Spaceborne Synthetic Aperture Radars (SARs) are commonly used in EO, offering fine imaging resolution by processing sets of consecutive acquisitions recorded while flying over the targeted area. Although SARs are well-established, research continues to seek innovative techniques to improve their performance. One such advanced technique developed as part of the SPACEBEAM project is the scan-on-receive (SCORE) mode. The SCORE-SAR paradigm overcomes a fundamental limitation of the conventional stripmap SAR, where the achievable spatial resolution along-track is inversely related to the swath width across-track and thus requesting multiple passes of the satellite to fully observe a given area with a high resolution.
The SCORE principle involves illuminating a wide swath in transmission and receiving the radar echoes through multiple smaller (i.e., higher-gain) receive beams steered across-track to cover the whole swath. To date, there are no X-band SCORE SAR systems in orbit or planned. This is due to the limitations of digital electronics in enabling multi-channel, wide bandwidth, real-time digital beamforming (from few hundreds of MHz up to 1.2 GHz per channel) in terms of size, weight, power, and costs (SWaP-C), which prevents its applicability on board of small and micro-satellites. To make the SCORE-SAR observation mode available for higher frequencies and smaller SAR missions, a compact and less power-hungry implementation of the beamforming is required.
Photonic technologies have already penetrated various satellite subsystems and functions, thanks to their broadband nature, insensitivity to electromagnetic interference (EMI), low loss, and data-transparent link capacity. In particular, microwave photonics (i.e., photonics applied to microwave systems) offer the possibility of easily performing frequency-agnostic operations to process radio frequency (RF) signals in any employed region of the RF spectrum. The impressive advances in photonic integrated technologies are very promising in reducing the SWaP of microwave systems. Photonic integrated circuit (PIC) technologies have demonstrated the capability to implement beamforming networks (BFNs) for 5G communications and wideband radar systems. The first photonics-assisted SCORE-SAR receiver module intended for EO from space has been carried out within the project SPACEBEAM: Space SAR system with reconfigurable integrated photonic beamforming, funded by the European Commission under the H2020 programme.
The core of the system is an extremely compact hybrid PIC, realised thanks to the advanced integration of a Silicon Nitride (SiN) chip, including extremely low-loss passive components, with few Indium Phosphide (InP) chips for active and opto-electronic components. The hybrid PICs have been designed, realised, and assembled by Lionix.
The controls of the SiN PIC are implemented using piezo-electric actuators in lead zirconate titanate (PZT), a patented solution of Lionix with the fundamental feature of consuming negligible electrical power, which is of paramount importance in space applications. As per the requirements expressed by OHB, the proposed SCORE-SAR photonics-assisted receiver is able to process 12 incoming RF signals in the optical domain, thanks to an analogue reconfigurable optical BFN capable of dynamically synthetising and steering three independent beams with continuity. Simultaneously, the PIC also performs the down-conversion to intermediate frequency (IF) of the X-band signals received by the antenna elements. In terms of number of functions and of input/output ports (I/O), the SPACEBEAM project hybrid PIC is therefore the most challenging photonic integrated circuit developed to date.
