Troposcatter for the Radio Amateur : Part 1

Tropospheric Scatter (a.k.a. troposcatter) systems were a very popular beyond line-of-sight communications system. Troposcatter is typically deployed over water, difficult terrain, or in a harsh climate. With less potential points of failure, troposcatter is nearly as reliable as line-of-sight communications if properly implemented. For these reasons, before satellites, troposcatter was widely used for military and government communications.

Tropospheric Scatter vs LOS

Tropospheric Scatter vs LOS

Troposcatter is similar to line-of-sight microwave with higher power and larger high gain antennas that point at a common spot in the sky. Irregularities in the troposphere scatters a small amount of signal back toward the receive antenna. Most systems operate from low UHF to microwave frequencies with 20dBi – 30 dBi directional antennas and power levels in the 1kw – 50kw range. In order to ensure high reliability, systems typically utilize some type of diversity.

Notable Troposcatter Systems

A few of the better known troposcatter systems were the White Alice and ACE High systems. White Alice was used to relay data from the Ballistic Missile Early Warning System (BMEWS) and also provided telephone service in Alaska. ACE High provided a hardened military communications system for NATO in Europe. Both systems used large high gain antennas and high power levels and operated in the 900MHz range.

Parabolic antenna used in the White Alice Communication System

Parabolic antenna used in the White Alice Communication System

Amateurs use troposcatter quite often for beyond line-of-sight communications above 50MHz. Most of the time if you hear a distant repeater well beyond the line-of-sight on 2 meters it’s caused by tropospheric scatter or it’s cousin tropospheric ducting. The difference between these systems and the typical amateur experience is the reliability of the link. Large systems that use diversity will have availability above 98%, while the 2 meter band openings come and go.

If properly planned, amateurs could build reliable troposcatter links. Hams could build a 150 – 200 mile 440MHz or 915MHz link using high gain antennas and modest power. Digital modes that offer error correction and automatic repeat request (ARQ) would increase the reliability of the link. Proper planning of the link power budget and tropospheric variations with the seasons is key for high reliability.

I found this excellent website by Palle Preben-Hansen, OZ1RH that goes into the details of setting up an amateur troposcatter system. On the other end of the price scale I found this link from Raytheon showing microwave troposcatter systems. Raytheon systems are capable of 100 mbps troposcatter using kilowatt power levels in the C-Band (3 GHz – 5.5 GHz).

Emergency Communications

Troposcatter lends itself to an EMCOMM (emergency communications) role nicely in either a fixed antenna configuration linking two points, or with movable antennas to setup links to multiple points. Here in the state of Kansas, a fixed system could be employed to link two cities separated by 150 – 200 miles. Packet radio could be utilized to provide terminal-to-terminal communications between Kansas City and Wichita in a disaster where land based systems have failed and HF ionospheric propagation is not reliable at this short of a distance.

I hope this write up gets the word out on troposcatter and how reliable it can be. I was always interested in how a beyond line-of-sight system could provide such a reliable path for critical communications. With modern digital modes, amateurs can put troposcatter to work easier and cheaper than ever before. In Part 2 I will go over some of the math involved in setting up a troposcatter path, and in part 3 I will discuss the equipment required.

Below are some additional links to information on troposcatter:

Wikipedia – Tropospheric scatter

Troposcatter Techniques For The Amateur W2NNT ARRL (PDF)

Understanding Tropscatter Propagation – Defense Technical Information Center (PDF)

Thirty Years of Troposcatter Communications – Defense Technical Information Center (PDF)