Case hi

Claim 1

IN A PULSE RADAR RANGE MEASURING APPARATUS ADAPTED FOR SURVEY AIRCRAFT USE TO INDICATE THE CLEARANCE HEIGHT OF THE AIRCRAFT ABOVE A TERRAIN SURFACE: A DUAL CHANNEL AMPLIFIER ADAPTED TO AMPLIFY A RADAR ECHO PULSE SIGNAL IN EACH CHANNEL THEREOF; A RANGE INDICATING DEVICE; MEANS PROVIDING A CONTROL SIGNAL PROPORIONAL TO THE RANGE INDICATED BY SAID DEVICE; MEANS PROVIDING A TIMING REFERENCE SIGNAL; MEANS PROVIDING AT PREDETERMINED SPACINGS IN TIME A CONTROLLED REFERENCE SWEEP SIGNAL RESPONSIVE TO SAID REFERENCE SIGNAL AND SAID CONTROL SIGNAL; MEANS RESPONSIVE TO SAID CONTROLLED REFERENCE SWEEP SIGNAL FOR INCREASING THE AMPLIFICATION OF ONE CHANNEL OF SAID AMPLIFIER WHILE PROPORTIONALLY DECREASING THE AMPLIFICATION OF THE OTHER CHANNEL THEREOF; MEANS NORMALLY BIASING SAID CHANNELS FOR EQUAL AMPLIFICATION THEREBY AT A PREDETERMINED TIMING IN EACH CONTROLLED REFERENCE SWEEP SIGNAL; MEANS PROVIDING AN ERROR SIGNAL RESPONSIVE TO THE RELATIVE AMPLIFICATION OF SAID ECHO PULSE SIGNAL BY SAID AMPLIFIER CHANNELS AND HENCE RESPONSIVE TO A LACK OF COINCIDENCE OF SAID CONTROLLED REFERENCE SWEEP SIGNAL AND THE CENTRE OF SAID ECHO SIGNAL; AND MEANS DRIVING SAID RANGE INDICATING DEVICE RESPONSIVE TO SAID ERROR SIGNAL TO CHANGE SAID CONTROL SIGNAL EFFECTING COINCIDENCE OF SAID SWEEP SIGNAL AND SAID ECHO SIGNAL THEREBY REDUCING SAID ERROR SIGNAL TO ZERO.