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Author Topic:   GNSS Position Fix: Horizontal and Vertical Precision
jimh posted 12-05-2013 06:09 PM ET (US)   Profile for jimh   Send Email to jimh  
It has been my observation that the position fix solution on a GNSS receiver tends to always be more accurate in the horizontal plane than in the vertical plane. Since boaters are mostly not concerned with their altitude above sea level--they are usually at sea level or know the level of the lake they are on above sea level--the altitude component of the position fix is usually ignored in marine applications. In many instances on marine GNSS receivers the altitude is not even shown on the main display; you have to dig into the submenus to find it.

When the altitude of a position fix is monitored, one sees that the accuracy of the altitude is generally much worse than the accuracy of the horizontal fix. Altitudes tend to vary over a much wider range than the 2D horizontal fix. The cause of this was not clear to me until recently. In reading about Dilution of Precision, I found a very succinct explanation for the difference in precision between the horizontal plane and the vertical plane for GNSS receivers:

In the horizontal plane, lines of position to satellites can be computed over a 360-degree arc. In the [vertical] plane, because radio waves from the satellites cannot pass through the Earth, the lines of position to satellites can only come from a 180-degree arc, that is, from one horizon to directly overhead to the other horizon. Also, many GNSS receivers do not use satellites with an elevation angle of less than 15-degrees for a ranging solution. This further limits the vertical plane to a 150-degree arc (180 - 2*15). Because the range of lines of position to satellites is much more limited, the dilution of precision in the vertical plane tends to be higher than in the horizontal plane.

For a good, readable, understandable explanation of the effect of satellite position geometry on dilution of precision, I recommend reading:

Marko888 posted 12-06-2013 02:57 PM ET (US)     Profile for Marko888    
Horizontal precision typically exceeds vertical precision by about a third.

A key point for all of us to keep in mind when relying upon our GPS chart plotters, is that the positional accuracy is not as absolute as we might think. As a land surveyor who owns and operates high-precision dual frequency GPS equipment, I still get caught at times staring at my chart plotter and treating the position illustrated as gospel, when I know better.

Garmin notes their autonomous positional accuracy at 15m, and 3m with DGPS corrections (such as WAAS).

Even when in open water, short term holes in the satellite constellation can lead to higher DOP values and reduced precision.

While DOP feedback is very useful, having some awareness of the environmental factors which erode GPS positioning can also be useful when on the water. When we are boating in steep, big mountain country, such as the fjords of BC, some satellite signals will at times be masked, and when we are near large stone bluffs, multipath conditions can exist which will also erode accuracy.

Jefecinco posted 12-06-2013 07:45 PM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     
The altitude reading on my Garmin 740S slowly creeps up from zero to 150 feet then descends back down to zero then repeats over and over. This happens when the boat is in the driveway at home approximately 60 feet above sea level. Not that I'm concerned, but I wonder what is going on.


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