Photonic Syntethic Aperture Radar (SAR)
The first civilian use of a Synthetic Aperture Radar (SAR) system on a satellite was SeaSat, launched by NASA in 1978 to control ocean movements. SAR system emits an electromagnetic signal toward a surface and record the backscattered signals. The advantage of moving in orbit is synthesizing a virtual wider antenna from the physical limited antenna in the direction of flight. In the early 1990s, SAR Interferometry (InSAR) proved to be a powerful instrument for reconstructing Earth’s topography because of its wide area and high spatial resolution images (10 m or less) without ground-based instrument needs. Moreover, differently by optical cameras, SAR systems can collect images day and night regardless of weather. These advantages opened the way to the development of innovative SAR configurations and architectures.
Thanks to the diffusion of nanosatellites, the number of Earth Observation (EO) missions has increased 7 times, pushing towards SAR payload miniaturization. However, reducing system size can be obtained only at the expense of performance parameters, such as image resolution, ground coverage, orbit duty cycle, revisiting time, and mission duration, due to electronic miniaturization limits. To overcome this barrier, monolithic SARs have been replaced by a constellation of smaller satellites, such as COSMO-SkyMed or Capella SAR, distributing imaging tasks all over the formation. In this field, the group is actively exploring Distributed SAR (DSAR) systems, focusing on configurations involving swarms of receive-only CubeSats. These satellites would operate as a phase-aligned array, collectively forming a virtual aperture to enhance spatial resolution. A key challenge lies in maintaining precise formation and phase coherence across the swarm. To address this, the team is developing methods based on RF inter-satellite links, leveraging transceiver-based communication to enable real-time exchange of synchronization data [1].
The need for miniaturization drives the employment of competitive technologies with intrinsic compactness as photonics. Photonics is a well-established enabling technology in several fields, such as telecommunications, aerospace and defence, life science and health care. Photonics for Space has become a strategic R&D sector for the global aerospace market [2]. The main benefits of photonics over the competing technologies are electromagnetic interference immunity, low power consumption, high immunity to vibration/shock, radiation and small footprint. These features are critical onboard satellites, and this is the reason why the research effort on photonic sub-systems on board satellites for Earth observation (EO) and telecommunications is quickly growing [3].
The photonic SAR payload architecture and its functional blocks are shown in the figure [4].
Since the transmission arm requires microwave pulse compression to ensure a wide resolution and a wide swath, it is necessary to use a frequency modulated microwave waveform generator (LCMW) capable of delivering a time-bandwidth product (TBWP) in the range of 102-103. The group in 2022 has developed a solution consisting of a combination of an opto-electronic frequency tunable oscillator (OEO) and a recirculating phase modulation loop (RPML), whose structure is shown in figure [5]. The calculated phase noise is −100 dBc/Hz, the time bandwidth product is TBWP ≈ 1078, with ultra-large pulse compression ratio (PCR) of 814.
The phased array antenna (PAA) system fed by beamforming network (BFN) plays a pivotal role in setting up an inter-satellite as well as satellite to ground communication, forming and directing the beam toward the target. The group has investigated several configurations of OBFN based on optical delay lines as in [6-7]. In 2024 it has been proposed a reconfigurable OBFN feed by efficient true time delay lines (TTDLs) consisting of cascaded coupled resonator optical waveguides (CROWs) which not only provide continuous delay tunning but also offer adjustable bandwidth so that same set of antennas can be used to send or receive signal at different bandwidths. The bandwidth can be tuned from 3 to 7 GHz whereas the maximum relative delay is found to be around 90 ps with a small insertion loss of 0.13 dB [8]. In 2024, the group has presented a novel configuration that overcome the need for tunable delay lines, thus reducing design and fabrication costs and overall system power consumption, exhibiting a directivity of the main lobe of about 45 dB while ensuring a squint-free wide bandwidth of 0.5 GHz and a θ of ±15° [4].
Taking into account the benefits of optical processing, there is ongoing investigation into novel architectures capable of directly processing Synthetic Aperture Radar (SAR) data onboard satellites. This approach aims to minimize the data transmitted to ground stations. A flexible and reconfigurable architecture based on LiNbO3 has been devised, capable of conducting full-optic real-time extrapolation and processing of each echo’s spectral component. The proposed solution has been investigated by taking into consideration features and constraints of current SAR payloads, guaranteeing 256 channels spaced 300 MHz apart in the Ka-band, with low propagation losses (2.8 dB/m), maximum insertion loss of 12 dB, maximum applied voltage of 7 V. maximum time delay of 0.98 ns, and tuners’ length of 4.1 mm [9-10].
