In border security, radar systems permit the continuous and systematic observation of the area of interest. Beside detection and tracking, imaging radars provide information for the target identification and recognition. Two-dimensional (2D) high-resolution images can be generated by exploiting the inverse synthetic aperture radar (ISAR) technique typically on monostatic systems (a single ISAR with co-located transmitter and receiver). Co-located bistatic radars, i.e. systems with two closely placed antennas for interferometric use, can achieve three-dimensional (3D) reconstruction, exploiting the interference signal due to slightly different imaging geometries carried out at the different antennas.

Significant limitations affect the backscattering measures form a single direction. Typical targets can generate effects of shadow, scintillation, etc. thus providing only partial imaging of the object under investigation. A more complete reconstruction of the scenario can be achieved exploiting multiple active and passive radar sensors in multistatic configuration (i.e., with multiple transmitters and receivers distributed on different sensors in different positions), able to perform advanced 2D and 3D ISAR imaging. Multistatic ISAR systems also outperform multi-monostastic ISAR systems (based on several independent monostatic ISARs). In fact, they provide a higher flexibility with different viewpoints for the incidence and scattered angles, thus collecting additional details of the target scattering matrix. However, the synchronization problem is slowing their real implementation. Frequency diversity sensing is a further advancement with respect to a single frequency sensing, allowing observing the target scattering at different frequency windows, and using the wavelength diversity for advanced interferometric processing. However, technological issues still limits the frequency flexibility of the current systems.

SOLE wants to face these two main limitations of distributed radar systems, by proposing a new technological solution based on photonics that enables a fully coherent multistatic (without autofocusing algorithms) and multiband system, exploiting the spatial and frequency diversities constructively for enhancing the detection, tracking and classification capabilities.

In particular, SOLE aims at developing a ground network of coherent multistatic and multiband software-defined radars for enhanced detection, tracking and classification of heterogeneous targets in maritime environments, and it aims to test the developed surveillance system in a real maritime scenario. The radar network is composed by:

  • a central core that exploits photonics for generation and detection of all the coherent multiband RF signals to be used in the sensor fleet,
  • optical fiber links for both RF signal and sampling clock distribution from the central core to the antenna sites and back, through radio over fiber technology that maintains the RF signal coherence
  • simplified and low power consuming heterogeneous radar peripherals at the antenna sites

From the observation point of view, the SOLE advances are:

  • flexibility to form a tailored coverage area to meet the specific requirements
  • increasing the system sensitivity, thanks to the augmented total received power (from several coherent radars) and fully coherent processing
  • enhanced position estimation and tracking, thanks to 2D velocity decomposition and higher cross range resolution due to different viewing angles and beam overlapping
  • reduced target signature scintillation glint and blind parts, thanks to the fusion of information acquired from different viewing angles and RF carrier that results in a more accurate target recognition
  • improvement of 3D target reconstruction via multifrequency interferometry
  • reduction of the clutter and jamming noise thanks to geographical decorrelation and use of different frequencies.

SOLE will consider the following three main radar network typologies:

  • a distributed radar network with enhanced system sensitivity and detection range
  • a multistatic multiband 2D ISAR system with distributed sensors for enhanced target recognition and classification and improved estimation of physical and chemical features.
  • a multistatic multistpectral 3D ISAR interferometry.

SOLE consortium includes CNIT (NATO country Project Director), CSEM (Partner country Project Director) and CNR-IREA with expertise respectively in microwave photonics and radar hardware, optical clocks, and radar systems and signal digital processing. The expected impacts are the radar fleet design, the adaptation of data processing to SOLE, a demonstrator implementation, and the verification of its enhanced features through field trials in controlled and relevant maritime environments in collaboration with Port Authorities and Companies involved in radar and security markets.