Radio altimeter system with reduced interference between altimeters - has high-Q filter outside band-pass to step altimeter frequency if interfering altimeter approaches

摘要

Apparatus for independently controlling a CWVFM radio altimeter, to reduce interference from another nearby altimeter, employs a high-Q filter outside the bandpass of the signals which are developed and decoded for altitude indication. The filter detects an approaching interference signal received from the other altimeter and produces an output to a threshold detector. When the output of the filter exceeds a predetermined level, the threshold detector causes the transmitted frequency of the altimeter to be stepped. In one embodiment, a digital counter cyclically counts up and down at a sawtooth modulation frequency. The output of the counter is applied to a digital-to-analog converter to produce an output to control the frequency of the transmitted signal. Additionally, the output of the counter is conducted to inputs of an adder containing a present add number. When the output from the threshold detector is produced, it loads the counter immediately with the sum of the present counter output and the preset add number. The counter then begins its count at the totalled count, thereby immediately stepping the transmitter frequency.