From: Dr Bruce Griffiths <[EMAIL PROTECTED]> Subject: Re: [time-nuts] Positional accuracy of the M12+T Date: Thu, 04 Jan 2007 11:03:01 +1300 Message-ID: <[EMAIL PROTECTED]>
Björn and Bruce, > [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. For many purposes the heigth value is a mear detail and has no real interest and then only the horizontal deviation would be of any interest (and here it should be more accurate). Also, one-sigma measures are good for reference, but two-sigma or three-sigma values is usually of more interest for those people. Also, that is somewhat of a worst case number. You are usually better, but as a good engineer, what value do you promise? :-) Cheers, Magnus _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
