[time-nuts] GPS altitude somewhat wrong?

Michael Perrett mkperrett at gmail.com
Wed Jun 8 18:33:19 EDT 2016

A couple of things come to mind:
1) Is this a single measurement or an average over at least 24 hours?
2) Did you get your elevation via the receiver survey mode (recommended)?
3) How close is your "nominal" elevation measurement and what makes you
think it is truth?
4) The vertical component of the GPS position solution is typically 50%
worse accuracy and a lot noisier than the horizontal measurement. If you
have a good horizontal measurement it is unlikely you have a "wrong answer"
on elevation since your receiver is using the same data, just solving the
equation for a different variable.
5) What is your satellite mask angle? The geometry (hence accuracy)
degrades as an increasing function with mask angle. Suggest for the survey
mode you use as low a mask angle as possible (typically 5 to 10 degrees).

Finally, your 214' error is outrageous. For a surveyed position the answer
should be with +/- 10'.

Michael Perrett

On Wed, Jun 8, 2016 at 1:33 PM, Van Horn, David <
david.vanhorn at backcountryaccess.com> wrote:

> I have just installed a Thunderbolt here to get our time and frequency
> equipment all on the same page.
> As I was looking at the display on Lady Heather, I was noticing that the
> GPS altitude seems rather wrong.
> We are in Boulder CO, which is nominally 5430' and the antenna is about
> 20' off the ground.
> The display (near overdetermined position) reads 1589.72991 meters or 5216
> and change in feet.
> Altitude is a big deal around here. :)
> I suppose 214' isn't that outrageous, but it does bring me to a question:
> How accurate is the altitude number really?
> Thanks.
> --
> David VanHorn
> Lead Hardware Engineer
> Backcountry Access, Inc.
> 2820 Wilderness Pl, Unit H
> Boulder, CO  80301 USA
> phone: 303-417-1345 x110
> email: david.vanhorn at backcountryaccess.com<mailto:
> david.vanhorn at backcountryaccess.com>
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