Abstract: A method of operating a system for monitoring the interior of a motor vehicle in which a rest signal amplitude is measured for the acoustic receiver or receivers in the absence of sound and is stored in memory while an actual reading is taken of the measured signal again in the absence of sound within the vehicle interior and this letter value is compared to the first and an error register is set if the measured value is outside a stored window.

Abstract: The interior of an automotive vehicle is monitored for incursion by launching primary sound wave pulses into the interior of the vehicle and picking up the primary sound wave pulse together with superimposed reflection and sound signals generated by incursion, etc. The picked-up sound waves are transformed into electrical signals which are decomposed in an A/D converter into a time sequence of digital measurement vector elements which in turn are compared with reference elements by a subtraction process to produce a different vector. Summation of the positive terms of the difference vector and summation of the negative terms of the difference vector to produce positive and negative sums allow the sums to be combined to form a gradient which is added to previous gradients to provide a gradient sum. The alarm is triggered when the gradient sum exceeds a threshold.