[TowerTalk] GPS Receiver Information Please

[Jim Lux]

It's not only the estimated position error you have to worry about.

The original questioner wanted height.  GPS is generally not as accurate in
the vertical axis as in the horizontal (highly dependent on the satellite
configuration).  You might do better by getting an accurate lat/lon and then
getting height from a map or database.

The other thing is the repeatability.  It's kind of site dependent, but
you'd need to do some checks to make sure that the positions are consistent
and stable.  For instance: Set 3 stakes in the ground some 20 feet or more
apart.  Stand over one stake, measure the position.  Stand over the other
stakes, one at a time, measure the position.  Check and make sure that the
positions are consistent (does the GPS position actually correspond... FWIW,
working in UTM is a LOT easier for this).  Then, go out the next day, or 6
hours later, and make another set of measurements (when the satellites are
all in different positions, the ionosphere is different, etc.)


True differential GPS is amazing.  However, unassisted GPS sometimes appears
spuriously accurate (you get a good fix, because the SNR is high and there's
limited multipath, but the position is "wrong" in an absolute sense).  The
position error displayed on your receiver is really only calculated from the
received SNR and a limited geometric dilution of position calculation.  WAAS
is a sort of half way to DGPS solution.

These are the sorts of things that separate the $100 receivers from the
multi-thousand dollar receivers used by surveyors.


Subject: Re: [TowerTalk] GPS Receiver Information Please


>
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> I have had excellent luck, using my Garmin eTrex Legend
>
> Is WAAS enable and using locks to within 9 feet or better pretty
> easily when I am sitting still
> Mobile I run about 12 feet on average, mainly because of the limited
> sky view it has then.
>
>
> 73
> - -----------------------------------
> John
> N5BSD

>
> I am in need of information about a GPS Handheld receiver which I can
> use at work to install a point-to-point 5 gig back bone.
>