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uk.rec.gps (UK Sat Nav) (uk.rec.gps) for the discussion of all aspects of the UK use of Global Positioning Systems and any other satellite positioning/navigation systems which may be developed. Also any improvements, or extensions to the above and radio navigation systems.

Variation in speed reported by GPS



 
 
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Old December 31st 12, 05:58 PM posted to uk.rec.gps
R. Mark Clayton
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Posts: 119
Default Variation in speed reported by GPS


"Mike Coon" wrote in message
o.uk...
R. Mark Clayton wrote:

Police radar guns can give a speed accurate to 0.001mph, although in
reality such accuracy should be taken with a pinch of salt (heartbeat
and breathing would have an effect much greater than that).

The position is resolved by calculating (solving simultaneous
equations for) x, y, z and t from four satellites to get an accurate
fix. By corollary the speed can be calculated as well. The only
problem is that the space vehicles are doing around 18,000mph
possibly in opposite directions so the speed of the receiver is very
small by comparison.
In practice taking the difference in two readings some time apart will
probably give a better result than Doppler measurement, however there
may be fluctuations when the constellation used changes.


I bet if I answered an exam question which said "show working" with the
words "By corollary" I would not get many marks!


Oh sorry: -

"The velocity may resolved by calculating (solving simultaneous
equations for) dx, dy, dz and dt from four satellites to get an accurate
vector.
"
which I think you will find means that working out the speed relative to
four satellites is analagous to working out the distance!



Surely the point of speed calculation using doppler is that it is distinct
from position calculations.


Sort of. The principle is the same - you work out your own speed from the
relative speed (doppler shift) of four satellites. Obviously you can take a
reference [zero] when you are stationary (as with comparing the calculated
position with a know fix - start or differential GPS transmission), or
reduce the calculation if travelling on the level in the same way as you
only need three satellites if on the sea


I find radar guns even easier to understand than the "doppler"
measurements of blood flow in my legs, in the local maternity clinic, a
few years ago...

Mike.


Is there something you are not telling us 'Mikela'?

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