I seriously doubt they can detect the azimuth. How should that work with just one antenna? Even with two, it would have to be a very controlled environment (antenna position, distance, etc.).
I've been using
PowerFLARM for some time with good results. Obviously, it's most useful in areas where gliders are equipped with FLARM (right below clouds, white against white, often not detected by radar). PowerFLARM cannot detect Mode C but they have promised this feature even though I've heard from several experts that it is barely feasible due to the way radar stations query transponders and the inherent ambiguity of assigning Mode C responses to the originating stations. However, Mode C is technically illegal in Europe (where transponders are required). The next firmware version (currently in beta) adds TIS output for showing traffic on e.g. Garmins.
I've also heard good things about the
Funkwerk product.
When EASA plan to come out with the next expensive, mandatory equipment change, they should require extended squitter. All Mode S capable transponders have to support this feature (part of the certification) and once transponders always send out the aircraft's GPS position, traffic monitors become a lot more useful. Today, all aircraft > 5.7t are required to do the extended squitter and they show up nicely on the PowerFLARM.