I understand why the pilots need to be able to switch off the transponder in case of malfunction. However, would it be possible to have a secondary transponder on board which would be automatically activated in the event that the primary was switched off? Perhaps the backup could even be battery powered, and could be set to squawk a particular "warning" code. (A warning code indicating there -could- be a problem, rather than an emergency code indicating hijack, etc.) Is there a reason that wouldn't work? It would be unlikely that two different transponders with two different power supplies would both have electrical malfunctions at the same time, so there would be no reason to shut both off.