[time-nuts] GPS altitude somewhat wrong?

Tom Van Baak tvb at LeapSecond.com
Wed Jun 8 22:26:21 EDT 2016

Hi Gary,

>From one NW GPS farm to another... I'm willing to help you debug yours.

> A Garmin 18x reports:
>    Altitude Err:    +/- 264 ft   

Something is terribly wrong with your setup. The Garmin 18x is much, much better than this. I know because the 18x was one of the GPS receivers I brought along on a recent mobile clock experiment.

Here is a plot of 2 hours of GPGGA raw altitude data while stationary in Tucson:

With clear sky view, the peak to peak is under +/- 8 m, and the (1-sigma) standard deviation is 3 m.
Even at the hotel lobby, with obstructed sky view, the (1-sigma) standard deviation stayed under 7 m.
Your 18x number, +/- 264ft (+/- 80 m), is 10x to 25x worse than this. It doesn't feel right.

Off-list, can you send me a day of NMEA from your 18x? Not gpsd output; but the raw serial ascii data from the receiver. I'd like to get to the bottom of this. We'll all learn something.


----- Original Message ----- 
From: "Gary E. Miller" <gem at rellim.com>
To: "Van Horn, David" <david.vanhorn at backcountryaccess.com>
Cc: <time-nuts at febo.com>
Sent: Wednesday, June 08, 2016 3:24 PM
Subject: Re: [time-nuts] GPS altitude somewhat wrong?

Yo David!

On Wed, 8 Jun 2016 20:33:58 +0000
"Van Horn, David" <david.vanhorn at backcountryaccess.com> wrote:

> I suppose 214' isn't that outrageous, but it does bring me to a
> question:

Looking at my GPS farm, fair but not great skyview.

A Garmin 18x reports:
    Altitude Err:    +/- 264 ft   

A u-blox 8:
    Altitude Err:    +/- 36 ft         

And an adafruit HAT (u-blox 6?:
    Altitude Err:    +/- 69 ft 

On a Skytraq I have seen +/- 12 feet.

> How accurate is the altitude number really?

Depends.  :-)

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
 gem at rellim.com  Tel:+1 541 382 8588

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