Quite the conversation. First off, I will caveat by saying I am not a proponent of ADSB, but not for most of the reasoning I see in this thread.
A system is needed to provide location and indentification of aircraft in the airspace. Mostly for ATC, but also for self separation and avoidance.
We have to face it, the airspace, like any other highway, is getting crowded and congested.
Unfortunately, the means and methods of tracking in that airspace, are stressed to the limits, and development around airports has significantly reduced RADAR capability.
Incoming ADSB. Currently, in my opinion, being touted as an intermediate /future step in airspace management, but actually, it is a technologically ancient system. ADSB uses the 800 Mhz system, recently moving ground ops to 1090 Mhz taking some of the load off. (you can see all of the 'commitees' working on the broadcast message)
The ADSB system has the capability, due to bandwidth, to manage 4000 'messages' per second. The message is each signal broadcast, but important, a signal is a single 32 bit message. 32 bits does not include one whole hell of a lot. (ie, a single message string includes aircraft id, but not location) Each broadcast string, per aircraft, to include ID and location, etc, will consist of many message strings.
Depending on the frequency of messages (typical is every 5 seconds) the data from the ac will be broadcast. So, long stry short, each aircraft, depending on the requirements, will be broadcasting multiple strings of data, at 5 second intervals.
It is also important to keep in mind that aircraft are not the only ones using this system and frequency.
Being an 800MHz system, (and 1090) there is a limitation to the bandwidth and sub frequencies available. This system cannot multipath, so when a freq/subfreq is used, it is unavailable to others. As a point of reference, many are familiar with push to talk hand held radios, ADSB is the same type of system that push to talk systems use, so when you push to talk, you use that bandwidth until you unpush. That is why the ADSB system for aircraft is limited to a 32 bit string.
That 32 bit string equates to 4000 ADSB messages per second. WOW that seems great!
Then reality creeps in once again.
Each aircraft broadcasting strings, ID, then location, then speed, etc..every 3 seconds, doesnt take long to determine in a crowded airspace, how many messages ADSB system will encounter.
A while back, we did a trial at Munich, well in 1995. When full capability was turned on, there were 40 THOUSAND messages per second. Remember above, the freq/system had capability of 4000/second. The ADSB was effectively shut down, hence the need for 'selective capability' of the system, or in regulatory speak, 'committees' to determine signal content and priority.
This is why mandates have been so far out. When some agencys, like the FAA, in 2005, set a deadline for 2020, it simply means, push off this deadline to after I retire so I dont have to make this work during my career.
Sooo, long story short.
In 2020, when the ADSB mandates and protocals are in full decided and put into operation, it will be the technological equivalent of using Morse Code on your cell phone.