[aprssig] How gps works

[Stephen H. Smith]

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.