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Re: [gpsd-dev] [gpsd] Altitude in TPV

From: Gerry Creager - NOAA Affiliate
Subject: Re: [gpsd-dev] [gpsd] Altitude in TPV
Date: Mon, 29 Oct 2018 14:09:01 -0500

Long delayed response (work got in the way of looking at hobby-related emails...)
On Fri, Sep 14, 2018 at 1:38 PM Gary E. Miller <address@hidden> wrote:
Yo Gerry!

On Fri, 14 Sep 2018 07:49:58 -0500
Gerry Creager - NOAA Affiliate <address@hidden> wrote:

> > I often put two GPS side by side.  Their lat/lon will agree
> > closely, but their altitude often differ by 20 feet.  I had one $8k
> > GPS that was off by 64 feet!
> > 
> For code-phase receivers, this makes sense, especially for single
> frequency models. Code phase has trouble resolving vertical at much
> better than 3-5 meters, and will bounce around a fair bit.

True, in the recent past.  I have at least two models of newer GPS that
do much better than that.

> multi-frequency carrier-phase postprocessing will resolve centimeter
> heights for HAE. Othometric heights require just a little more
> processing to accomplish.

So I thought.  But my recent practical experience has not shown that.
At least without post processing.

> At least that's the academic version of things.
Yes, academic, not experimental.

> I did some work using
> L1C, and multi- and long-baseline L1/L2 code and carrier phase work on
> orthometric height determination a few years ago.

In this case, the academic version was based on a field program and rather rigorous analysis. Some of my work eventually was utilized by the National Geodetic Survey.
I have an L1/L2/L5 GPS.  I was expecting great things, but never saw
them for real time measurements.

Not so much in code phase. Long term averaging of data collected at a fixed interval (e.g., 10-30 sec) then decimated would give a better representation. We can talk about the need to decimate if you'd like.

Gerry Creager
“Big whorls have little whorls,
That feed on their velocity; 
And little whorls have lesser whorls, 
And so on to viscosity.” 
Lewis Fry Richardson (1881-1953)

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