Monitoring system for scanning-beam microwave landing apparatus

Claim 1

In a scanning-beam microwave landing system of the time reference type, the combination comprising: first means including a transmitter and a directive antenna for transmitting a continuous wave narrow beam of microwave energy and for scanning said beam through a predetermined angular sector in successive scan cycles each consisting of a sweep from a starting angular position through said sector to a predetermined angular limit and a return sweep from said angular limit through said sector to said starting angular position; second means including a receiver and duplexing means, said receiver and said transmitter being connected to said antenna through said duplexing means to permit contemporaneous operation of said transmitter and said receiver through said antenna; third means comprising a plurality of transponders each located at a discrete angle within said sector, said transponders including means responsive when said beam scans across each of said discrete angles, to modulate and retransmit to said antenna, a signal discretely modulated as a function of the corresponding discrete transponder angle, said receiver being responsive to each retransmitted signal during said sweep of said beam across said angle of each corresponding transponder; and fourth means responsive to the output of said receiver for providing an angle reference for each discrete transponder angle and for comparing the scanning angle at which each of said modulated transponder signals is received with said reference angle for each corresponding transponder, to provide an output signal indicative of any non-correspondence between any of said reference angles and the scanning angle at which the corresponding modulated retransmitted signal is received.