[Openstreetmap] GPS reception in narrow city streets

[Matt Amos]

On Thursday 03 March 2005 11:42, Frank Mohr wrote:
> just to repeat my question from january
>
> I still have some problems with narrow city streets where
> the GPS accuracy gets realy bad.
> (several meters offset, left/right jumps on straight streets)
>
> The "London Free Map" page mentions a "window of opportunity".
> How do i calculate this time window for my location?
> Is there any tool available?

your reception will be slightly better if a satellite is in direct 
view, as the LFM page is suggesting. however, its not going to help 
the multi-bounce situation very much (thats probably whats causing 
your offset) and its only mildly going to help your accuracy.

the trouble is you really need at least 2 good locks and 2 more 
(possibly quite poor) to get a decent postion. in an urban canyon you 
can only see a small strip of sky and given the orbits of the 
satellites its very unlikely that two satellites will occupy the 
strip at the same time.

the best solution to this is to map the street multiple times and put 
a best-fit curve through the data. alternatively, carry an 
accelerometer and use von kalman filtering to help. (added advantage 
that it works in tunnels, too!)

cya,

matt
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