On Sun, 05 Mar 2006 14:15:42 GMT, chuck wrote:

While opinions about accuracy vary, no numbers have been posted. Come
Spring, it would be great to see a couple of sailors with RDF's take
them out of hiding and check their accuracy against their GPS.

I used to rely on an RDF back in the 70s and have a good feel for both
accuracy and the source of errors. I'm not talking about aircraft
type VHF/VOR, but rather the traditional marine HF radio type with
rotatating antenna sensor.

RDF works by providing a bearing to a known object, in this case the
radio tower which is plotted on a chart. The bearing provided by the
RDF was typically relative to the heading of the boat except for the
hand held RDFs with built in bearing compass. Either way, the
accuracy of the bearing was only as accurate as the compass used,
typically plus or minus 2 or 3 degrees.

A single bearing to a radio tower provides only a single line of
position, i.e., your boat is located somewhere along that line but you
don't know exactly where without a second LOP from another radio
station. Ideally the second bearing should cross the first at 90
degrees but in real life that hardly ever happens. The narrower the
crossing angle, the more that any bearing error is magnified, and the
more uncertainty that is introduced in your position plot.

Back to accuracy. An error of plus/minus 3 degrees at 10 nautical
miles translates to an uncertainty of plus/minus 3/10ths of a mile or
about 3600 feet in total. Taken with a 90 degree crossing angle of
the same error magnitude, you now have an uncertainty box about
6/10ths of a mile on each side. With less than ideal crossing angles
the uncertainty box elongates to as much as one or two miles in real
world situations.

Compare that to LORAN-Cs 100 yard accuracy, or GPS/WAAS at 10 feet and
make your own decision regarding RDF accuracy.