References
[1]. P. Tortora, D. Modenini, A. Locarini, A. Curatolo, G. Paialunga, A. Caruso, M. Lombardo, A. Ponti, A. Fonti, G. Acierno, M. Matarrese, P. L. De Rubeis, G. Trinchero, L. Simone, G. Cucinella, A. Negri, V. Fortunato, D. Chirulli, L. M. S. Pascali, C. Ciminelli, G. Brunetti, F. Giordano, S. Longo, G. Leccese, A. Terracciano, S. Natalucci, “The INNOVATOR CubeSat Mission and the development of its intersatellite link transceiver (ISL‐T),” In Small Satellites Systems and Services Symposium (4S 2024), Palma de Mallorca, Spain, May 26-31, 2024, Vol. 13546, pp. 483-495. doi: https://doi.org/10.1117/12.3061553
[2]. C. Ciminelli, “Cutting-Edge Integrated Photonics in Space,” In European Conference on Integrated Optics (ECIO), Aachen, Germany, June 17–19, 2024, pp. 295-300. doi: https://doi.org/10.1007/978-3-031-63378-2_49
[3]. G. Brunetti, N. Sasanelli, N. Saha, G. Campiti, F. Hassan, A. di Toma, M. N. Armenise, C. Ciminelli, “Integrated photonics for NewSpace,” in Applications in Electronics Pervading Industry, Environment and Society (ApplePies), Genoa, Italy, September 26-27, 2022, vol 1036. doi: https://doi.org/10.1007/978-3-031-30333-3_39
[4] A. di Toma, G. Brunetti, M. N. Armenise, C. Ciminelli, “Synthetic aperture radar payloads: migration towards photonic approach,” In Small Satellites Systems and Services Symposium (4S 2024), Palma de Mallorca, Spain, May 26-31, 2024, vol. 13546, pp. 1372-1383. doi: https://doi.org/10.1117/12.3062644
[5] G. Brunetti, M.N. Armenise, C. Ciminelli, “Chip-scaled Ka-band photonic linearly chirped microwave waveform generator,” Frontiers in Physics, vol. 10, p. 785650, 2022. doi: https://doi.org/10.3389/fphy.2022.785650
[6] T. Tatoli, D. Conteduca, F. Dell’Olio, C. Ciminelli, M. N. Armenise, “Graphene-based fine-tunable optical delay line for optical beamforming in phased-array antennas,” Applied Optics, vol. 55, no. 16, pp. 4342-4349, 2016. doi: https://doi.org/10.1364/AO.55.004342
[7] C. Ciminelli, G. Brunetti, D. Conteduca, F. Dell’Olio, M. N. Armenise, “Integrated Microphotonic Tuneable Delay Lines for Beam Steering in Phased Array Antennas,” In 2018 20th International Conference on Transparent Optical Networks (ICTON), Bucharest, Romania, July 1-5, 2018, pp. 1-4. doi: 10.1109/ICTON.2018.8473592
[8] C. Ciminelli, N. Saha, G. Brunetti, A. Di Toma, M. N. Armenise, “Reconfigurable Optical Beam Forming Network for Telecom Payloads,”. In 2024 24th International Conference on Transparent Optical Networks (ICTON), Bari, Italy, July 14-18, 2024, pp. 1-4. doi: 10.1109/ICTON62926.2024.10647596
[9] M. N. Armenise, A. di Toma, G. Brunetti, N. Saha, C. Ciminelli, “Flexible photonic integrated circuits: a new paradigm to process data on-board satellites,” In 2023 23rd International Conference on Transparent Optical Networks (ICTON), Bucharest, Romania, July 02-06, 2023, pp. 1-1. doi: 10.1109/ICTON59386.2023.10207537
[10] A. di Toma, G. Brunetti, M. N, Armenise, C. Ciminelli, “LiNbO_3-Based Photonic FFT Processor: An Enabling Technology for SAR On-Board Processing,” Journal of Lightwave Technology, vol. 43, no. 2, pp. 912-921, 2025. doi: 10.1109/JLT.2024.3453670.