[time-nuts] GPS altitude somewhat wrong?
Van Horn, David
david.vanhorn at backcountryaccess.com
Wed Jun 8 19:11:57 EDT 2016
From: time-nuts [mailto:time-nuts-bounces+david.vanhorn=backcountryaccess.com at febo.com] On Behalf Of Michael Perrett
Sent: Wednesday, June 8, 2016 4:33 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] GPS altitude somewhat wrong?
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)?
This is what's currently being displayed in LH after a 60 hour survey
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).
Currently set to 5 degrees, which I know is low, but I wanted to see what the whole sky map looks like.
I will dial it up as we get things settled in.
Finally, your 214' error is outrageous. For a surveyed position the answer should be with +/- 10'.
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?
> 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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