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Can
you tell me how to convert Loran numbers to GPS numbers? R.S.,
via e-mail
There are
several methods, but none are perfect. The problem is that old bugaboo
of Loran, the Additional Secondary Factors (ASFs). ASFs correct for the
fact that theoretical Loran readings, which are based on the speed of
radio waves over salt water, are different from actual Loran readings
when they travel over landmasses of various characteristics.
Since
ASFs are fairly constant over time, they don’t affect the repeatability
of a Loran set. They don’t even matter much for plotting positions
using the Loran TD (time difference) lines overlaid on many charts, since
the lines are adjusted for ASFs. Where they really matter is in the complex
formulas used to translate TDs into latitude and longitude. Most Loran
sets didn’t have the computational horsepower to properly figure
large databases of ASFs into these calculations; that’s why we all
learned to distrust those lat/long conversions and work with TDs. Even
the many modern GPS units that can create a lat/long waypoint from input
TD numbers may not do the best possible conversion.
There
are PC software programs that can do the conversions using the full ASF
databases (with corrections every five miles). Two I know of are Coastal
Loran & GPS at www.weak-industries.com and LoranGPS at www.andren.com.
Both can organize and map your waypoints, and the latter program can upload
to the GPS units of several manufacturers (easier if you have a system
as described in my column).
But
even the best-tabulated ASF corrections are not perfect, so the best possible
way to do your conversions is to fire up that old Loran set, go to each
of your TD hot spots, and mark it using a differential or WAAS GPS. It
will be time-consuming, but heck, you’ll be boating instead of computing,
and you could even drop a line over. (By the way, there’s technology
in development that promises to eliminate lat/lon conversion problems
in the next generation of Loran; I’m hoping that it will also mean
the end of those pesky TD lines on paper and raster charts.)
—B.E.
Got
a marine electronics question? Write to Electronics Q&A, Power
& Motoryacht, 260 Madison Ave., 8th Fl., New York,
NY 10016. Fax: (917) 256-2282.e-mail: PMYElectronics@primediamags.com.
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