A signalling system includes a plurality of active transponders (10),
an
interrogation station (12) and a source (14) of switching signals.
Each of the transponders has a microcontroller (20), a radio transceiver
(34) for responding to radio interrogation signals from the interrogation
station, the radio transceiver being coupled to the controller by way of switching
means (36) which switches the transceiver on or off whilst leaving the microcontroller
active, and a passive receiver (38) for receiving switching signals from
the source of switching signals which signals are used by the microcontroller for
switching-on or -off the transceiver. Transducers (26, 28, 30) may be coupled
to the microcontroller for monitoring environmental features, such as ambient temperature
and humidity, and data relating to the features is stored in a RAM (32)
in readiness to be relayed to storage means (56) in the interrogation station
(12) as and when required. Signals received by the passive receiver enable
the radio transceiver to be de-activated in locations, such as in an aircraft,
where spurious radiation from the transponder is not permitted and to be re-activated
when removed from such a location.
