Radarbox/SBS-1 only show aircraft with full ADS-B, mostly newer Boeings/Airbus. CRJs, Dash8s, ERJs, biz jets, mil, GA etc don't show (normally) so it wouldn't be any use in the air.
Yep. I'm not the worlds expert on mode-S, but I seem to recall there's "Elementry mode S" and "Enhanced mode S".
Elementry mode S is what you will find in a typical spamcan. It's to a very large extent a mode A/C transponder (4-digit ident plus altitude encoding), plus the following:
- Transmission of a flight ID (e.g. your callsign)
- Transmission of the 24-bit (?) ICAO ID
- Selective responsiveness to interrogations: It only responds when the radar head wants to specifically interrogate your transponder, or when the radar head emits the 'all stations' signal.
Enhanced mode S is what you will find in the big iron. This transponder is integrated with the FMS and will include in its response various parameters of that FMS, including, but not limited to:
- Current pressure altitude and selected pressure altitude (stop climb/stop descent)
- Current heading and selected heading
- GPS/INS derived position
It's this last parameter, the GPS/INS derived position which is used by SBS-1 and similar "virtual radar" setups.
So as a TCAS-replacement it's not going to be much use unless your main worry is to be hit by "big iron".
And as far as ADS-B is concerned: I think (but am not entirely sure) that ADS-B is simply the encoding mechanism for the parameters that are both required under Elementry mode S and Enhanced mode S. So both types of mode-S transponder will emit an ADS-B signal, but Elementry mode S will only use that to encode a few parameters, and that list does not include the GPS/INS derived position.