, this impairs achievable bandwidth for most conventional signal modulation schemes. Where performance data has been published for troposcatter systems, there is a pronounced reduction in achievable data rates with increasing link range, due to the cumulative effects of dispersion and declining power at the receiver. Multipath fading effects due to ground bounce at the receiver will also not contribute to achievable channel capacity.

A range of design techniques have been adopted since the introduction of this technology, to reduce the adverse impact of the medium. These include frequency diversity, including the use of adaptive techniques, but also more recently modulation techniques which are better suited to dispersive media have been used, in addition to robust Forward Error Control (FEC) techniques. Recent research suggests that newer modulation techniques, including Coherent Orthogonal Frequency Division Multiplexing (COFDM/OFDM), which is highly resistant to multipath effects, are well suited to such an application (Hu et al).

Current state-of-the-art US equipment provides data rates of 8 - 22 Megabits/s at unspecified ranges, operating in the 1.7 - 2.3 GHz and 4.4 - 5 GHz bands, and using conventional Quadrature Phase Shift Keying (QPSK) and FEC techniques, with quad frequency diversity. An example is the US designed and built Comtech troposcatter component of the British Cormorant battlefield network, cited at a data rate of 8 Megabits/s to a range of up to 300 km.

While troposcatter systems no longer occupy the prominent position they did during the early Cold War period, being used primarily in niche applications, this technology may yet see a revival as orbital and spectral congestion place increasing constraints on satellite systems. With Moore's Law providing ever cheaper computational power over time, the cost penalties of using modulation and coding techniques capable of overcoming highly dispersive and time variant propagation media will decline. Whether this results in a major renaissance for troposcatter remains to be seen.

AN/TRC-170 being deployed (US DoD).



AN/TRC-170 (US DoD).