Mobile Crossing WayPoint Map Loader Betreibershandbuch Seite 27

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SeaMarks manual V. Additional Information
27
The user-adjusted table is, by the way, filled-in more than the DMA tables. The DMA tables are blank over land. This
filling-in was done by extrapolation. It was done to help the program avoid a kink in the Loran lines for a map or chart
when they extend over land. It was not meant to provide accurate conversions over land.
When examining the ASF tables, note that they are arranged in 1-degree Lon/Lat blocks where the block designator is
the upper left-hand corner of the block. This means that to see 74 20.0W, 39 35.0N, you need to look at the +75 +40
block (round up both numbers). You have choices of any of the 7 basic Loran GRIs (7930 - 9960) and any of the up to 4
slaves (W - Z).
An examination of all the DMA ASF tables show that the ASF factors never get to be greater than +/-5.5 microseconds
and generally never change more than +/-1.5 microseconds from one 5 minute block to the next. A more normal case is
an ASF of 2 microseconds or less and no more than 0.2 microseconds change block to block. If the ASF pattern shows
much variation over the area of interest, then a single ASF value per file is not a good choice. Some areas show wild
variations and must have the more complete user ASF tables. Experiment with the results to see which best suits your
situation. The graphic below shows some Canadian data on ASFs in an area where we do not have complete tables.
This graphic will give you a feel for the variability of ASFs. It amply shows how much land areas influence the Loran
signals.
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