Abstract
Apparatus for classifying the activity of an aircraft including the geophone proximate an airfield used by jet airplanes for generating a signal indicative of the existence of the jet aircraft. Amplifier means are provided for enhancing the signal. The signal is then passed through a low pass filter means for receiving the signal and passing only that portion having a frequency less than about 30 Hz. A high band pass filter also receives the signal and passes only that portion having a frequency ranging from about 90 Hz to about 120 Hz. Two demodulator means are provided which receive the low pass signal to provide a low pass envelope and receive the high band pass signal to produce a high band pass envelope. Threshold signal generating means are provided to receive the low pass envelope and the high band pass envelope, and generate a signal when either or both envelopes reach a predetermined threshold. Finally, comparator means are provided for comparing the time sequence of the signals generated by said threshold signal generating means to determine the activity of the jet aircraft. In a preferred embodiment, a means for indicating the existence of a taxing aircraft is provided when the time the high band pass envelope exceeds its threshold is more than one second after the time the low band pass envelope exceeds its threshold, provided the low pass threshold, is still exceeded at that time. In addition, an indication signal of a take-off activity is provided when the time the low pass envelope exceeds its threshold is no more than one second before the time the high band pass envelope exceeds its threshold. The time the low pass exceeds its threshold can be after the high band pass exceeds its threshold. Finally, the device includes means for indicating a fly-over condition when the time of the closest point of approach (CPA) of the aircraft is less than the time the low pass envelope exceeds its threshold, provided the high band pass threshold has been exceeded.
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