Redefining Electronic Navigation...
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 1)There is the actual location where we are on this planet, and then there are;
Totally confused; Oh Boy, you say. Let us try to explain.
. 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.The GPS system has a design technical resolution capability of approximately 100 meters
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.
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.
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.
All LORAN units will convert the Time Delay (TD) numbers to a L/L expression, but 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 â€œAverage Tablesâ€ 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 â€œAverage Tablesâ€.
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
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. 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.
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