[balloon-makers] GPS Altimeter

Keith Sproul ksproul at rci.rutgers.edu
Thu Mar 7 08:27:41 CST 2002

The GPS units with built-in Baro Altitude are GREAT!!

I have the Garmin eTrex VISTA, and the new Magellen Meridian Platinum 
also has baro altitude.

Both of these also have max ascent and max descent rates, so after 
you land, you can find out how fast you went up and down...  You can 
even watch this in real time. I used this when I did my terminal 
ascent.  (1231 fpm)

The altitude in 'normal' GPS units isn't as good.  This has to do 
with how GPS works.

To get a good position report (x/y), you want signals from sats on 
the horizon.  To get a good altitude, you would want signals from 
sats overhead.  These two 'wants' are 90 degrees (literally) out of 
phase with each other.,   So, getting a good horizontal position 
makes getting a good vertical position less accurate

Keith Sproul

>In my experience, GPS altitude is pretty good since they turned off
>Selective Availability in April 2000 - something I like to much I
>consider it the crowning achievement of Bill Clinton :-) Basically, it
>is almost as good as the results I was getting using local home-build
>differential correction transmitters. Greg and Phil both noted how
>surprised they were to find their GPS altitude very closely matched
>their altimeter on their gas flights last Fall.
>The problem with using GPS for altitude is that a GPS has no clue where
>the surface of the Earth is. Our planet isn't a sphere. A GPS has to
>"imagine" that the surface of the Earth is some built-in mathematical
>model. This is why we use WGS-84 in the US. It is a survey model of the
>Earth that models the Earth's curvature for North America rather well.
>There are hundreds of other similar models you can select with your GPS
>that model other parts of the world more accurately. It is possible that
>the altitudes could be significantly off from that based on atmospheric
>pressure (and calibrated...)
>Also, the altitude rate-of-change period which most GPS receivers use I
>think is too long for serious balloon use.
>GPS's are fun in the basket, but I don't really think they will replace
>an altimeter or vario.
>Disclaimer: I've got some burner time, but I don't claim to be a pilot.
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Keith Sproul	ksproul at rci.rutgers.edu		WU2Z
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