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

[Randy Warner]

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: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] 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.




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