Mobile Crossing WayPoint Map Loader Instrukcja Operatora Pobierz pdf (Strona 20)

2 Calibrating the Conversions

The program uses calibration waypoints to achieve the best accuracy in Loran to Lat/Lon conversions. It has the full set

of Government Defense Mapping Agency (DMA) Additional Secondary Factor (ASF) tables that are stored in 5-minute

(5-n-mile) increments. These tables have been derived from the printed books for the chains: 5930, 5990, 7960, 7980,

8970, 9940, and 9960. They were scanned into the computer and processed by optical character recognition (OCR). Be

advised that there may have been some errors in this process even though we were very careful in examining the tables

after the OCR process. There is a feature available during the calibration process to examine them. They are all that is

available, and their accuracy is only about 250 ft. You may get better accuracy since the program has been pre-

calibrated for areas where customers have donated appropriate data to us. Keep in mind that there are two sets of station

parameters for chains 9960 and 8970. They are carried in the program as, for example, 9960b (for before 1/17/07) and

9960a (for after 1/17/07). The government made a timing change on that date. Calibrating with the wrong set will put

you off by about 400 ft. Only one set of ASFs is currently held, so make sure your data is entered with the right versions

of these chains. That is, enter your old (pre-2007) data with 9960b and any newer measured calibration data with 9960a.

There are three optional calculation methods used in the program. Calculation method 2 uses these DMA tables as-is

in the conversion calculation. Using these tables will provide about 250 ft of conversion accuracy compared to up to

1500 ft without using any ASFs. Of course, calibration does not effect conversions using this method.

To get even better accuracy, the program has the capability to learn from your reference waypoints and use them to

either calibrate the DMA tables or calculate a new set of ASF values to use. These are called calculation methods 1

(and 3 explained later). To do calibration, you need to enter waypoints taken offshore with both a GPS and a Loran on

the same spot or get this data from someone else. These simultaneous waypoints of Loran and GPS are the ‘truth’ by

which you will calibrate the program and also check to see that it is converting the waypoints correctly.

You can enter this calibration data into the program as new waypoints using both Loran and Lat/Lon or you can just

correct existing waypoints. For the latter, you correct the Lat/Lon on the spreadsheet, which then promotes that

waypoint to calibration status. In either of these procedures, you will be asked where you got the Lat/Lon. If it was from

a Differential GPS (DGPS) or WAAS, it will be treated with greatest respect and given a high quality factor. If it came

from a less accurate source, a lower quality factor will be assigned to it. Calibration waypoints will never have

conversions done on them unless you demote them to non-calibration status.

A two-digit chain number identifies calibration waypoints where the leading digit is assigned based on the source. For

example, a chain number of 96 indicates that the waypoint is a calibration waypoint using a DGPS or WAAS and Loran

chain 6. Generally, the lower the leading digit, the less the program trusts the data. Changing the chain number back to

a one-digit number (such as 6) demotes the waypoint to non-calibration status and indicates that loran chain 6 is being

used as the source data. It could also be changed to 0 to indicate that GPS data was entered.

When you push the Tools>Calibrate Conversions button, several operations are performed. The calibration data is

first scanned for errors. The ASFs of all the calibration waypoints are calculated. Then the program calculates an

average of the ASFs for each Loran slave station for all the calibration waypoints weighted by the quality factor. If any

of the ASFs is in gross disagreement with the average value, that waypoint is brought to your attention for possible

demotion. All demoted waypoints are ignored in the calibration process. They are usually demoted to be loran

waypoints.

As a part of the calibration process, a file that contains all the calibration waypoints is created and maintained. It will

contain all the calibration waypoints of all the files you have calibrated. Therefore, it will apply calibration waypoints

from one file to another if they are in the vicinity. If you suspect that this file has gotten corrupt or contains waypoints

you no longer want in the calibration process, you are given the option to erase the old calibration file (Calib~ASF.DBF)

and begin anew. You can also open it and edit it directly to delete questionable points.

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