[BC] GPS recommendations

[Cowboy]

On Thursday 14 June 2012 11:21:46 pm Broadcast List USER wrote:
> 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.

 This is and has been known from near the beginning.

 One of the ways this is "corrected" is time averaging.
 The longer a GPS receiver remains in one spot, the more accurate
 its reported location.

 The satellites themselves also wander a bit in the sky, introducing
 a positional/path/timing error.
 Once every ten minutes or so, I forget, might be once an hour,
 correctional information is transmitted, based on what's being
 received from the satellite at a known ground receive location,
 as well as corrections for the on-board clock.

 The issue isn't really the accuracy.
 It's being able to know when it's accurate, and when it's not, 
 and when it's not by how much.

 The "snap to map" functions can provide a rather dramatic example
 of Heisenberg compensation in a vehicle especially when transitioning
 from moving to stopped, and vice versa.

-- 
Cowboy