The dominant mode of communication today uses antennas to transmit information carrying high frequency signals using antennas which at the receive site produce an electrical signal which is amplified and demodulated by the receiver equipment. However this form of communication is limited by line of sight propagation limiting the range to slightly more than the line of sight.
Tropospheric scatter (also known as troposcatter) is a method of communicating with microwave radio signals over considerable distances – often up to 300 kilometres (190 mi), and further depending on terrain and climate factors. This method of propagation uses the tropospheric scatter phenomenon, where radio waves at UHF and SHF frequencies are randomly scattered as they pass through the upper layers of the troposphere. Radio signals are transmitted in a narrow beam aimed just above the horizon in the direction of the receiver station. As the signals pass through the troposphere, some of the energy is scattered back toward the Earth, allowing the receiver station to pick up the signal.
Troposcatter systems are ideal for crisis response. Responders facing a lack of reliable communications after a natural disaster can deploy a troposcatter system to quickly establish networks even when no power is available. Raytheon IIS vice-president Todd Probert said: “Think about any natural disaster over the last several years; one of the biggest problems is a lack of reliable communications. “A troposcatter system easily can be deployed anywhere in the world and can transfer data at extremely high rates, the equivalent to streaming ten high-definition videos simultaneously at a range of more than 115 miles.”
Military is interested in Troposcatter communucations as it delivers beyond-line-of-sight communications up to a distance of about 200 miles, providing secure, reliable wireless data networks with 100Mbps data transmission speeds almost anywhere in the world, regardless of terrain or operational condition. Raytheon has secured a ten-year contract worth up to $663m to deliver troposcatter communication systems to the US Army. Under the indefinite-delivery indefinite-quantity contract, Raytheon will ensure troops have access to secure voice and data communications in contested environments.
“Not only does troposcatter save the service money, it also serves a very important strategic function,” Vigil said. “It offers a bigger pipe for data—more than satellites supply—as well as less latency. In combat, milliseconds count.” Troposcatter fits an interesting niche,” said Cedric Vigil, Raytheon’s troposcatter program manager. “Tropo doesn’t replace radios, fiber or satellite communications. It’s another tool for network planners to use when the conditions warrant it.”
Raytheon Intelligence, Information and Services president Dave Wajsgras said: “Soldiers on the front lines can’t afford dropped calls. Our solution, a secure, reliable and wireless troposcatter system, allows troops to communicate in areas that would otherwise be dead zones. “Importantly, it also gives the military a way to communicate in satellite-denied environments.”
US-based Comtech Systems has received a contract to supply the Swedish Defense Material Administration (FMV) with deployable troposcatter systems for the Swedish Armed Forces (SwAF). The latest $2.2m contract follows the assessment of commercial off-the-shelf (COTS) Comtech troposcatter products. As part of the evaluation, rigorous field-testing was carried out in various operational environments, including high northern latitudes.
Under the terms of the contract, the company will supply MTTS deployable troposcatter terminals customised to meet SwAF requirements. MTTS uses Comtech’s CS67500 50Mb/s troposcatter modem, CS4400 frequency converter system, and 500W solid state power amplifiers (SSPA).Comtech Telecommunication’s president and CEO Fred Kornberg said: “This contract highlights the growing need of militaries around the world to augment their existing communications toolkits with high-bandwidth, long-range alternatives to satellite communications in order to enable critical communication links under a wide variety of operational scenarios.
Earlier Raytheon tested its satellite communications system terminals and demonstrated that it can also carry out long-range “troposcatter” communications without a satellite relay. Raytheon has also utilized its GaN technology to troposcatter communication systems to increase its range while reducing the size of terminals.
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.
Frequencies of transmission around 2 GHz are best suited for tropospheric scatter systems as at this frequency the wavelength of the signal interacts well with the moist, turbulent areas of the troposphere, improving signal to noise ratios.
Troposcatter communications typically are secure because their radio waves are difficult for enemies to intercept, hence their interest for military communications. Among the companies that have been involved in troposcatter communications are General Dynamics, Raytheon Co., Microwave Radio Communications (MRC), Lockheed Martin Corp., ITT Exelis, and Comtech
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.
“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. 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.
US Army requirement of troposcatter communications
U.S. Army researchers are also looking at tropospheric scatter (troposcatter) communication technologies for fixed-site and on-the-move long-range military communications as an alternative to satellite communications (SATCOM). Army officials issued the RFI on behalf of the Army Communications Electronics Research & Development Engineering Center (CERDEC), Space & Terrestrial Communications Directorate (S&TCD) at Aberdeen Proving Ground.
“Army researchers are interested in troposcatter communications technology for worldwide operation that is reliable in a wide variety of environmental conditions, reduces or eliminates the need for communications relay sites, works in rugged terrain, automatically adjusts data rates due to atmospheric changes, communicates IP voice, data messaging, and multimedia services simultaneously over ranges of at least 25 miles, has an easy-to-use graphic user interface, and that is small, lightweight, and man-portable,” writes John Keller.
US navy requirement of Ship based Troposcatter
US Navy has also published its requirement of Developing troposcatter control algorithms and control software that can compensate for ship motion and can overcome the communications limitations imposed by Anti-Access Area Denial (A2AD) environments.
