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UPDATING OLD LORAN TDS
TO GPS LAT/LON

Fishermen are facing a big problem; the government will probably shut down the LORAN system within the next few years. GPS or Global Positioning System will then be the only navigation system in operation. Before that time, all LORAN fixes in TDs (time differences) must be converted to the GPS system which uses LAT/LON (latitude and longitude). A number of difficulties exist in the conversion process. Every fisherman has a collection of LORAN fixes for favorite fishing holes, wrecks etc.. With LORAN, these spots could be relocated with an accuracy of about 30 ft. using pre-established TDs originally set on the same machine. When the LORAN system fixes a position it does so in TDs and converts these to LAT/LON mathematically. The LAT/LON values calculated cannot be expected to be accurate to the degree required to find a small wreck or fishing spot. The user therefore cannot simply transfer this information to the GPS memory and expect it to be useful. It will get you to the general neighborhood but be off by up to 100 meters. Not very good if you are looking for a small piece of wreckage in a big body of water.

The other problem that exists is that the GPS signals are purposely degraded by the military for national security with something called Selective Availability or SA. With this system working, GPS fixes can be off up to 100 meters. They are usually much better than this figure but 30 meters is about the best obtainable. This error may vary in any direction at any given time and is obviously much too great for precise location. The public and GPS manufacturers have been raising a stink and the government has said they are considering removing the SA degradation. If they do, GPS without SA has a working error with satellite fixes of up to 30 meters. This is still not as good as the loran system could fix a position in TDs. In addition, you can bet if there is any military action, the SA feature would be turned back on. They couldnt care less if you could not find your favorite honey hole for a day of fishing.

The answer to this dilemma is DGPS or Differential Global Positioning System. This system uses an additional set of broadcast stations that are land based. These stations send out signals that continuously add corrections to the GPS fixes. It requires the use of a small secondary receiver unit and antenna called a "Differential" or DGPS. This unit attaches to an already existing GPS unit that has "Differential" capability. Almost all GPS units include this feature. With a DGPS operating, fixes of less than 10 meters are guaranteed and 5 meters are common with just average price range units. This kind of accuracy is even better than loran. Some higher priced DGPS units can guarantee fixes within only a few centimeters! Getting back to the original problem of converting LORAN fixes to GPS, the only sure method is to run both LORAN and GPS units side by side. Relocate your old favorite fishing spot using your loran and then store that position in your GPS unit while using the DGPS. Once saved in this way, the fixes will be absolutely accurate and repeatable and even more precise than your old LORAN numbers. In addition, these new DGPS numbers will be just as accurate if used by another person using DGPS. In other words, you can share your spots with others or may get new fixes from friends but only if they were originally determined using DGPS.

Copyright: Al Lorenzetti 1997 Published in "The Fire Island News" 1997

 

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