Final Report - Strategic Environmental Research and Development ...

Figure 25. Classification performance using restricted decay time ranges.Library matching methods employing various procedures to compare polarizabilities of unknowntargets with those of targets of interest are commonly used for classification. In general, librarypolarizabilities derived from signature data collected with sensors using different pulse lengthswill not be exactly the same. The measured response may be distorted due to interfering signalsfrom leading and trailing edges of pulses in the Tx sequence. This is referred to as run-on. Theeffect of shortened pulse length is illustrated in Figure 26, which compares the measuredresponse of a 4 inch diameter aluminum ball with different transmit sequence pulse lengths. Theexpected signal for a perfect step response is shown with the dashed curve. In this case, the8.3 ms and 2.8 ms pulses do not capture the complete response. In general, the pulse lengthrequired to approximate the ideal (step response) polarizability depends on the target. Recall thatthere are two time scales for a target’s response – the surface mode time scale t S associated withthe t -1/2 to t -3/2 decay transition and fundamental body mode time scale t B corresponding to thelate stage exponential decay. Both scale in proportion to size 2 . The body mode time scale is alsoproportional to the permeability, so steel objects require longer pulses than nonmagnetic ones.Table 6 lists the time scales for various objects. Sensors using pulse lengths shorter than theseneed to have their own library which includes the distortion due to the run-on effect.31