Re: Metric Instruments

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" <ian@xxxxxxxxxxxxxxxxx> wrote in message
news:1124920830.685720.42150@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> 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
> ian@xxxxxxxxxxxxxxxxx
>

.

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