Trick your GPS into reading Loran-C numbers with surprising accuracy.
Originally Published April 2011 print edition.
Loran-C transmitting stations are now officially off-line, but don’t throw away those old numbers yet. Instead, learn to use the “phantom” Loran-C TDs (time differences) supplied by most modern GPS navigators and chartplotters—with one caveat: You need to know that these phantom Loran TDs are generated by mathematically converting GPS latitude and longitude coordinates. Also, be aware that the GPS-to-Loran algorithms used in each machine are different and the majority will not usually supply the accuracy needed to locate small targets like a wreck. Generally, and without fine tuning, you shouldn’t bank on accuracy better than about 1,000 feet.
Why is it so difficult to convert GPS coordinates to Loran TDs accurately anyway? Well, the answer lies in a number of factors but one important one is the fact that in Florida, Loran-C radio signals passed over a variety of terrain prior to reaching your vessel. Since these were ground wave signals, this imparted errors that are virtually impossible to calculate mathematically over wide areas. This is also at least part of the reason Loran really never had very good absolute accuracy; it was only designed to be accurate to about a quarter mile.
What Loran did have, though, was outstanding repeatability, which meant it would take you to the same place over and over with remarkable consistency. The level of accuracy was often better than 100 feet, which, considering the technology involved, was rather astounding. All most fishermen cared about was that when they went to a set of Loran TDs for a fishing spot, the structure would show up on the sounder, every time, and it did.
The problem of converting GPS coordinates to Loran TDs has never really been solved with any easy-to-use method or machine, at least not to the accuracy we are used to today with GPS, or were able to obtain with Loran repeatability in the past. So, therein lies the problem with phantom Loran TD usage.
Fortunately, Loran-C had an additional secondary factor, or ASF, correction capability built into the system designed to correct for possible signal errors that could be introduced for a variety of reasons. On some Loran-C units an ASF correction could be added to each Loran secondary (meaning secondary transmitting station) by the user. Many current GPS makers carried over this correction factor or renamed their own correction factor and allowed the user to input data as needed to modify the TD readings. We will use this correction to trick the GPS into giving us phantom Loran TDs that closely match those that a Loran-C receiver would have displayed over the same spot. Keep in mind most but not all current GPS units allow the user to enter their own correction factor.
Correct those TDs
Here is how you do it. You will need Loran-C TDs for at least one and preferably two spots in your area that are immovable and that you can locate either visually or by GPS. Your dock and a well-known wreck would be two good choices. With your boat sitting at the dock, configure your unit to display the same GRI (ground repetitive interval, a designator for the respective loran network) and pair of Loran TDs that you normally would have used in a Loran unit. Then compare the readings displayed in phantom TDs with the original Loran-C TDs that were once displayed by your old Loran receiver at this same spot. Now enter a correction factor for each TD so that the phantom TD numbers match your old Loran-C numbers. Record the corrections that you entered to each TD. Do this procedure again at the second location and record the corrections again. Hopefully the two pairs of corrections will be numerically close; now average them, and then enter the corrections for other Loran waypoints that you don’t have GPS numbers to locate.
Keep in mind when doing this that you will be dealing with phantom Loran and not real Loran, so don’t expect perfection. But you should expect, with a little sounder searching and a little luck, to be able to locate your long-lost wreck. FS