Measuring velocity of a target by Doppler shift, using improvements in calculating discrete Fourier transform

摘要

The invention is embodied in improvements in calculating discrete Fourier transform (DFT) using recursive digital filtering in a method for determining the velocity of a target located in a medium for vibratory energy. In the method a transmitter electrical signal of prescribed frequency is generated. Coherent vibratory energy, the frequency of which is in fixed relationship with said prescribed frequency, is transmitted into the medium and is directed toward the target. The transmitting is done in recurring pulses of prescribed duration. During range gate intervals each of prescribed duration, the transmitted coherent vibratory energy is received from the medium after its interaction with the target. The vibratory energy which is received during the range gate intervals is converted to a receiver electrical signal. The transmitter and receiver electrical signals are mixed together to obtain a demodulated electrical signal through heterodyning or homodyning. The demodulated electrical signal is digitized and then digitally filtered on a recursive basis to separate it into digitized spectral components in accordance with a discrete Fourier transform procedure. Squaring the absolute value of each of the digitized spectral components generates digital electric signals descriptive of the power spectrum of the interaction of the vibratory energy with said target. After performing a comparative analysis of the digital electric signals descriptive of that power spectrum, the velocity of the target is calculated from the results of the comparative analysis.