at a junction 25 which may comprise a mixer which functions as an adder this makes the output line 34 d 34 from the junction 25 carry a pulse train represented by 34 b c 34 as illustrated on line 34 d 34 of fig5 the control switch 24 is operated by a parameter monitor 26 which is responsive to occurrence of some event which is desired to be signalled by this system this event may of course be any of countless conditions or happenings which a predetermined transducer is set to record such as the attainment of a specific flight performance figure the sighting of a certain object by a tracking system the position of a control element and so on the pulses on line 34 d 34 of fig5 are fed to a laser firing circuit 27 which controls the triggering current to a laser such as a gaas laser diode 29 each electrical pulse produces one laser pulse the laser pulses are beamed by the collimating lens 15 substantially directly to the rear of the projectile 1 through the passage 7 in this system upon firing of the projectile the thermal battery 20 is activated to start the reference oscillator 21 operation of the circuit as previously described will thus result in the sending and receiving of laser beam pulses at a frequency represented by 34 b 34 this will indicate that the telemetry system is functional but the condition to be monitored by control switch 24 is not.