Time frequency processor for radar imaging of moving targets

Abstract

Conventional radar uses the Fourier transform to generate a radar target image. Constraints on the use of Fourier methods requiring point scatterers to remain in their range cells and requiring Doppler frequency shifts for point scatterers to be stationary are impractical due to a moving object's inherent non-uniform motion and rotation. Time varying motion-induced Doppler frequency shift spectra represented with Fourier transform methods smears radar target image. Representing time-varying Doppler spectrum using joint time-frequency transform methods is desirable. Replacing conventional radar Fourier transform with a high resolution time-frequency transform, a 2-D range-Doppler Fourier image frame becomes a 3-D time-range-Doppler cube. By sampling in time, a time sequence of 2-D range-Doppler images with superior resolution can be viewed. Smears from time-variance of Doppler spectrum may be removed to enhance target image.

I claim: 
     
       1. In a radar image signal processing system, an improved method for processing radar target image information comprising: generating a series of pulses for transmission in a target range,   receiving target image backscatter information generated from said series of pulses transmitted in said target range by reflecting from a target in said target range, and   processing said target image backscatter information received in said receiving step into target information using a series of processing steps comprising: generating frequency domain signatures from said target image backscatter information,   range processing said frequency domain signatures and generating first transformed target information,   receiving said first transformed target information and generating target range profiles therefrom,   performing a Joint Time-Frequency Transform upon said target range profiles and generating second transformed target information,   sampling said second transformed target information in the time domain and generating time sampled target information, and   displaying a target image using time sampled target information.     
     
     
       2. The method of claim 1, wherein said series of pulses further comprises individual narrow band pulses of increasing frequency. 
     
     
       3. The method of claim 1, wherein said target image backscatter information further comprises target image backscatter information from a plurality of point scatter sources on said target. 
     
     
       4. The method of claim 1, wherein said step of range processing further comprises removing a motion component further comprises removing extraneous phase term from a spatial frequency signature of said target. 
     
     
       5. The method of claim 4, wherein said step of range processing further comprises removing said motion component using an Inverse Discrete Fourier Transform. 
     
     
       6. The method of claim 1, wherein said first transformed target information further comprises N range profiles. 
     
     
       7. The method of claim 6, wherein said target range profiles further comprise M target range profiles comprising N range profiles. 
     
     
       8. The method of claim 1, wherein said step of performing a Joint Time-Frequency Transform upon said target range profiles and generating second transformed target information further comprises performing M Joint Time-Frequency Transforms upon M target range profiles and generating M groups of said second transformed target information. 
     
     
       9. The method of claim 8, wherein said step of sampling said second transformed target information in the time domain and generating time sampled target information further comprises sampling said M groups of said second transformed target information and generating M frames of said time sampled target information. 
     
     
       10. The method of claim 9, wherein said step of displaying a target image using time sampled target information further comprises displaying an enhanced target image in the Range Doppler plane, said target image comprising M frames of said time sampled target information displayed on a display over a predetermined time interval. 
     
     
       11. A radar system comprising: means for generating a plurality of radar bursts, each said burst comprising a plurality of radar pulses;   means for receiving echoes of said pulses in said bursts;   means for time aligning pulses across said bursts to form an aligned echo data base; and   means for transforming said aligned echo database by a preselected joint time-frequency transform.