Defense

DVB-S2-based passive radar imaging

SABBIA system.
© Fraunhofer FHR

SABBIA system.

Cooperative target  Porpora ship.
© Fraunhofer FHR

Cooperative target Porpora ship.

DVB-S2-based ISAR-Image of Porpora ship [dB].
© Fraunhofer FHR

DVB-S2-based ISAR-Image of Porpora ship [dB].

Reliable and high resolution radar imaging play an increasingly important role in the identification of non-cooperative targets in military scenarios. Passive radar guarantees at the same time also discreet surveillance. SABBIA, a system that was developed at Fraunhofer FHR, uses the signals that are sent by satellites (DVB-S2) to image non-cooperative targets. SABBIA is able to achieve  resolutions below 2 meters and also allows operation with different orthogonal polarizations.

Passive radar systems are gaining in importance in the area of defense technology. Apart from the conventional air-space surveillance systems, a number of new reconnaissance techniques were developed in the past years. These include the highly rated passive radar imaging techniques SAR and ISAR. The image capability of such radars normally depends on the wavelength and the instantaneous signal bandwidth of the illuminator of opportunity used as transmitter. Conventional transmitters in the VHF and UHF frequency ranges greatly restrict the available signal bandwidth. Digital TV satellites (DVB-S2), on the other hand, which are operating in the Ku-band, allow the utilization of a higher bandwidth. Range resolutions of just a few meters can be achieved and this is a very important prerequisite for imaging techniques. In addition, the utilization of DVB-S2 for military applications offers two significant advantages. For one, the DVB-S2 signals offer high illumination and are therefore available in remote areas and open seas. Secondly, a deactivation of satellites in the event of war is much more difficult than is the case with ground-based transmitters. One disadvantage of DVB-S2-based passive radars is the low power spectral density at the receiver as this requires the utilization of high gain antennas and long integration times when processing the signals. Long integration times are, however, not an important disadvantage for passive target imaging as these are traditionally required from the ISAR technique to achieve a high resolution capability in the cross-range dimension.

Fraunhofer FHR developed the experimental system SABBIA to allow passive radar imaging for non-cooperative airborne and maritime targets. Here, the DVB-S2 signals that are transmitted from geostationary satellites, e.g. the signals from the satellite ASTRA 19.2°E, are exploited. SABBIA basically consists of two structurally identical receive modules – a reference module and a tracking module – as can be seen in Figure 1. The reference module acquires the direct satellite signal which illuminates the scene and the tracking module is used to receive the target echo. After the analog-to-digital conversion, both received signals are recorded with high-speed data recorders.

The receive unit comprises an 85-centimeter dish  antenna, a custom designed antenna horn and a Quattro LNB. This has a low noise figure and also has an internal low phase noise oscillator as well as an external 10 MHz reference input. Both receive modules are equipped with a GPS-IMU (Inertial Measurement Unit) so that they can receive precise information about position and antenna pointing direction. The LNB can demodulate DVB-S2 signals simultaneously in horizontal and vertical polarization as well as in both the low and the high band. This enables fully polarimetric operation of the system.

Field tests were carried out with SABBIA in 2017 within the framework of the MAPIS (Multi-channel passive ISAR imaging for military applications) project of the EDA (Cat. B). In the course of the tests, the military vessel Porpora (Figure 2) was measured and the resulting signal is then processed to obtain an ISAR image (see Figure 3). The bistatic geometry and the alignment of the vessel allow the imaging of the target from a top view (bird´s eye perspective) . Here, the vessel echo is squeezed in a single range cell, as a higher range resolution could not be achieved due to the low signal bandwidth. The cross-range direction (shown in Fig. 3 as Doppler axis) is, however, well imaged. This result can be used to derive the size of the target. This information is essential for  detection and classification of the target.