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It took almost exactly 100 years from Maxwell's first conception of the theory of electro-magnetic waves and the subsequent experimental proof by Hertz in the nineteenth century until the first patent on Doppler Beam Sharpening (DBS) was issued to Wiley in 1965. DBS was the original attempt to improve the Doppler resolution of moving pulsed radar by exploitation of a synthetically spanned aperture compared to the physical antenna with its very limited resolution. Although somewhat rudimentary because of the neglected quadratic phase history and thereby still a range-dependent azimuth resolution, it is considered the cornerstone of Synthetic Aperture or imaging Radar (SAR). The fundamental idea here is that through the motion of the radar-carrying platform a long synthetic aperture is spanned, along which the radar raw data are focussed, to achieve a high resolution in the flight direction. Besides the essential platform motion, the other vital attribute of SAR is that the recorded data by no means constitute the desired image but instead must first be processed using suitable focussing algorithms, which among others require a precise knowledge of the platform trajectory. At the onset of SAR, owing to the absence of digital signal processors, the focussing could only be achieved using systems of optical lenses whereby the SAR raw data were recorded on film. This technology was not flexible and only permitted rather moderate results that typically suffered from low resolution, defocussing and other artefacts.

Chapter Contents:

  • Introduction to imaging radar
  • Onset
  • Twentieth century
  • Twenty-first century
  • Outlook
  • Acknowledgement
  • References

Inspec keywords: radar imaging; synthetic aperture radar

Other keywords: imaging radar; SAR; synthetic aperture radar; radar-carrying platform; Doppler resolution; Doppler beam sharpening; moving pulsed radar; optical lenses; range-dependent azimuth resolution

Subjects: Radar equipment, systems and applications; Optical, image and video signal processing; Radar theory

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