[time-nuts] Positional accuracy of the M12+T

[Dr Bruce Griffiths]

bg at lysator.liu.se wrote:
> On Wed, January 3, 2007 13:40, Stephan Sandenbergh said:
>
>   
>> The datasheets mention <25m SEP (haven't got a clue what 'SEP' stands for)
>> positional accuracy. However, I am sure this is relative to the actual
>> datum. Does anyone know where I could find information on relative
>> positional accuracy within multi channel common view configurations?
>>     
>
> GPS have strange error metrics, Spherical Error Probability, iirc is 50%
> of the 3d positions within a sphere  with a radius (diameter?) of 25m.
> Maybe the metrics come from the military "background" of GPS. SEP says
> nothing about gaussian error distribution and 50% is even less than 1
> sigma, whatever that says.
>
> Hmmm... with 25m position accuracy (and 100ns is about 30m), how do they
> really get time down to a few ns. Clever engineering!  :-)
>
>
> --
>
>    Björn
>
>
>
>
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Björn


The metrics have nothing to do with the military origins of GPS. Similar 
metrics are used in 3 dimensional surveying. They are a consequence of 
the 3 (neglecting time) dimensional geometry involved in determining 
position. A spherical error volume is a crude approximation, actually it 
is an ellipsoidal with  as the height error is usually significantly 
larger than the other positional errors which also may have different 
rms errors. The errors are assumed to have a gaussian distribution with 
different standard deviations for each coordinate axis. There may also 
be some correlation between the errors for each axis.

The concept of spherical error probability seems to have been introduced 
for those who insist on a single error measure.

Bruce