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SeaMarks manual V. Additional Information •

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V. Additional Information

1. What Are ASF Correction Factors?

ASFs (Additional Secondary Factors) are correction factors used in the Loran to Lat/Lon conversions to make it more

accurate. They compensate for the fact that radio propagation over land and over water will distort the Loran signals as

they travel to you. This distortion can be as much as + 5 microseconds and can cause the Loran to Lat/Lon conversions

to be very inaccurate. The ASF tables created by the government in the 1980s assumed the land features then existing.

Much coastal development has taken place since then and the new structures effect the accuracy of Loran signals. The

program has the ability to upgrade the government tables by using calibration waypoints to teach the program what

corrections are necessary.

When using the program to convert Loran numbers using calibrated ASF tables, it is possible to achieve an accuracy of

better than 0.2 microseconds for "C" waypoints (about 100 ft.) and 2 microseconds for old "A" numbers (about 1000 ft.).

Keep in mind, that, the old "A" numbers were never very accurate to begin with and the calculations do not improve on

that accuracy even if they show lots of decimal places.

Where do you get the ASF numbers? First, the program has a built in table of ASFs derived from government tables.

These tables are reasonably accurate, and can provide an accuracy of around 250-ft. They were derived in the early

1980's before the availability of GPS so they contain lots of educated guesswork. Also, the tables are not available for

all Loran chains. ASF tables calculated from your own calibration waypoints are much better and achieve an accuracy of

better than 100-ft in most cases (but no better than the original Loran repeatability).

Land based calibration waypoints are not desirable since Loran signals are distorted near shore and are strongly

influenced by overhead power lines, buildings and bridges. A Differential GPS (DGPS) or WAAS waypoint is the best

source of the Lat/Lon part of a reference waypoint, whereas a standard GPS waypoint has about the same repeatable

accuracy as Loran in the better cases.

When comparing the Loran to LAT/LON conversions of your GPS or other sources with the program you may see

differences. Only if they are using the same ASFs and the same formulas will they be in close agreement. Also keep in

mind that various Loran brands did not always agree with one another and might differ by as much as 0.2 microseconds.

If the program comes pre-calibrated to one specific Loran, it might require more calibration to make it better agree with

your Loran.

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Mobile Crossing WayPoint Map Loader Manuale dell'Operatore Pagina 19