Troposcatter Bounces Back
Written by Adam Baddeley
MIT 2010 Volume: 14 Issue: 6 (July)
COMMS TECHNOLOGY LINKS FORCES
IN RUGGED AREAS WITHOUT USING
EXPENSIVE AND SCARCE SATELLITE TIME.
Once seen as a nearly forgotten technology that was never fully exploited by the military, Troposcatter is back and being used to meet nearterm needs, linking distributed forces in Afghanistan and elsewhere, without recourse to expensive and scarce transponder time using satellites.
Troposcatter, or “Tropo,” is a nonline- of-sight system that scatters the signal off the troposphere. It differs from high frequency communications’ use of refraction in the ionosphere. In Tropo, signals scatter off variations in the troposphere that extend up to about seven miles. This allows scattering of these radio signals to locations far beyond line-of-sight. Unlike many SATCOM links, it not only performs very well in adverse conditions, it positively thrives.
Scattering the signal, as its name suggests, requires the signal to be stitched back together at the receive site. This is done through digital signal processing. The better the digital signal processing is accomplished, the further signals can be sent while retaining coherency.
For many years, the AN/TRC-170 Troposphere Scatter terminals were the only such capability in this category of U.S. service, being used to link air defense batteries with a very specific, low latency application supporting missile firing data. The systems and the underlying technology were largely overlooked in favor of SATCOM for significant periods of time, with little in the way of upgrade throughout the 1980s and 1990s.
New investment in the technology is now being made, with Tropo being seen not as an alternative to SATCOM but as an important and integrated supplement, part of a tool kit of communications capabilities.
DEPLOYABLE EQUIPMENT
The TCS Tactical Transportable Tropo (3T) is the latest addition to the TeleCommunication Systems (TCS) line of deployable communications equipment for the U.S. military.
The 3T capability is a technology enhancement to the SIPRNet/NIPRNet router access point (SNAP) terminals. The SNAP systems are very small aperture terminals (VSATs) that use commercial off-theshelf equipment to provide secure beyond line-of-sight (BLOS) communications to battalions and smaller units.
The demand for reliable, secure BLOS communications to smaller maneuver units originated in the military’s need for small, highly equipped units to operate in very remote locations, such as the mountains of Afghanistan.
To meet this demand, TCS, in conjunction with Comtech Systems, developed and introduced a technology enhancement that provides next-generation Troposcatter communications capability using existing SNAP terminals.
One of the driving forces behind this groundbreaking capability is James Snyder, an account executive at TCS. “The SNAP suite provides interoperability among three different SATCOM terminals [1.2M, 2.0M, and E500 BGAN] and utilizes three different frequency bands [Ku-, Ka- and X-band] and three interchangeable baseband solutions to provide warfighters with compact, lightweight and highly flexible NIPR/SIPR communications packages—all while providing a common logistics tail. With the introduction of the TCS 3T, warfighters will have unparalleled technology at their fingertips.
“With the extensive deployment in Afghanistan,” Snyder continued, “TCS customers began reporting recurring communication challenges, particularly their need for over-the-horizon capability in mountainous terrain. This challenged us to identify a technology that could provide robust communications for anywhere between five and 100 kilometers. Further, satellite bandwidth is expensive. Consequently, our customers needed a communications capability that was also low expense.”
The rationale driving the development of on-the-quick-halt Troposcatter systems was therefore simple: provide low-cost, shortto- intermediate range, beyond line-of-sight capability without satellite transponders.
After industry and technical evaluation, TCS developed a partnership with Comtech Systems. An entrenched leader in Troposcatter communications, Comtech already possessed technology that addressed the military’s portability needs and was adaptable to the modular SNAP architecture.
By enhancing the SNAP VSAT terminal, TCS and Comtech were able to develop a solution that customers are already familiar with, yet one that provides them with a seamless upgrade path to a tactical VSATbased Troposcatter system. Cost-effectiveness and extended capability of this product were obvious benefits.
“What’s compelling about the 3T solution is that it doesn’t require significant modifications to existing SNAP hardware,” said Snyder. “Enhanced feed cartridges, along with modem and highpower amplifier HPA upgrades, convert basic SNAP VSAT terminals into deployable transitcased Troposcatter systems.” The TCS 3T combines modularity and ease of use, allowing field personnel to switch from geosynchronous VSAT connections to terrestrial BLOS Troposcatter communications with ease.
The system utilizes the latest Comtech IP Troposcatter modem to power the 22 Mbps bandwidth capability. Comtech’s technology provides improved throughput, adaptive power control and code rating optimization. This means that the operator is not required to configure an undue number of parameters to establish a solid, high-bandwidth link.
