Detection means

Apparatus for determining the time of the closest point of approach of a taxiing jet aircraft to an observation point, comprising:

• microphone means for producing an input signal in response to the sound energy of operating engine noise of a taxiing jet;
• low pass filter means adapted to receive said input signal and to provide an output comprising low signals having a frequency less than or equal to 1.0 KHZ;
• high pass filter means adapted to receive said input signal and to provide an output comprising high signals having a frequency greater than or equal to 1.5 MHZ;
• demodulator means for receiving said output signals from said high pass filter and said low pass filter, said demodulator means being adapted to produce a low frequency envelope from said low signal and a high frequency envelope from said high signal;
• and
• comparator means for receiving said low frequency envelope and said high frequency envelope, said comparator means being adapted to provide a signal at the point in time when the amplitude of the low frequency envelope exceeds the amplitude of the high frequency envelope to thus signal the closest point of approach of said taxiing jet aircraft.