Anti-Access Area Denial (A2AD) environments impose significant communications threats from traditional jamming interferers to kinetic attacks on any communications relay vehicles. The A2AD threats continue to grow significantly each day rendering certain counter-counter measures less effective and, potentially, ineffective in the very near future. A2AD environment threats can be partially overcome via communications systems that do not rely on communication relays; these include Line of Sight (LOS) and Beyond Line of Sight (BLOS). Shipboard LOS communications, however, are generally limited to 15 miles which provides very limited ability to overcome A2AD environments for a vast majority of mission scenarios.
Troposcatter typically utilizes narrow channel beams which provides inherent jam resistance and Low Probability of Detection (LPD). Troposcatter, therefore, can effectively provide communications capabilities for ships in A2AD environments. However, due to the amplitude fluctuation associated with mobile platform dynamics, troposcatter communications have seen limited deployments in Navy ship platforms.
Recent work by Draper Laboratories indicates that the issue of platform dynamics can be addressed. “Comtech has an existing troposcatter solution for relatively low dynamic marine oil and gas platforms to stationary shore site communications. There is no commercial system to the best of PMW/A 170â€™s knowledge that can address two dynamic platforms (ship to ship) troposcatter communications,” writes US navy SBIR.
The navy wants novel troposcatter control algorithm and ship motion compensation software that utilize existing shipboard Commercial Broadband Satellite Program (CBSP) C-band without imposing significant ship alternation is desired. The troposcatter control algorithm should be designed to maintain at least 10 Mbps of throughput with Bit Error Rate of 10^-5 or better. The troposcatter motion compensation software will also need to compensate for ship motion dynamics that range from World Meteorological Organization (WMO) sea state 0 (calm) to 5 (rough).
The US Navy’s Office of Naval Research (ONR) has awarded a contract to TrellisWare Technologies for the construction and testing of a new tropospheric scatter (troposcatter) radio prototype.
In August 2020, The US Navy’s Office of Naval Research (ONR) awarded a contract to TrellisWare Technologies for the construction and testing of a new tropospheric scatter (troposcatter) radio prototype. This prototype is expected to address the communications challenges of the US Navy. The goal of ONR is to use the Beyond Line of Sight (BLoS) troposcatter communications capability to support ship-to-ship and ship-to-shore mobile naval operations rather than satellite communications (SATCOM).
The prototype troposcatter radio of TrellisWare will use new optimised waveforms to maintain reliable and stable convection troposcatter communications, even under difficult and variable propagation conditions. Under a previous ONR contract with TrellisWare, the waveform was developed and tested successfully over-the-air between San Diego and Los Angeles County in 2019. TrellisWare’s troposcatter radio will offer reliable communication over the horizon, and its Size, Weight, and Power (SWaP) are greatly reduced compared to conventional convective-dissipation capabilities.
The combination of stronger waveforms and lower SWaP terminals will be able to better integrate with antenna pointing (acquisition), tracking and stabilisation (PTS) technologies, which will permit mobile troposcatter capability. TrellisWare will verify the performance of the prototype troposcatter radio in aerial testing and demonstration activities in the second half of 2021.
Raytheon combines sat, tropo communication
A U.S. military satellite communications system has showed it can also carry out long-range “troposcatter” communications without a satellite relay. Raytheon said it established a high-bandwidth troposcatter link using its DART-T tactical terminal and a modified Joint Network Node satellite terminal. The achievement took place at Southern California’s Camp Pendleton Marine Corps base during a test of lightweight and mobile tactical communications terminals.
Raytheon Vice President Jerry Powlen said Friday the test showed that U.S. expeditionary forces could count on troposcatter technology in the field both as a means of easing congestion over satellite channels and as an alternative in the event satellites are unavailable.
Col. Kirk Bruno, technology officer for the I Marine Expeditionary Force at Pendleton, said in the Raytheon release that combining troposcatter and satellite systems would reduce the amount of gear required by leathernecks in the field and expand its communications options. “Increased data throughput without relying on already stressed satellites is critical, and unlike satellite communications, TROPO systems provide cost-free access,” the colonel stated.
Raytheon uses its GaN technology for troposcatter
Raytheon’s modern troposcatter capitalizes on the company’s gallium nitride-based semiconductor technology, which delivers superior performance. Raytheon troposcatter is offered in vehicle-mounted equipment as well as a portable, grab-and-go system. The latter comes in as few as three cases, and can be up in running in less than an hour. The system automatically establishes the link with a push of a button.
That marks a big improvement over the military’s older systems, which require “the operator to align the antennas at both ends,” Vigil said. “Imagine trying to get the rabbit ears just right on your dad’s old TV and then having somebody on top of the TV station tower trying to point its transmitter at your rabbit ears 80 miles away. ‘OK…now a little bit to the left…wait, go back a little…’ That’s what it used to be like.”
Troposcatter is almost like a water main for any type of Ethernet traffic. Soldiers can set up voice, video, data and wifi networks. They can also collect intelligence from drones, and electro-optic and other sensors. “It will integrate with any number of comms and sensor packages,” Vigil said. “It allows commanders to get a clearer picture of the battlespace, managing their assets and helping protect our forces.”