At 09:04 AM 7/10/2009, you wrote:
HA
There's an interesting and very technical article in a recent Cross
Country magazine by an international comps judge on GPS altitude
errors. From memory the error can be as much as +/- 175 metres, a bit
too much to be used for airspace violations or record claims.
You can't use GPS altitude *directly* for airspace violations as
airspace boundaries are defined by pressure altitudes either by
reference to Area QNH or 1013.2 hPa or some other agreed altimeter
subscale setting. If you have the temperatures in the air layer under
you and the location you can derive a "pressure altitude" from the
GPS altitude but ALWAYS any measurement and correction should have error bars.
I've sat in a boat where the GPS altitude was 165' ASL which at the
time seemed unlikely.
Depends on your location and the deviation from the WGS 84 model of
the Earth at that location and whether your GPS receiver is in 2D or
3D mode and what the superstructure of the boat is doing to the GPS
receiver's view of the sky (or somebody may have fiddled with the GPS
settings and be using other than WGS84 reference). We set our GPS
receivers to only report a fix if it is a 3D fix. Boat GPS units
probably don't do this and deep in the setups, which not may be user
accessible after manufacture, the unit may not be optimised for 3D
use. Some early GPS units weren't very good on altitude anyway which
led to things like some early Colibris having very long smoothing
times on the GPS altitude record leading to false impressions of what
the glider was doing at any particular moment.
An awful lot of people are using GPS for final glides. I've never had
a problem with doing that and I also have a GPS antenna on the roof
of my house with retransmit into the workshop and the house doesn't
seem to move up and down more than about 10 metres.
As ever it would be nice if more people learned about measurement.
+/- 175 metres was what GPS altitude used to be like before SA was
turned off in 2000 and the receiver deep setups can make a big difference.
Here's a nice discussion of part of the problem.
http://bhgc.wikidot.com/tutorials:differences-between-pressure-and-gps-altitude
Mike
Additionally, most of the manufacturers like Flytec, Brauniger and so
on who make low cost GPS, logger, vario, ASI, barograph units have
currently not got igc certification because they claim it is too
expensive.
So we have a situation where to keep a device low-cost, it is not
certified but they do function as very good position recorders.
DMcD
_______________________________________________
Aus-soaring mailing list
[email protected]
To check or change subscription details, visit:
http://lists.internode.on.net/mailman/listinfo/aus-soaring
Borgelt Instruments - manufacturers of quality soaring instruments since 1978
phone Int'l + 61 746 355784
fax Int'l + 61 746 358796
cellphone Int'l + 61 428 355784
email: [email protected]
website: www.borgeltinstruments.com
_______________________________________________
Aus-soaring mailing list
[email protected]
To check or change subscription details, visit:
http://lists.internode.on.net/mailman/listinfo/aus-soaring
