Carrier tracking and tunable passband filters for TDM-LDV mine detection

Abstract

Using laser Doppler vibrometers (LDVs) to find buried land mines has been shown to have a high probability of detection coupled with a low probability of false alarms. Equally good results have been achieved using a 16-beam LDV. Time division multiplexing (TDM) of this multiple-beam LDV has also been investigated as a means of increasing the scanning speed and potentially allowing the sensor to move down the road at speeds faster than that allowed using stop-and-stare LDVs. A moving platform induces Doppler shifts in the LDV beams that are not perpendicular to the motion vector. This shift can be much greater than the modulation bandwidth of a stationary LDV signal; therefore, the demodulation must allow for the shift either by increasing the processing bandwidth, which increases the system noise or by tracking the Doppler offset and adjusting a band pass filter's center frequency. A method has been developed to track the carrier frequency to compensate for the Doppler offset for each of the 16 channels caused by the moving platform and then adjusting the center frequency of a digital band pass filter. This paper will present the basic filter structure and compare the noise statistics from two different carrier tracking methods that were investigated.