Clearedtoreenter said,
Of course it does require the other aircraft to be fitted with a transponder that works and is switched on, which means it possibly detects about half of the aircraft where I fly.
AND and it is a big one,
Provided there is an external interrogation source such as SSR radar or a Multi-Lat array or TCAS equipped aircraft within electronic range.
If there is no external interrogation source for the TXPDR then the unit does not work.