The widespread existence of digital wireless communication installations and digital broadcasting stations makes the application of passive radar using these stations as illuminators attractive. Passive radar systems using analog FM radio transmitters have been designed and systems are now commercially available. The disadvantage of FM-systems is that the autocorrelation function of the transmit waveform, which determines the detection and resolution properties of the radar, is dependent on the emitted radio program. Modern digital audio or video broadcast (DAB/ DVB) signals have an autocorrelation function practically independent of the information content due to digital coding and are therefore much better suited for passive radar purposes. Currently there is a widespread activity in developing DAB/ DVB passive radar systems [1, 2]. However, the usage of identical frequencies causes a number of problems (e.g. ghost targets) which are not easy to solve. Base stations of GSM mobile phone systems are also suitable illuminators, because they are widely spread (even in sparsely populated regions), have digital coding, and use frequency diversity. The large number and availability of base stations makes such systems very attractive. However, to this day the number of applications of such radar systems is still comparably small. The key problem of passive radar is the detection of the weak echo in the presence of the strong direct signal. Modern space-time array signal processing methods for interference suppression and signal enhancement are required. Therefore antenna arrays are mandatory for passive radar. Another factor to improve system performance is the exploitation of multiple transmitters.
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