Although the 3T system is not packaged with encryption functionality, an external inline encryptor (such as a KG-175) can be added where secure communication is required. Most SNAP terminals are already utilizing the same encryption devices in the field.
The 3T system is capable of communications of up to 22 Mbps. When used in conjunction with the 2.0 m system and scalable high-power amplifier options, it can span distances of up to 100 miles. The range can be extended with additional configurations, since the system’s technology enables the modem to act as a relay between multiple 3T terminals.
“TCS’ expertise in sustaining VSATdriven networks, paired with Comtech’s industry-leading Troposcatter modem technology, has produced the first true IPcapable Troposcatter solution for the mobile warfighter,” Snyder concluded.
TRANSPORTABLE ANTENNA
Comtech Systems digital Tropo equipment and solutions are in service with the U.S. and international militaries and commercial users around the world. “Since Comtech’s first 8 Mb/s digital systems in the 80s, which was considered a very high data rate for that time, the Tropo market has evolved to equipment capable of 22 Mb/s” said Mark Yamamoto, director of government marketing for Comtech Systems.
As a result of demands from users for higher data rate modems and additional capabilities, Comtech Systems in the early 2000s developed a family of Tropo modems that provide data rates up to 22 Mb/s, forward error correction, automatic data rate change capability to compensate for degrading Tropo paths, and automatic control of the transmitted power keep the transmitted power to only the level needed for the grade of service required thereby reducing the probability of unfriendly intercept or interference.
Comtech’s new family of high rate Tropo modems met the U.S. military’s requirement for more data throughput from the TRC-170, and Comtech has received multiple contracts to provide in excess of 320 high data rate modem upgrade kits to replace the old Tropo modems. These kits have given new life to the TRC-170 and are currently in use by all service branches.
The Marine Corps requirement for a new TRC-170 antenna system that is easier to deploy and maintains the current level of performance has resulted in a contract for Comtech’s Transportable Fast Link Antenna (TFLA). TFLA is a rapidly deployable Troposcatter antenna that can be set up by only two people without any special tools and become operational in 30 minutes. TFLA operates in the 4.4-5.0 GHz band and is compatible with the AN/TRC-170A (V)2/ (V)3/(V)5 and a wide range of other Troposcatter communications terminals.
TFLA consists of a three meter carbon fiber reflector mounted on a highstrength aluminum trailer, which provides the optimum balance between lightweight design and ruggedness. Key features of the TFLA include an automated antenna pointing sub-system, four-port angle diversity feed subsystem, and an optional self-contained electrical power subsystem. With this innovative four-port angle diversity feed subsystem, the TFLA can support either dual or quad diversity operation with only one reflector rather than two. As a result, rapid deployment and solid link performance can be obtained at a significantly lower life cycle cost than traditional systems that rely on separate antennas for space diversity.
TFLA has been designed to allow future insertion of all the Troposcatter electronic hardware to provide a simple upgrade path to eliminate the TRC-170 radio shelter altogether, thereby reducing the logistical footprint and manning by 50 percent. With optional electronics and generator, the unit provides a highly mobile, quickly deployable Tropo terminal. These systems are going into service in the Middle East next year.
In addition to the need for more capability from Tropo systems, the military has expressed a need for smaller easier to deploy terminals. “The development of a modular transit case Tropo terminal capable of being used as a line-of-sight radio, or a beyondline- of-sight radio that is scalable to meet mission requirements, will have a considerable positive impact on deployment. Transit case Tropo is easy and faster to deploy and will allow Tropo systems to be used in new applications at lower echelons than ever before,” said Yamamoto.
The transit case Tropo has even more flexibility when combined with a transit case satellite system. Comtech teamed with TCS to jointly develop a joint transit case satellite/Tropo system.
One of the key differences in Comtech Systems’ thinking is the frequency band used, Yamamoto explained. “The performance of Tropo in Ku-band is unproven and not used because of frequency congestion and interference with existing military systems. Tropo in C-band is well proven and has been in service for many years in a band reserved for this type of application.”
PERFORMANCE BOOST
General Dynamics SATCOM Technologies’ Tropo solutions are based around the new TM-20 Troposcatter modem, according to Chief Technology Officer Tim Shroyer, who described it as the first 20 Mbps capable troposcatter modem. Improved time-space coding and techniques led to improved signal- to-noise ratios, boosting tropo’s performance.
While noting that Tropo is not without its weaknesses, Shroyer commented, “The new modems are better than they used to be, but they are not the answer to everything and you cannot get 100 percent availability. They are a useful adjunct to other systems such as SATCOM. Unlike with SATCOM, you don’t pay that continuing cost of leasing satellite capacity, and if the distance is not too extreme and spectrum is available, then troposcatter systems are a good fit.”
