Loran & GPS, Now and the Future
Redefining Electronic Navigation...
Capt. Rod and Susie Stebbins

It is easy to understand how electronic navigation works, provided we do not try to mix the LORAN-C and GPS electronic systems in the process! 

Let us explain. For all intents and purposes there are six (6) possible Latitude and Longitude (L/L) positions for every actual position, each being derivatives from some form of measurement and all are trying to tell where we are! 

Remember that a Latitude / Longitude may always be just that, while at the same time it may not describe the destination of your choice

1)There is the actual location where we are on this planet, and then there are;
2)the L/L that a GPS device with Selective Availability (SA) offers;
3)the L/L that a GPS device without SA offers;
4)the L/L that a Differential-GPS (D-GPS) combination offers;
5)the L/L that a LORAN-C with the Additional Secondary Factor (ASF),(click to see a typical ASF page), turned OFF, and lastly;
6)the L/L that a LORAN-C with the ASF turned ON offers.

And they are each different, and all of them can vary greatly!

Totally confused; Oh Boy, you say. Let us try to explain.

1)  above is where we actually are, expressed in L/L as might be determined by a physical survey taken with actual position measurements as a Civil Engineering Survey crew might do. This by the way is the challenge for electronics... find this spot, for it is this spot on the planet that we are trying to find, right?

2)  above is the derived L/L position from a GPS unit while the government is "dithering" or altering the clock used to calculate positions by inducing errors randomly. We have plotted these variations to be as much as 2,300 feet and as little as 18 feet. The trouble with this is that when the SA was really working, we the navigator never knew just how much we were being affected by the system. Shucks, we might be navigating to the wrong county and not realize it!

3)  above is the derived L/L position from a GPS without any SA degradation affect.  The GPS system has a design technical resolution capability of approximately 100 meters.  This amounts to a circle of about 328.083 feet in diameter.  What this means is that GPS positions not being degraded will most always appear randomly inside of this size area.  This makes sea buoys and navigation markers easy to find most all of the time, even in the mist and fog.

Boaters Beware…

Note about the government’s use of Selective Availability.  The government on May 1, 2000 issued a News Release entitled “STATEMENT BY THE PRESIDENT REGARDING THE UNITED STATES’ DECISION TO STOP DEGRADING GLOBAL POSITIONING SYSTEM ACCURACY.”  This Press Release states in part “To meet these goals, I committed the U.S. to discontinuing the use of SA by 2006 with an annual assessment of its continued use beginning this year (2000).”  Some interpretations of this memo have stated that SA is discontinued completely and others have stated that the government may use SA at any time when they feel a need to, which is what the Press Release clearly infers.  And, as before, the government will not notify users of the system when SA is or is not in effect.  We have attempted to get a clarification regarding the use of SA, and to date we have not received any clear explanation.

4)  above is the derived L/L position using GPS with a Differential GPS unit.  Differential GPS amounts to a land based station that knows exactly where it is by a land survey.  This system offers a big improvement in derived L/L positioning.  When your boat is so equipped, it will send your position derived from your GPS to the Differential station.  The Differential station will review your position L/L based on its derived and corrected L/L.  The differential station will then transmit to your GPS a correction and your GPS will display a corrected L/L.  These corrections amount to approximately a times ten accuracy factor.  This means if your GPS originally thought it was 230 feet from a destination the D-GPS would correct that L/L to display your position 23 feet away.  Similarly if your GPS shows you to be 40 feet away, the Differential derivative would amount to 4.0 feet from your destination.

Note that when SA is in effect the Differential Factor of times ten still applies.  If the degradation error puts your boat 500 feet from your destination because of SA your position will then be corrected to about 50.0 feet or, as in the case of extremes, if your GPS position is 2,300 feet from your destination it will be corrected to 230.0 feet accordingly. 

The random reporting feature of GPS applies whether in the GPS or D-GPS modes or with or without the effects of SA. 

Understand that some manufacturers make their Differential unit as a separate piece of equipment while with others the Differential feature is built into the GPS unit.  Usually the Differential unit will have its own antenna while other combined units will use a single antenna for the GPS and the Differential system.

