> Basically what I want to know is this:  can I use a pressure
> sensor with an absolute range of up to 30 PSI to measure within
> 10 to 20 feet the difference in altitude between the current position
> of a sensor package and the position at which the package was
> initialized?  At altitudes of 1000 feet or less?

Yes you can get resolution of 10-20 feet with an absolute pressure
sensor using something like a Motorola MPX series sensor.  However,
you also need to know barometric pressure, and to get barometric
pressure the user needs to input that number (either in millibars or
inches Hg).  If you don't know the barometric pressure, a weather
change will fool the device into thinking it has climbed or descended.

If you were using a GPS to get altitude information, the variation is
+-300 feet, not good enough for your application.

In aircraft, when landing at an airport there are two ways to set the
altimeter: 1., set it to the airport's elevation (which whill change
the barometer setting in the altimeter) or change the barometer
setting in the altimeter which will change the altitude, and that
altitude should agree with the field elevation.  Some airports have
radio frequencies to dial in to receive automated current altimeter
setting in barometric pressure, so you can listen in and adjust the
altimeter.

Regards,
Bob
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