Joint bistatic low frequency airborne radar experiments

The French-Swedish bilateral research project "Joint Bistatic Low Frequency Airborne Radar Experiments" is described and the main results summarised. The project has developed, tested and demonstrated bistatic synthetic aperture radar (SAR) operating in the VHF/UHF-band (222-460 MHz) with transmitter and receiver on two airborne platforms. The concept was first tested in France 2009 and then an extensive data collection was performed in Sweden 2010. The data collection in 2010 demonstrated for the first time that bistatic SAR images can be generated with similar quality as monostatic SAR images in the VHF/UHFband. The techniques developed to synchronise the radar systems worked successfully and images with nominal resolution were obtained. The geometrical accuracy of the bistatic images is slightly inferior compared to the monostatic images which reduces change detection performance. The cause of this problem is not fully understood but it is likely due to residual phase drift between the radar systems. Most of the flights were conducted with the two aircraft flying side-by-side with the same velocity along parallel but offset tracks. Bistatic elevation angles between 0° ("quasi-monostatic") and 10° were realised in this manner. A few other geometries were also used, i.e. circular tracks, combinations of circular and straight tracks as well as parallel tracks with different speeds. The main test site of the experiment was the Swedish Army Combat School at Kvarn where targets were deployed in the open and concealed in forests. The site also included an area with buildings under construction. The second test site used was Remningstorp, west of the city of Skövde in Västra Götaland, which is a forest research park with an extensive data base of forest parameters. Results from both Kvarn and Remningstorp show that the bistatic images (horizontal polarisation) show a decrease of the forest clutter. The effect is more pronounced when the bistatic elevation angle increases and a suppression of up to 10 dB is observed. The concealed targets are less affected which means that the target-to-clutter ratio generally increases and hence detection performance. Results from the building complex in Kvarn show that penetration through nonmetallic roofs of buildings and imaging of building interiors are possible using VHF/UHF-band SAR.