Monday, May 18, 2015

Driftnet Radio Buoys

A radio buoy is not a piece of hardware that the average person needs to use. In other words, you probably can't buy one refurbished on Woot. They are a type of battery powered, radio beacon used extensively used by fishing boats operating on the open sea. Popular models are made by Kato Electronics, Taiyo, Sea Star and Newstar, and Ryokuseisha Co.  More here.

By definition they are non-directional, and usually broadcast on a 4-minute loop repeating an ID three times followed by a steady carrier and a stop. They are most frequently found between 1600–2850 kHz and are very common on the 160-meter band. The ID has nothing to do with the location, it's just a unique ID to distinguish one driftnet buoy from another. They usually broadcast with a power of 4-15wats though tmuch higher strengths have been recorded. To extend battery life there is a type of radio buoy called a "SelCall buoy." These only broadcast a signal in response to one from their ship.  More here.

As you might imagine Hams  located on the coasts encounter them a lot, and other shortwave users consider them a source of RFI. They are quite difficult to manage having no fixed location, unknown power and a variable radius. VK2DX classifies them in "Endemic Groups" an elegant classification he created. [LINK] I'll just quote his explanation here:
"Due to the fact that fishnet beacons do not have a fixed location, finding the exact coverage radius could be a difficult task. The starting point into any distance or location-related analysis would be the establishment of Endemic Groups. An EG is a group of distinctive call signs (ID’s) identifiable at a particular location. In other words, EG beacons are your local beacons. For example, a study of VK2DX log reveals that the most often logged ID’s are Class 5b beacons starting with 2Axxx. The most frequent daytime loggers are 2AEGH (1807khz), 2AFYZ (1771khz) and 2ACVW (1753khz). It is therefore reasonable to conclude that 2Axxx beacons would constitute an Endemic Group based on the coast of central part of the state of NSW (VK2). Naturally, a group of beacons with different ID classes might be coexistent in the region. "
But why are they in use at all? Drift net buoys are used in situations where fishing nets aren't anchored to the boat, or another fixed anchor, like the sea floor. In other words, the nets move. They're called drift nets. They hang vertically in the water column without being anchored to the bottom. The nets are kept vertical in the water by floats attached to a rope along the top of the net and weights along the bottom of the net.There are numerous criticism of the technique [LINK] but Greenpeace had not begun jamming them yet.