wrote in message
ups.com...
Thanks, but how exactly does the TCAS get the coordinates of the
transponder it talks to, in determining whether or not an evasive
mechanism needs setting in? Anything like a +/- 50 meter accuracy there
would be disastrous.

TCAS typically uses the transponder output to determine altitude of the
other traffic, and basic radio direction and range-finding processing to
determine direction and distance.

For the purpose of TCAS, +/- 50 meters is perfectly sufficient. Alerts are
provided at distances much greater than that, and an error of 50 meters
would affect the timing of the alert by a second or less. When in a
congested area, where TCAS is most typically important, airplanes are
traveling on the order of 50-100 meters per second.

You seem to be under the impression that flying involves a very high degree
of accuracy with respect to position information. That's simply not true.
Even in a differential GPS precision approach or a Cat III ILS, accuracy is
only to within a meter or so, and for anything else tens, hundreds, or even
thousands of meters is sufficient (depending on the exact situation).

In addition, while you are relying on your GPS receiver for altitude,
airplanes almost never do. They have barometric altimeters that provide a
different reference (not necessarily more accurate...just different) that is
more appropriate for the operation of an airplane (generally speaking). So
the GPS data is used only in two dimensions, in which the instantaneous
accuracy is generally much better, as compared to 3D accuracy including
altitude.

Your demand for accuracy in construction is FAR greater than any need
aviation has. As far as the difference in the GPS receiver goes, it is
likely that the basic operation of the GPS receiver you are using is
identical to that of an aviation handheld.

That said, I'm surprised you are using sample averaging to gain the accuracy
you need. Surveyors do use differential GPS with great success, and it
provides similar accuracy to what you're getting, only in a matter of
seconds rather than hours. (DGPS used for surveys has better accuracy than
that used for aviation because the reference point is so close to the
measured location in the case of surveying, whereas it may be quite a
distance away for aviation).

Pete