[BC] GPS recommendations

[Broadcast List USER]

>From: Alex Hartman <goober at goobe.net>
>
>I'm curious of this as well. My phone does have GPS stuff on it, but it's not nearly as accurate as i'd like (~10-20ft). I'd like something that's within a few feet and a cold boot time under a millenia. :)

Here is the problem, and no one wants to discuss it.

The speed of radio waves from the satellites is not constant!  This  
means the timing is not constant.

So, no matter what you do (without differential signals, which are  
land based), you cannot get better than about (give or take) 30 meters.

The best example I can give are fresnel zones on STL paths.  The  
signal wanders around.  Now, imagine how much more the path wanders  
when the path is tens of thousands of miles instead of tens of miles!   
The only way to improve on this accuracy is through frequency  
diversity, which is what the military does.

GPS receivers use a number of ways to "improve" accuracy.  The most  
basic is to use map snapping.  They snap you to a road.  I don't turn  
that on when I have a chance.

Your phone might use WiFi to try to pin things down better, but you  
are not generally going to get 10-20 feet on a consistent basis.

--chip