We often receive questions
about Loran-C receivers, or about conversion of coordinates from Loran-C
to GPS, or from Loran-C TDs to latitude and longitude. This page started
with some references collected by ILA member Bill Brogdon. We'll add to
this list from time to time. Your contributions are welcome!
Scroll down to see the entire list.
a link to Loran-C Coordinate Conversion Software from Dr. Paul Williams,
from Loran-C to GPS - Bill Brogdon, ca. 2000
Suppose you already have good, measured Loran-C waypoints, and want to
enter them in a GPS receiver. This is a good idea; it allows you to use
the two systems as a check on each other. But Loran-C waypoints are best
described by time delay (TD) readings; the latitude and longitude that
a Loran-C receiver calculates differs from the correct values by up to
a few hundred yards. Using the latitude and longitude of a Loran-C waypoint
as a GPS waypoint thus introduces large errors. Going to each Loran-C waypoint
and storing the measured position with the GPS receiver gives excellent
results, but is time-consuming.
Some GPS receivers can calculate Loran-C TD numbers from the GPS latitude
and longitude. You can also enter a waypoint in Loran-C TDs in these receivers.
However, these TD calculations also have the same inaccuracies that cause
the latitude and longitude calculated by a Loran-C receiver to differ from
the correct values.
Don't confuse these GPS receivers with ones that measure Loran-C TDs with
an internal loran receiver. Trimble, Raytheon, and others have produces
units that include both Loran-C and GPS receivers. They give TDs as accurate
as any stand-alone Loran-C receiver.
If you are going to a waypoint well clear of hazards, marked by a large
buoy, you can simply use the calculated latitude and longitude as the GPS
waypoint. The problem occurs when it is foggy, or if you are heading to
a waypoint marking an underwater wreck or rock. You must be as accurate
as possible in these cases, or you will miss them entirely. We have worked
out a way of converting Loran-C waypoints to GPS waypoints that has shown
good accuracy in our tests.
First go to a convenient position clear of obstructions, and stop the boat.
It's best to be in open water rather than in the marina. Large bridges
can distort the Loran-C signals, and big buildings can reflect the GPS
signals. You want to make a comparison free of such disturbances. Check
both Loran-C and GPS receivers to be sure that the numbers aren't ``jumping
around.'' Check that the Loran-C is locked on the usual TDs, that the SNR
is high, and that the GPS has low PDOP and high signal quality. Save a
Loran-C and a GPS waypoint at the same time. Shift the Loran-C receiver
to indicate latitude and longitude, and compare them with the GPS position
at the ``common waypoint.''
This is sample data taken in our area:
N 34? 42.040’ W 76? 59.172’
34? 41.97’ W 76? 59.13’
With these two receivers, in this area, adding these differences to Loran-C
waypoints gives the latitude and longitude to be entered into the GPS receiver.
It has shown good accuracy.
There are some precautions. First, use your own receivers. Two other receivers
at the same spot gave corrections of +.23 minutes of latitude and and +.16
minutes of longitude. Second, the corrections are valid over relatively
short distances. Due to the way that most Loran-C receivers apply ``ASF''
corrections, they will be most accurate within the one-degree square containing
the common waypoint. The corrections in the example are most accurate from
34? N to 35? N, and from 76? W to 77? W. Only a few hundred yards to the
west, beyond 77? W, new corrections are necessary for the highest accuracy.
Third, when you do go to each waypoint, save it in the GPS receiver. That
is the most accurate GPS data.
Some GPS receivers allow you to enter a waypoint as a range and bearing
from a known position. You could use the Loran-C receiver to find range
and bearing from the common waypoint to other waypoints, for entry as GPS
waypoints. This is not a good idea; the bearings are only accurate to one
degree. At ten miles, there will be an error of about 350 yards, at twenty
miles, 700 yards. It's more accurate to use corrections to Loran-C receiver-determined
latitude and longitude to establish GPS waypoints.
(c) 2000 W. J. Brogdon
From Captain Brogdon’s
book Boat Navigation for the Rest of Us, International Marine
available from the author:
Avmar Technical Services
1541 Silverlace Ct.
Gulf Breeze FL 32563.
They repair Micrologic receivers
and have parts, manuals, etc.
We don't know of any aviation
Loran-C receivers being manufactured. There is one marine receiver
on the market, and it is a good one:
It is available from the
usual discounters such as:
From Bill Brogdon: The conversion
from Loran TDs to latitude and longitude is quite complex and requires
ground conductivity data that isn't available readily.
All Loran receivers except ancient
ones can convert TDs to Latitude and Longitude. Just change the display
from TD to L/L. Many GPS receivers can make the same conversion by
entering a waypoint in Loran TDs and then displaying it in latitude and
longitude. However, the error in these conversions are up to 500
yards or so. That's fine for clear-weather navigation, but not for
foggy weather or for finding underwater objects.
There is a good commercial program
to convert Loran TDs to latitude and longitude with reasonable accuracy,
store waypoint data, and print charts showing the waypoints.
ANDREN SOFTWARE CO.
906 S. Ramona Ave.
Indialantic, FL 32903-3435
Look at http://www.andren.com/
There is a public-domain program available
here on the International Loran Association home page at http://www.loran.org/
Click here to download gptotd.zip and GPtoTDNotes.txt
The NOS and USCG programs run under
DOS, and aren't as accurate as the Andren program, or as easy to use, and
do not create data bases, but they are free.
For somewhat improved accuracy with
the NOS and USCG programs use the ASF tables available from NOS at http://chartmaker.ncd.noaa.gov:80/MCD/loranc.htm
For the highest accuracy with any of
the programs you must establish local corrections by simultaneous Loran-C
and GPS observations near the area of interest.
Bill Brogdon - 9/2002