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[aprssig] GPS question

Stephen H. Smith wa8lmf2 at aol.com
Sat Dec 8 21:36:45 UTC 2012


On 12/8/2012 4:26 PM, Andrew Rich wrote:
> I am not sure I have asked this before but I will ask it again.
> I have a GPS on the side of a building and it can only see half the sky - to 
> the north
> What is curious - is that when plotted on google earth - the postion is north 
> of where it should be
> I will move it into full view of the sky and see what that does
> - Andrew -
>

A gps receiver should be able to function normally WITHOUT seeing the entire 
sky; they do this all the time on car dashboards.

Most likely you either have the unit set for a datum other than WGS84 (the 
GLOBAL grid system created by and for GPS).

Or the device is being confused by reflections from nearby structures.  Since 
GPS works by timing the difference in time of arrival of signals from multiple 
satellites, multipath reflections that add to path length (and thereby delay 
the time of arrival of signals) will translate directly into a shifted 
position.  In downtown large-city "urban canyons" of glass-faced high-rise 
buildings that bounce the 1575 MHz GPS signals every which way, it's not 
uncommon to see GPS "go nuts" and jump back and forth several blocks.



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