of current with intrinsic safety by applying envelope detection to the mf current however in this case phase inversions that occur when the interrogation assembly travels over the responder are detected at the same times as the instants at which they occur thereby making it possible to calculate speed with a digital responder demodulation of the digital signal representative of the identification message must take the phase inversions 0 u03c0 of the low frequency signal into account as explained above the phase inversions of the low frequency signal can be processed safely by using safety electronics of known type such as that used in the automatic on board controllers in the val transport system where the same function is required for detecting passages over the servo control lines of the transmission mat placed on the crossings of the track when necessary it is also possible to establish a low data rate communication link between the interrogation assembly and the marker to do this the low frequency signal can be phase modulated at a low modulation rate of about 1 kb s that modulation may be used as a return channel which is advantageous in certain responder locations e g in a station such a link can be established without requiring an additional antenna on the interrogation assembly it is possible to use responders having lengths in the track direction which depend on their locations it is generally desirable to have short responders in the ordinary portions of the track in contrast it may be desirable to use long responders e g having tuned circuits that.