The presence or absence of objects is determined by interrogating or
exciting transponders coupled to the objects using pulsed wide band
frequency signals. Ambient or background noise is evaluated and a
threshold adjusted based on the level of noise. Adjustment may be based
on multiple noise measurements or samples. Noise detection may be
limited, with emphasis placed on interrogation to increase the signal to
noise ratio. Match filtering may be employed. Appropriate acts may be
taken if detected noise is out of defined limits of operation, for
example shutting down interrogation and/or providing an appropriate
indication.