Multi-channels do better than single channel units, but in general GPS
(in open areas) can get you within 10m. Ground reference calibrations (what
you get with the mentioned auxiliary systems) can do better. It also helps
if you happen to have a Cesium time standard attached to your iPhone. :-)
The accuracy factor is known as GDOP (Geographic Dilution Of Precision)
and is mostly due to variations in atmospheric propagation, moisture, etc.,
which is why the local calibration systems at airports and such can take out
most of the uncertainty.
(Speaking as someone who has done 4th order phase-lock-loops for GPS
tracking of MX missiles.)
On 11/11/09 8:38 PM, "cocoa-dev-requ...@lists.apple.com"
<cocoa-dev-requ...@lists.apple.com> wrote:
>
> On Wed, Nov 11, 2009 at 4:41 PM, Glenn L. Austin <gl...@austin-soft.com>
> wrote:
>> Consumer-grade GPS is still limited to 30 meters (stationary) or 10 meters
>> (in-motion).
>>
>> The reason for the better accuracy for in-motion has (basically) to do with
>> doppler effect.
>
> Well DGPS or WAAS can improve beyond 30 meters but the iPhone doesn't
> contain such a receiver to the best of my knowledge.
>
> -Shawn
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