"The system provides a "picture" of all aircraft fitted with a mode S/ADS-B(Automatic Dependent Surveillance Broadcast) SSR transponder system within a line of site distance..."
It's almost straight from the advertising blurb, which is cleverly worded.
Track Mode-S/ADS-B equipped aircraft in real time*
...
The Civil Aviation Authority (CAA) requires aircraft to be Mode S equipped for flights operating as General Air Traffic (GAT) in designated UK airspace from March 2005 and for all categories of flights in all other airspace from March 2008.
What it doesn't say is that it only tracks the ones that do ADS-B, and there's no requirement for that anytime soon.
Don't get me wrong -- I'm not knocking the kit. I'm sure it's a super little innovation. But I think one has to be realistic about its capabilities.
The problem with unreliable position information can, I believe, be resolved by placing a network of Mode S receivers on the ground... the signal received from aircraft can be cross-checked (bit like DF auto-triangulation?), and in the event of error the aircraft's navigation equipment can be updated... i.e. no cost GPS?
Not sure you need a network. Mode S ground stations are designed to track the position of Mode S transponders using azimuth and range, just like any other radar. It's only when you take away the directional capability that they need the position sent to them.