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

[Gerry Creager - NOAA Affiliate]

Gary

On Thu, Sep 13, 2018 at 3:18 PM, Gary E. Miller <address@hidden> wrote:

Yo Peter!

On Thu, 13 Sep 2018 19:45:30 +0000 (GMT)
Peter Liu <address@hidden> wrote:

> I cannot find any definitive answer to whether the Altitude is HAV or
> MSL. Also, which datum is assumed here?

No real definitive answer is possible.  Different GPS report altitude
differently.  gpsd has no way of knowing how your GPS is programmed
and/or configured.  So it just passes on what your GPS told it.

Most GPS will provide MSL calculated internally from ECEF and WGS-84.
But many can, and do, use other datums.

We burned this bridge in the early days of GPS hacking, too. Virtually all calculations are ECEF, then transformed to a cartesion system of some sort. The standard datum for GPS is WGS84, as that was the codified worldwide datum for GPS. Other local datums may be used, and some receivers do a good job of the datum transformation. Others, respond like a bored graduate student who doesn't understand matrices. One reason Motorola put fairly significant processors in the PVT6 originally was to handle the datum transformationson the fly. 

Check your GS doc, and then compare to a known benchmark.

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

At least that's the academic version of things. 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.

gpsd tries to program u-blox to output ECEF.  Then gpsd uses the ECEF
and its own WGS-84 conversion to get to MSL.  When compared to USGS
benchmarks the agreement is very good.

So, as always: YMMV.

RGDS
GARY
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V/r

Gerry

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Gerry Creager

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