[time-nuts] Averaging Location for Position Hold

Miguel Gonçalves mail at miguelgoncalves.com
Tue Sep 13 13:55:25 UTC 2011

Hi all!

I am installing a timing GPS unit over a new location where I already have a
NMEA GPS with PPS (let's call it unit A). The NMEA GPS is logging every 16
seconds its GPGGA string.

I have a log with around 213400 samples. Each line is similar to this one

55817 48494.500

I averaged the minutes of latitude and longitude, the MSL (278.9) and the
Geoid Separation (51.2) and got the following

40º 55.2155' N 8º 29.6044' W (MSL 268.195m and Geoid Separation 51.2m) (for
unit A)

I entered this location on Google Maps I got a position that is in the
middle of a close street (see footer for image; real location is the red

I also have another GPS unit (let's call it unit B) that is on the roof and
did the same calculations (36500 samples):

40º 55.2228' N 8º 29.6103' W (MSL 262.66m and Geoid Separation 51.7m) (for
unit B)

When I plot this with Google Maps I get the exact location of the antenna
(within 1 meter; see footer for image; real location is the red dot). I am
assuming the first unit has some kind of internal error. Or, does Google
Maps has an offset?

I need this information to enter the precise location in the Oncore UT+ unit
I will be using at this location. I will be using the location of unit A.


The images available here: http://dl.dropbox.com/u/11210443/GPS%20A.JPG and
http://dl.dropbox.com/u/11210443/GPS%20B.JPG. I did not attach these to the
message to reduce traffic on the mailing list server.

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