>> In other words, the calibration method will probably
>> have to involve multiple known distances at several
>> frequencies, AND this will all probably shift with
>>temperature.
>
> And pressure and to a lesser extent humidity, basically
> anything that effects the speed of sound in air.

Important is the current phase of Moon. Moon/Sun induced tidal process
in the atmospere cause local air pressure variations thus affecting
the speed of sound in air.
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist