gps altitude accuracy

On Thu, 10 Jul 2003 08:52:52 +1000, Mike Borgelt
wrote:

On Wed, 09 Jul 2003 11:00:12 +0100, Martin Gregorie
wrote:

On Wed, 09 Jul 2003 10:27:00 +1000, Mike Borgelt
wrote:

...snippage...

For badges and records ballooning and I think the rest of aviation
converts pressure readings from barographs etc to geometric altitude.
It is long past time we did this in gliding. The Ballooning people
have a nice worksheet to do this. FR's would get considerably cheaper
if the pressure altitude requirement was dropped.

Mike, do you mean that non-gliding FRs and barographs also record
ambient air temperature?

If that's not the case then surely they can only do what we do and use
the calibration chart to correct the FR altitude to the standard
pressure altitude without temperature corrections.

- Curious of Essex


No, the worksheet asks for the mean temperatures in the layer in
question by interpolation from met office temperature soundings at two
or three nearby stations. Also QNH values at the stations at the time
in question.
The whole process is done properly with error bands etc and you get
credited with the minimum after the errors are accounted for. All very
proper and obviously designed by someone who knew what he or she was
doing unlike anything official I've seen in soaring.

Thanks for your explanation.

Pressure altitude in soaring barographs and FR's is a joke. The
calibration chart was done in the lab at room temperature and we
expect all this to be the same at -40 degrees at 30.000 feet.
Lotsaluck!

Thanks for that, too. I had wondered if that might be the case.

I have to ask, though, does that matter? Unlike the situation in the
real atmospheric column the temperature in the chamber can't affect
the pressure unless there's a temperature dependency in either the
chamber's pressure measurement or (more likely) in the FR's pressure
sensor.

I'd appreciate your thoughts on this too.

GPS altitude will give this directly, the only matter for discussion
is what error band we put on it. I'd suggest add 100 feet to the low
point and subtract 100 feet from the high point. This is probably
conservative in the direction of crediting you with smaller altitude
gains.

I've no argument with that!

BTW, there's been a lot of discussion of the effect of EPE error on
height measurements, but are there any systematic GPS errors that
don't show up in the EPE figure? What about satellite clock drift and
ephemeris errors? I've been looking at

http://vancouver-webpages.com/peter/

but it doesn't appear to answer this question although it does give
all the error sources and their magnitude. Judging by the EPE figure I
usually see, my GPS II+ may only be calculating the EPE from
ionospheric and P-code error estimates.


--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :