Passive radar on mobile platforms
Passive radar plays an increasingly important role, particularly in the area of undercover reconnaissance. A research group at Fraunhofer FHR is currently investigating the implementation of this technology on mobile platforms.
Stationary radar sensors and radar networks have demonstrated their ability to locate air and ground targets in various field tests. Inflatable boats and small aircraft can be detected reliably. As opposed to emitting signals of their own, passive radar systems use existing signals, e.g. from DVB-T or mobile communications. Hence, the possibility of locating the radar can be virtually ruled out. Passive location therefore proves to be very interesting for undercover reconnaissance. In the future, this application will also be available for mobile platforms.
Due to increasing miniaturization, passive radar systems can be greatly reduced in size this making them more attractive for mobile platforms. The frequency range used by the transmitter networks (VHF/UHF) is particularly favorable with regard to the wave propagation characteristics and the backscatter behavior of the object that is to be detected. In this way, objects that are not in the line-of-sight of the receiver can, under certain conditions, also be detected. This leads to intensified research on optimal operating conditions for the successful implementation of passive radar sensors, e.g. on ships in coastal areas.
In contrast to stationary operation, the proper motion of the carrier platform has to be compensated when engaging in mobile applications. In the presence of proper motion, the platform creates a Doppler effect which distorts the signal and adversely affects the performance of the system. To detect a target within the surveillance range of the radar system on the basis of the »overheard« echo, the reference signal received directly from the transmitter must be reconstructed independent of motion. Fraunhofer FHR solved this problem by determining the motion-induced Doppler effect that is created by the platform on the basis of the analysis of the pilot carrier of the DVB-T direct signal. This paves the way for the required compensation of the signal distortion.
The DVB-T networks in the Federal Republic of Germany are operated as single frequency networks, i.e. all transmitters in the network coherently transmit the same signal at the same time. This calls for special efforts as a single target may reflect several echoes to the passive radar system when illuminated by different transmitters. The precise localization of the target direction and target tracking is therefore necessary to clearly allocate the target echo to a transmitter. This ambiguity problem must be solved to ensure that the passive radar can subsequently determine the correct distance and speed of the target.
Suitable phased array antennas can control the line of vision of the passive radar system by means of beamforming, thus facilitating the measurement of the target direction. Thanks to digital beamforming, the radiation pattern and hence the formation of the beams of an array antenna can be digitally modified. Each antenna element has its own digital receiver to ensure that the required phase shifts and amplitude scaling can be carried out on a software basis. A further advantage of array antennas with digital beamforming is their high flexibility, as the receiving pattern can be quickly modified using software. During the digital signal processing of the data, the radiation pattern can therefore be simultaneously calculated for several independent main beams to realize coverage in various directions at the same time.