ADS-B Exchange uses ADS-B and MLAT data. MLAT is derived from multiple receivers receiving Mode S messages from the same aircraft. Mode S is somewhat of a precursor technology to ADS-B and includes the six-hexadecimal digit “ICAO” code assigned to the aircraft but does not include the lat/lon coordinates or other information.
We calculate these coordinates for MLAT targets by timing when the signal arrives at the various participating receivers. The even older “Mode C” transponder technology does not incorporate the uniquely identifiable ICAO hex code, so Mode C only aircraft will not show up. Fortunately, ADS-B is required in a large (and growing) amount of airspace, so more aircraft show up every day. In fact, in most US and European airspace, ADS-B transponders are mandatory.
Note: transponders can sometimes be misconfigured and transmit the wrong hex code due to human error. Military planes sometimes on purpose use a bogus/invalid/unassigned hex id.