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[aprssig] How gps works

Andre aprs at pe1rdw.demon.nl
Sun May 6 14:12:40 UTC 2012


On Sun, 06 May 2012 08:34:19 +0200, Stephen H. Smith <wa8lmf2 at aol.com>  
wrote:

> On 5/6/2012 1:49 AM, Andrew Rich wrote:
>> Does seeing "part" of the sky make a difference ?
>>
>> If I sit beside a buidling were I can only see 1/2 the sky does my  
>> position skew ?
>>
>
> NO.  It's a very binary process.  Either the part of the sky you can see  
> has enough satellites in view for a fix.  Or, if there are not enough  
> receivable, the unit "spins it's wheels" endlessly in the acquisition  
> phase with no fix.
>
> Typically as the pattern of satellites visible from your location  
> changes, and the number visible fluctuates above and below the magic  
> minimum, the unit will keep shifting between a 4-satellite 3-D fix  
> (altitude as well as lat/long), a 3-satellite 2-D fix (no altitude info)  
> and no fix at all.
>
> When you get into the "urban canyons" of high-rise big city downtown  
> areas, GPS units frequently "go nuts" due to signals being blocked, and  
> by apparent time delays for given satellites abruptly changing due to  
> multipath reflections off glass-faced buildings.
>
> The same thing happens frequently on winding mountain roads, especially  
> deep in canyons.   The receiver is constantly acquiring satellites only  
> to lose them again when you go around a curve, forcing the receiver to  
> search for and acquire other satellites, as the visible part of the sky  
> keeps changing.
>
It's not just visabilety of the sky that can make gps go nuts, I have  
worked as an assistant surveyer for a while in Rotterdam and everytime we  
where in range of the airport we would get big deviation errors, sometimes  
more then half a meter and this was with a DGPS system and at least 8  
satellites locked, waiting a few minutes would bring the deviation back to  
a few CM making at acceptable again for surveying pipes.
We never figured out what it was but we assumed it was the airport radar.

-- 
73 Andre PE1RDW



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