[EMAIL PROTECTED] 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 > > > > > _______________________________________________ > time-nuts mailing list > [email protected] > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > >
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 _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