5)  above is the L/L derived from a LORAN-C.  (There is a feature within some LORAN units that allow for the Additional Secondary Factor calculation within the LORAN to be turned ON or OFF.)  In any event a L/L derived from a LORAN-C unit should NOT be used with GPS or D-GPS if you expect to find your destination.  

Note, we said that any L/L derived from the LORAN unit itself should NOT be used with GPS or D-GPS if you expect to find your destination.

 Oh, the Mysterious ASF…

All LORAN units will convert the Time Delay (TD) numbers to a L/L expression, but with very little accuracy.  The L/L derived from a LORAN unit can be anywhere from a couple of hundred feet to a few miles off!  This will depend entirely which algorithm is being used and if ASF Factors are used.  We suspect that there is not a LORAN unit that has the storage capacity to contain all of the ASF factors required for the United States coastal waters and the Great Lakes.  This file, in simple text form would amount to approximately 28 megabytes.  There are about 640,000 individual correction factors within the LORAN system considering only the following LORAN-C GRI (grids) 5930, 5990, 7960, 7980, 8970, 9940, 9960 and 9990.

It seems that LORAN-C manufacturers used what is referred to as the Average ASF tables.  In actuality each GRI Secondary has 481 correction factors for every one (1) degree of Longitude and three (3) degrees of Latitude. There are 41 pages of factors per secondary for the 7980 GRI and there are 4 secondary stations (W, X, Y and Z).  This totals 164 pages at 481 factors per page with a grand total of 78,884 factors for just one GRI.  The “Average Tables” use only a total of 492 factors for their calculations, or 3 per page resulting in considerable discrepancy over many of the areas. 

These books of Factors were published by the Defense Mapping Agency Hydrographic / Topographic Center and have been out of print ever since the government made the statement they were going to discontinue the use of LORAN-C in the mid 1990s.   

The preferred arithmetic algorithm to accomplish TD conversions is the one developed by the North American Defense Mapping Agency.  There are two other algorithms that are used by some LORAN-C manufacturers that do not do well in converting LORAN-C Time Delays to L/L.  Remember that LORAN-C does take signal measurements and the Time Delay numbers are the time it takes the signal to get from the transmitting antennas to your boat. So the challenge is to make conversions that amount to the same position of Latitude and Longitude. 

There are several products on the market that advertise making accurate conversions, when in fact there are only two, as most are using the “Average Tables”. 

6)  above amounts to the same as Five above for most LORAN-C manufacturers in that many did not include any ASFs in their conversions and those that did used the Average Tables.   

When LORAN-C was LORAN-C only the derivative L/L from the unit was satisfactory as it gave a position approximation for navigating.  For those of us that used LORAN-C a lot, learned early that a TD was not the same as the L/L, but close enough for estimating purposes.  In every case the LORAN-C units did not use their L/L expressions for calculating navigation headings, times and distances so the accuracy of the L/L expression was never an accuracy factor.  LORAN-C positioning was the result of using TDs only.

Then came the GPS units and novices and non-boat drivers assumed that a Latitude / Longitude was in fact always correct… not true, as is the case of using LORAN-C derived L/L in the GPS.


Contrary to popular belief, GPS is not the miracle navigation tool that many sales promotions would have you believe. If you are navigating on the water and trying to find small features, GPS without differential will not do the job that you hope for.
There are and will be for some time long lists of LORAN-C / TD numbers that can be purchased from pennies to many dollars apiece.  It just seems to be that the difference is in the experience of the purchaser.  One must NEVER assume that a L/L accompanied with a TD is correct and useful to GPS.  You can be assured, for the most part, that L/Ls shared between friends are incorrect unless taken with a D-GPS after May 1, 2000 and/or TD numbers that have been converted using the preferred algorithm and all of the appropriate ASF Factors.  All other coordinates will frustrate your search for that nice ledge, rocks, reef or wreck, and many times be unsafe as in the need for serious navigation to a sea buoy or marker after dark or in the fog without radar.

©Rod and Susie Stebbins, Weak Industries, Inc.