On Feb 23, 7:10*pm, Marc Ramsey wrote:
Papa3 wrote:
On Feb 23, 1:08 pm, "Bill Daniels" bildan@comcast-dot-net wrote:

Simularly, fusing pressure data with GPS altitude data could work the same
way. *For example, a millisecond scale spike in GPS altitude, not confirmed
by presure data could be ignored with impunity. *A slow drift of the
pressure signal from the smoothed GPS signal is almost certainly a synoptic
pressure change and can be canceled by the long term stability of the GPS
signal making the pressure signal more useful in detecting airspace
incursions. *(Think automatic Kollsman settings.)

Bill Daniels

Which, interestingly, is exactly what the $200 Garmins with Pressure
Sensor do!

Garmins do not have calibrated pressure sensors, in the "auto
calibration" mode, you get smoothed GPS (geometric) altitude, not
calibrated pressure altitude. *Without additional information from
outside weather sources, it can't be used for detecting airspace
incursions. *An altimeter setting is a temperature corrected "sea level"
pressure setting for a specific reporting station. *There will still be
discrepancies between pressure and geometric altitude at anything other
than the elevation of the station, as the average temperature of the air
column between will almost never match that modeled by the International
Standard Atmosphere. *This discrepancy can amount to over 1000 feet at
10000 feet above the station on a hot summer day...

Marc- Hide quoted text -

- Show quoted text -

Who said anything about airspace incursions? What we are talking
about is filtering "noise" from spurious GPS signals. It presents an
interesting possibility for providing real-time (as oposed to post-
flight) noise cancelling.

I should have snipped the second part of Bill's post, as that again
gets us back to the GPS vs. Pressure altitude debate.

P3