A post above suggested that monitoring all the local transmissions would be overwhelming, but effectively, you would only be aware of those stations that are transmitting on the area of the GPS map-screen you are currently viewing.
Just to clarify...Your receiver will be hearing everything within line of sight on 144.39. It doesn't matter what map you're looking at. If two stations transmit at the same time, the data packets will collide and both beacons will be lost. If there are 10 stations, no problem, if there are 200, we have a problem. Why would two stations transmit at the same time? Because they are probably at an elevation of 3' AGL and they can't hear each other. Unfortunately, if you're at 10,000' you're going to hear everything.
BTW, the other issue with hearing all of this congestion is that your tracker is waiting for a break in the action to transmit its packet. Imagine being in a crowded room, and you're unable to speak until everyone else is quiet.
Unfortunately, in some areas of the country APRS is a victim of its own popularity.
Here's one example of APRS congestion with only 50 stations:
http://mm.aprs.net/map.cgi?map=APRSworld&lat=41.02117&lon=-96.36550&call=*&range=1
For you guys in Fort Worth, I didn't bother counting:
http://www.findu.com/cgi-bin/map-near.cgi?zip=76137
Ok, enough with the doom and gloom. There are things that can be done to reduce the congestion.
1) Transmit less often (smart beaconing)
2) Reduce the number of digi's in your path (WIDE 1-1, 2-1, don't use RELAY)
3) Use a shorter beacon (mic-e compressed)
(Of course, this is a community effort)
How will it work in flight? Hard to tell.
Paige