As Mode S is required for a TCAS System all airliners have to have it. However that does not mean that they have a Mode S Enhanced Surveillance system. Those are mandatory from 2007 at least in germany but most probably in all of europe. Since that sends a lot of information including position you can create a radar like picture by just receiving those mode s packets, no moving antenna required. There is a system that has a receiver box and a bit of pc software to do just that, however it depends on the local laws regarding radio stuff if that is legal, over here owning it is legal, using it is not (weird but true).