Originally Posted by Ron Black
Did anybody ever seen the duplication effects or knows why they occur?
The 24 bit Mode S address should uniquely identify an aircraft. A Mode S radar sends addressed interrogations to which only the transponder with the corresponding address will reply. This is the major difference to traditional Mode A/C interrogations to which all transponders in the radar beam will reply.
Internally, a Mode S radar maintains a database of all tracks in the coverage area of the radar. This allows the system to schedule interrogations for the moment the radar beam will cross a track. The track database is then updated with information acquired from the transponder replies. The radar then reports to downstream ATC systems with data from the track database.
When two aircraft with the same address are in the same azimuth of the radar it is difficult to determine which aircraft replied to the interrogations. This results in data from one aircraft being associated with the track data of the other.
A lot of data is not updated every radar rotation. In order to reduce the amount of transmissions, most data that doesn't change frequently is only downloaded from the aircraft when it has recently changed. To facilitate this, the transponder announces data changes by setting a specific flag in all replies, which triggers the radar to interrogate for the updated data.
Now imagine two tracks with the same Mode S address crossing the same radar azimuth. One aircraft announce a new flight ID, the radar interrogates for it, and the other aircraft replies. This causes the two tracks to have the same flight ID until the radar interrogates for flight ID again, which may be a long time.
The same applies to interrogations for squawk code, although that is not as persistent.
Originally Posted by Ron Black
Can all equipment do code-to-callsign conversion?
The label assignment (code to callsign conversion) is not done by the radar itself, it is done by the flight data processing system. It works on squawk code (and flight ID if squawking 1000), so if the radar supplies wrong data things get messed up.
Originally Posted by DaveReidUK
While that would affect the Mode S information that the controller sees, both aircraft would still be responding independently to Mode A and C
Note that Mode S radars, when interrogating Mode A/C, add an extra pulse so that Mode S transponders will not reply. The Mode S radar will do an addressed interrogation for altitude (Uplink Formats 4 / 20) and identity (Uplink Formats 5 / 21) for Mode S transponders. If the duplicate address aircraft are close to each other (in distance and azimuth) there is a change that the replies are swapped difficult to read due to overlap.
For TCAS II, the problem is even bigger. Since the TCAS algorithm does not track the direction of the replies, a duplicate address may cause a single target to be formed, with fluctuating distance. The distance measurement will be so noisy that TCAS will not be able to work correctly.
For ADS-B receivers, duplicate addresses will cause problems because the 24 bit address is used to correlate ADS-B messages to an aircraft.
I don't know about the effects on ADS-C.