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Chesapeake Sailor
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579 Posts
EA,

There is a fair amount of accuracy/repeatability difference between GPS with selective availability off and WAAS. Have a look at this URL an I think you will agree.

http://www.raymarine.com/recreational/latest/index.html

The graph was origianlly built as part of the (former parent company Raytheon) work with the FAA on WAAS.

The Garmin line of GPS receivers is/has gone exclusively to WAAS.

Wayne
 

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Chesapeake Sailor
Joined
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579 Posts
1) The resolution of the display has nothing to do with the accuracy or repeatability of the geolocation calculations. The delta in successive calculations alarms the anchor warning not the pixel location on a chartplotter.

2) the quoted accuracys are CEPs. The boat''s position could be calculated in the LR portion of the CEP at time 1 then at time 2 (possibly minutes later) calculated in the UL portion of the CEP. Both calculations within the 95% probability - but disbursed by as much as the diameter of the CEP not the radius from "ground truth". In this case 15 meters is now nearly 30 - enough to keep you awake at night listening to anchor warning alarms from the unit regardless of manufacture... unless of course you have a very big boat.
 
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