Airborne Imaging GPR

Synthetic Aperture Radar (SAR) is a technique for achieving high resolution while using small antennas. It is more complex than most GPR designs, since it requires all elements of the radar to be phased locked, and in addition requires precise track recovery for position correction. When a SAR is operated below 400MHz or so, it starts to penetrate soils, forest cover, ice and snow, and some man-made materials. It is therefore an ideal technique for rapidly surveying large areas to locate target objects which are concealed by soil or forest cover. In the 1990's there was a surge of interest by DARPA and the intelligence communities in imaging GPR systems. The images below show some data from that time where vehicles were parked up against thick forest cover, and the radar was flown so that it had to penetrate the trees to detect the vehicles.

Applications for UWB radars fall into two technology categories, - those using UWB images (UWB Synthetic Aperture Radar) and those using profiles. The imaging (SAR) applications use the UWB radar's ability to cover large areas per flight hour, (typically 200-400 sq. km /hour). However, the depth of penetration is currently limited by surface clutter which competes with the subsurface image rather like a double exposed photograph. Processing is possible which will separate these two, but as yet has not been developed for commercial applications. Typical depth of penetration for imaging UWB radar is 2-5 meters. Typical applications that use UWB imaging include;

  • Search and Rescue
  • Search for shallow buried military targets
  • Imaging buried minefields
  • Site selection planning for construction and pipeline projects
  • Detection of vehicles and artillery under foliage
  • Detection of illicit human activity under foliage

  • Imaging SAR can also be used to detect illicit activities such as drug processing operations when hidden in trees. The left image below shows two buildings (4m x 5m) under dense foliage. On the right a single building can be seen as a very bright group of pixels in a mass of forest returns. and the brightness of the image is possibly due to the radar catching a specular reflection, since other buildings are not always that easy to detect.

    Airborne imaging GPR can also collect dramatic images of mines and minefields in which it is common to see every single mine of the A-T class, and a high percentage of metallic A-P mines of the Valmera class. The British MINESEEKER radar, which trades off speed for high resolution, is capable of seeing even small A-P mines. THe images below show a minefield set out in Yuma AZ and imaged at 2000ft AGL with a single polarisation SAR. THe mines on the left were buried 30cm and invisible, the ones on the right were surface laid. In this test, the mines were 30vm diameter M-20 A-T metallic mines. With full polarisation, and signature matching algorithms, it is possible to remove much of the battlefield clutter, and leave just the mines. This has been shown by a number of groups in the late 90's. It is now possible to build a full polarisation GPR that will fly higherand faster, and detect smaller targets making the technology very desirablefor development into mine and other buried device detection/location.

    AS well as mine detection, the techniqueis ideal for rapid area surveys for shallow buried bunkers, munitions caches, and buried archeological features. In dry soils and with sophisticated processing it would be simple to exceed the presentlimit of around 2/5m penetration capability.

    For Airborne radar applications, the reader is directed to ;