At 22:06 24 August 2005, Ian Strachan wrote:

snip

Bill Daniels wrote:

snip

GPS provides highly accurate, although not ATC compliant,
altitude.

I am afraid that the claim that GPS altitude is recorded
'highly
accurately' in IGC files from IGC-approved GPS recorders,
is
unfortunately not true.



This is not an attack on the accuracy of the GPS system
or even its
altitude recording capability. It is a reporting of
results of GPS
altitude recording in IGC flight data files derived
from a number of
low-cost GPS boards made by a number of different companies
from
different parts of the world. I guess that in more
expensive
'professional aviation standard' GPS boards, and in
differential-GPS
systems with local beacons, the GPS altitude figures
are more accurate
and with less anomalies. But such (expensive) systems
do not apply to
the current 27 types of GNSS flight recorders that
are IGC-approved
(from 11 manufacturers) and whose IGC-approval documents
appear on the
IGC gliding/gnss web site:

http://www.fai.org/gliding/gnss/igc_approved_frs.pdf

The truth is that it is possible to record altitude
very accurately with GPS, suyveyors who produce our
maps use GPS both for lat/long and elevation with a
resolution in height of less that 15mm over 10Km. Perhaps
the reason that the manuafacturers mentioned above
do not upgrade their equipment is that there is no
demand as the IGC refuse to consider using GPS altitude.
However good a baro recorder is it can never approach
the accuracy of GPS.