RE: [Paparazzi-devel] distance measurement for landing.

[gisela.noci]

If differential measurements could be done, perfectly, with no errors, (one
sensor at the landing, one in the aircraft), then this would might care of
local condition changes and in a perfect world, you might land. Loacl
pressure at my abode (at sea level) changes easily by 2 to 5 millibar over a
day, and easily changes 2 millibar within 10 minutes. 2 millibar is approx
16meters....

However, perfect implies - sufficient resolution, better than 1/2meter,
appropriate accuracy, better than the resolution, Low sensor hysteresis,
better than the resolution, and a temperature compensation map matched in
the ground and airborne sensors. And then, you need to ensure that the Pitot
is properly mapped ( even more difficult to do than mapping the sensor),
since if the Pitot pressure versus speed and angle of attack curve is not
properly mapped, incorrect pressure will be read as the A/C slows and
changes angle of attack of the probe in the airflow. Hiding the sensor in
the fuselage to eliminate the Pitot is the worst approach, since you cannot
map the pressure changes in the fuselage at different airspeeds, throttle
settings and angles of attack. Repeatable and safe Autoland by Baro-Alt
alone is not feasible. 

The big guys have $20k US fully calibrated and compensated sensors and
upwards of $50k US Pitot's on the big jets and jet fighters and they all
rely on other methods of sensing for autoland.


Joe


-----Original Message-----
From: address@hidden
[mailto:address@hidden
] On Behalf Of Electro-Technik
Sent: Friday, November 20, 2009 4:17 PM
To: address@hidden
Subject: Re: [Paparazzi-devel] distance measurement for landing.

Hi,

I have been following with great interest the discussion about height
measurement for landing.

To steer further research effort in the right direction, I would like to
pose the following question.....

        "Even if local atmospheric pressure could be measured perfectly
accurately, with no errors, would this be the best parameter to use to
facilitate an automatic landing?"

Just a thought...........

Regards

Alan K.

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