General Dynamics SATCOM Technologies is pursuing Ku-band Tropo because of the higher gain using the same-sized antenna. There is, however, an absence of complete data about Ku-band Troposcatter’s signal propagating characteristics. “We are confident that it will work over lots of other distances with various power levels, but we can’t guarantee availability,” said Shroyer. “The customer is used to SATCOM link performance, where you engineer it for 99.99 percent availability.
“The big advantage of Ku-band Tropo was the gain of the antennas. With that, we can use a common antenna like a 2.4 SATCOM antenna, and we can switch over from a satellite system using the same terminal. That gives us a lot of flexibility with the terminal, and that is what led us down this path,” he continued.
“To get the best performance from Tropo, you need diversity in either frequency or space,” Shroyer said. “By space, I mean different angles so the signals are going different places. The way we do it at Ku-band is that we change the feed of the antenna. We have patented a new feed design at Ku that actually gives us two beams out of the same antenna. With two beams, we can get that space diversity with a single antenna with Ku-band. With C- or S-band, you need two complete antennas for that capability.”
General Dynamics has supplied Troposcatter systems to a number of specialized programs. General Dynamics and Raytheon have worked together on upgrades to the TRC-170, with both firms also working jointly on the Dual-mode, All-band, Relocatable, Tactical Terminal (DART-T) program.
ERROR-FREE TRAFFIC
Raytheon began work on modernizing legacy Troposcatter systems in 2004 in response to military interest to meet requirements emanating from operations in Southwest Asia. “We needed to step up and introduce new technology to utilize troposcatter to its fullest capability,” said Dave Hegarty, director of ground systems in Raytheon Network Centric Systems’ SATCOM and Space Systems group.
“Our first look was to make it smaller, simpler and lighter, because traditional Troposcatter systems are large, temperamental and require a lot of manpower. We wanted to make it as simple to use as a commercial satellite terminal. We have led with the technology and spent the last four years in field, testing it in over a dozen different U.S. military and international sites and in all different environments,” Hegarty said.
Tests have included the 2009 “Earth, Wind and Fire” air defense communications exercise at Fort Bliss and Fort Sill. Hegarty reported that the Tropo system provided by Raytheon ensured round-the-clock error-free IP traffic over several weeks. Raytheon has also participated in the annual Army Expeditionary Warrior Experiment at Fort Benning and C4ISR exercise at Fort Dix.
The new Tropo modem technologies used by Raytheon operate at new frequencies. Air Force studies at Hanscom AFB by Bob Crane, a recognized expert on atmospheric effects on propagation at Dartmouth College, identified the feasibility of moving to a higher frequency band, the Ku-band.
“They found it very favorable, because by going to Ku-band you are able to move to smaller components, smaller antennas and use less power with similar performance at C-band. That was our key focus,” Hegarty said. “We married the technologies of smaller antennas and high power amplifiers so that we were using less input power and were getting more output power. Also, we developed a new FPGA based modem, a new angle diversity feed and automatic alignment and adaptive bandwidth software.”
Hegarty argued that while others have imitated these techniques, they have implemented them in C- rather than Ku-band solutions, resulting in performance challenges. This is not a case of one band being better then another, he said, but rather of exploiting the advantages of one band—Kuband— to achieve maximum flexibility for a small tactical terminal, for ranges up to about 100 miles, that can also operate as a satellite terminal.
The reinvention of Tropo has led to its active consideration in mainstream programs, including perhaps as part of later increments of the Warfighter Information Network-Tactical (WIN-T) program. “We have had these in the field for over three years and we have had good response from the Army,” said Hegarty. “We have participated in a large number of Army exercises over the past three years with a lot of field time. We have honed the unit’s potential performance and have also done some qualification testing for the Army to prepare for deployment later this year.”
Raytheon’s initial approach was the DART-T, a trailer-based Ku-band SATCOM solution that also provides a Tropo solution. Subsequently the company developed TELOS, a miniature solution able to be carried in three transit cases. TELOS was an Office of the Secretary of Defense acquisition challenge program designed to accelerate new technologies by bypassing conventional procurement. It beat over 200 other submissions to win the award and is currently going through qualification testing.
“That has attracted an awful lot of attention from a lot of customers, particularly in environments where there is rough terrain or a shortage of satellite bandwidth. Our system is designed to be lightweight, easy to use, mobile and used by non-signal troops so that almost anybody can be trained to operate it very quickly. We are filling a gap in that requires reliable high bandwidth communications and without the cost of satellite and without the latency and vulnerability of satellites,” Hegarty said. ♦
