If you have ever driven Card Sound Road between Florida City and the Keys, you may have noticed some large reflectors and a building that was the U.S. end of an AT&T troposcatter link to Cuba which for many years was the only way Cuba connected by telephone to the outside world. That system was shut down several years ago and I thought use of this propagation mode for transmission had been largely relegated to history with the widespread use of satellite communications. A recent press release from Radyne and General Dynamics shows troposcatter is still in use. The two companies announced a new product, the TM-20 modem that allows data transmission up to 20 Mbps using troposcatter!
Troposcatter propagation works, as the name implies, by refracting RF from small variations in the index of refraction of air and from particles present in a volume of air in the troposphere that is common to both ends of the path. As you can imagine, the path loss with this type of propagation is quite large.
Radyne developed the TM-20 troposcatter system in partnership with General Dynamics C4 Systems. Bob Fitting, Radyne's CEO, described his company's contribution, "The TM-20 represents the first significant advance in over-the-horizon technology in almost three decades. The use of modern signal processing techniques and a patented channel-directed-equalizer provides for link performance and configuration flexibility previously unheard of in Troposcatter systems. Today the concentration of technology in the TM-20's single, two-unit chassis (four inches high, by 19 inches wide and 18 inches deep) provides functionality and performance well beyond what was achievable using entire racks of equipment only a few years ago."
Jeff Porter, general manager of General Dynamics' VertexRSI Antenna Products division, added, "The TM-20 provides a technically superior core for the next generation of beyond-line-of-site communications terminals, which will include highly configurable antenna and RF systems. Our experience and broad portfolio of existing antenna and RF products, coupled with the technology
Radyne brings in the TM-20, yields field-configurable mobile and tactical terminals capable of unsurpassed performance in satellite communication or troposcatter modes of operation."
I couldn't find any technical specifications on either the Radyne or the General Dynamics C4 Systems Web sites. I did find details on the AN/TRC-170 tropospheric scatter microwave radio terminal that the TM-20 is to replace. This system works over distances of 100 to 150 miles and uses frequencies in the 4.4 to 5.0 GHz band and appears to provide a maximum data throughput of 5 to 6 Mbps.
For comparison, the Cape Sound Road AT&T facility operated at 900 MHz, using two 10 kW transmitters with enough analog bandwidth to transmit 900 telephone circuits or one black and white video signal each.
Why use troposcatter instead of satellite or conventional microwave? For these over-the-horizon hundred mile or so paths, satellites are an expensive option. Conventional microwave requires multiple relay sites to cover over-the-horizon paths, which could be difficult to install in hostile areas. In both cases, not only do the end points have to be secured when used for critical command and control operations, the satellite or relays points have to be protected as well. However, if you think a troposcatter system might be handy for bringing back video from that distant ENG receive site, forget it as the path loss for troposcatter is very high and I don't think it would work at the maximum effective radiated powers allowed under Part 74 and Part 101 of the FCC rules. If I'm wrong about this, please let me know.
For additional information on this topic, see AT&T troposcatter link to Cuba Web site and follow the links to read about its interesting history. The AN/TRC-170 Web page provide technical information on the previous generation of troposcatter equipment. Information on the TM-20 came from the General Dynamics C4 Systems press release General Dynamics and Radyne Corporation Introduce Over-the-Horizon Modem.
