Hal, The height error is larger mainly because you don't have any satellites UNDER the antenna. If you run your GPS antenna in a window you will see a similar effects on the X/Y accuracy as you are only seeing satellites in a narrow area of the sky and the DOP goes way up. Without any satellites under the antenna your "altitude DOP" is huge. Note that this is a simplification of what is going on, but it gets the point across. The altitude error is normally about 1.5X the X/Y errors, depending on whose receiver you are using.....
The "ideal" geometry (again, this is just a generalization) is to have 1 SV directly overhead and three others about 10 degrees over the horizon and evenly spaced (120 degrees apart in azimuth). Some people will argue that having the 3 low satellites even lower would decrease the PDOP, but you run into ionospheric error problems that outweigh the decrease in PDOP. Randy ________________________________________________________________________ ______________________________ -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Hal Murray Sent: Wednesday, January 03, 2007 2:23 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Positional accuracy of the M12+T > 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. Why is the height error usually larger? Is that just geometry? Do I get good height data if there is a satellite close to overhead? -- These are my opinions, not necessarily my employer's. I hate spam. _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
