Larry wrote:

> News <News@Group.name> wrote in
> news:d5idnf8SFKwgUmfanZ2dnUVZ_t3inZ2d@speakeasy.ne t:
>
>> Correct, which voids the OP's accuracy and error probabilities.
>
> With the zero setting, and the fix not moving around constantly, the
> accuracy of GPS is most dependent on having a clear view of the majority of
> the satellites with minimal multipath propagation caused by high buildings,
> bridges, mountains, towers and anything else reflecting the incoming
> signal.
>
> In a clear parking lot, with more than 6 satellites in direct view, it will
> place the GPS in its location in a single parking space with no multipath
> in flat country. In city canyons of high rise buildings where the actual
> direct view of the sky is only a few degrees in most directions, you're
> lucky if it stops going crazy in the area of the whole aforementioned
> parking lot at the mall. It'll be all over the place as you move around,
> even walking and really screwing it with multipath going in a dozen
> directions at once.
>
> GPS is very dependent on TIMING, the time it takes the signal to get from
> the precisely transmitting bird to your receiver's fixed delay.
> http://electronics.howstuffworks.com/gps.htm
> This website does a great job, in a series of sections, to show how GPS
> works and that TIMING is EVERYTHING in the GPS system. The text is nicely
> detailed without becoming the math monster of spherical trig the scientific
> websites quickly become. Just take the calculations for granted.
>
> It's really quite neat how the cheap GPS gets an atomic reference clock for
> free (displaying it on your screen, quite handily).
>
> To really see what TIMING means to your GPS accuracy with awful multipath
> making the paths from the birds go crazy, just take your GPS mapping box
> into an open restaurant with a metal roof to block out the direct signals
> and large windows on at least 2 sides so the signals can bounce around off
> the traffic, buildings, etc., the GPS can see through the window. Move
> away from the windows far enough so you cannot see the sky from your table.
> Sit, eat, and watch the crazy patterns of a system gone mad.....

That's where "assisted GPS" (aGPS), as used on many cell phones like my
V3m helps out. By getting much of the satellite data via the cell signal
using data from the tower's GPS, the accuracy is enhanced, even within
buildings or other situations where the sat signals are partially
obstructed. Your location is also pinpointed much faster, since a lot of
the computations are handled by external computers instead of in the
handset. It's still much more accurate outside in the clear of course...

--
Dutch