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   Re: [xml-dev] Round 2: Identifying Data for Interchange

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At 11:28 AM -0500 1/7/03, Roger L. Costello wrote:
>DERIVED DATA: Distance represents data that is the output from doing a
>calculation on raw, position data.  The distance data is called derived

You've got this backward. Position is always derived from distance, 
azimuth, or elevation measurements and there may not be enough 
measurements to calculate position. Even GPS calculates the position 
by measuring the distances from satellites. Most ATC radars only give 
range and azimuth, the altitude is generally provided by transponders 
in the aircraft that may or may not be giving accurate readings. Some 
aircraft may have collision avoidance radars and what they basically 
measure are the direction (where the radar is pointing, some don't 
provide this), range (how long the signal takes to get to the target 
and back), and range rate (the doppler shift of the return signal).

It's been a while since I worked in this area, but it is generally 
dangerous to rely too heavily on position; it may not be available 
and it's generally inaccurate (yes, even GPS can be inaccurate when 
the DoD decides to place the system in war mode and inertial 
navigation systems are subject to alignment errors and human errors). 
In some systems the rough position might be acceptable (an airline or 
a military air operations control center wanting to know 
approximately where most of its planes are), but for most of the 
critical processing it's not going to work. Keep in mind that you 
also need to worry about accidental and malicious aircraft or other 
flying objects in your system.

In summary, my opinion is that making position your "lingua franca" 
is a major mistake.

Don Bate

Don Bate               | Specializing in Consulting and Mentoring in
Bate Consulting, Inc   | Object-Oriented Technologies,
                        | Software Architecture, and Software Process
(972) 618-0208 voice
(972) 618-0216 fax


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