[time-nuts] GPS altitude somewhat wrong?

[Van Horn, David]

-----Original Message-----
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?

60+ 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?

Damifino.  :)

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'.

Ok..



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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