By my checking, WAAS enabled, dual antenna DGPS receiver boards are cheap -
on the order of $10US in OEM quantities. The specs say 1 meter RMS in Lat
Long and 6 meters RMS in altitude when a DGPS signal is available. Of
course, they probably aren't in approved loggers.
6 meters in altitude is a lot better than a barometric altimeter on a
non-standard atmospheric day.
I wouldn't want to change ATC's reliance on barometric altimeters. On a
hot day, they understate the real altitude, giving us western US guys
another 1000 or so feet to play in below the floor of Class A airspace.
I checked GPS altitude a couple of times by putting a hand held Garmin GPS
on a prime US Geodetic Survey marker. The marker said 10,346 feet. The GPS
said 10,350 feet + or - 70 feet. The + or - error estimate seemed pretty
pessimistic. Those are pretty typical numbers.
Bill Daniels
"Ian Strachan" wrote in message
ups.com...
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.
The second part of the statment above IS true, that the GPS altitude
datum is not the same as the pressure altitude datum used worldwide in
aviation for altimeter settings for aircraft separation and for
controlled and restricted airspace.
In theory, due to the angle of cut of the lines-of-position from the
satellites, GPS altitude errors should be, on average, about 1.8 times
those for horizontal position or lat/long. Measurements over many
years by the IGC GNSS Flight Recorder Approval Committee (GFAC) show an
average lat/long error of 11.4 metres, taken from a moving vehicle at
surveyed points at about 51N 001W (Southern England, near Lasham
Gliding Centre). Going on this, an average GPS altitude error could be
expected as about 20.5 metres.
However, in a significant proportion of IGC-format flight data files,
there are significant anomalies in the GPS altitude figures that have
been recorded, in excess of the 20 metres mentioned above. Only today
I was commenting in another email on aspects of an IGC file from a
recent glider flight in the USA that had a 1500 foot overshoot in GPS
altitude (compared to the much more reliably recorded pressure
altitude) for reasons unknown.
The problem seems to be, particularly in low-cost GPS boards, that,
rather than processing a fix in three dimensions, it is processed
separately as lat/long and then separately as altitude. The algorithms
for lat/long and for altitude appear to be different, hence the regular
occurrence in IGC files or GPS altitude anomalies despite few lat/long
anomalies. Naturally, more attention seems to be paid by GPS board
manufacturers to lat/long rather than altitude.
In a survey made in year 2000 after the deliberate Selective
Availability error was removed from the GPS system by Presidential
Decree, no less than 27% of over 400 IGC flight data files analysed
from 7 countries in both hemispheres, had anomaliesof one sort or
another in the GPS altitude recorded in the file. From IGC files that
I have seen since, there is no reason to believe that this proportion
is much improved today. Just look at a large selection of IGC-format
flight data files and see for yourselves. In my database, I have
literally hundreds of IGC flight data files that show major anomalies
in recorded GPS altitude data. Fortunately, anomalies in lat/long data
in the same IGC files are very rare.
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
Ian Strachan
Chairman IGC GNSS Flight Recorder Approval Committee