sophistication and computational power. Much current research is currently focusing on the exploitation of modern digital broadcast signals. The US HDTV standard is particularly good for passive radar, having an excellent ambiguity function and very high power transmitters. The DVB-T digital TV standard (and related DAB digital audio standard) used throughout most of the rest of the world is more challenging— transmitter powers are lower, and many networks are set up in a "single frequency network" mode, in which all transmitters are synchronised in time and frequency. Without careful processing, the net result for passive radar is like multiple repeaters jammers. Target imaging Researchers at the University of Illinois at Urbana–Champaign and Georgia Institute of Technology, with the support of DARPA and NATO C3 Agency, have shown that it is possible to build a synthetic aperture image of an aircraft target using passive multistatic radar. Using multiple transmitters at different frequencies and locations, a dense data set in Fourier space can be built for a given target. Reconstructing the image of the target can be accomplished through an inverse fast Fourier transform (IFFT). Herman, Moulin, Ehrman and Lanterman have published reports based on simulated data, which suggest that low-frequency passive radars (using FM radio transmissions) could provide target classification in addition to tracking information. These Automatic Target Recognition systems use the power received to estimate the RCS of the target. The RCS estimate at various aspect angles as the target traverses the multistatic system are compared to a library of RCS models of likely targets to determine target classification. In the latest work, Ehrman and Lanterman implemented a coordinated flight model to further refine the RCS estimate. Ionospheric Turbulence Studies Researchers at the University of Washington operate a distributed passive radar exploiting FM broadcasts to study ionospheric turbulence at altitudes of 100 km and ranges out to 1200 km. Meyer and Sahr have demonstrated interferometric images of ionospheric turbulence with an angular resolution of 0.1 degrees, while also resolving the full, unaliased Doppler Power Spectrum of the turbulence. Space Debris Detection and Tracking Silentium Defence has launched the first operational ground passive radar specifically designed to track LEO. The Oculus Observatory is based in Swan Reach, South Australia with plans to scale the technology globally. Silentium has a range of products that support both tactical and strategic applications ranging from drone detection, maritime surveillance to long-range air and space search.