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Miniaturised Photonics Enabled Next Generation SAR

Periodic Reporting for period 1 - RETINA (Miniaturised Photonics Enabled Next Generation SAR)

Reporting period: 2018-11-01 to 2020-04-30

There is a natural trade-off between spacecraft size and functionality in all current satellite applications, independently of orbit and mission. Therefore, advances in both miniaturization and integration technologies are required to increase satellites’ lifetime and performance, simultaneously reducing their cost. In case of the next generation of Earth Observation satellites, one of the key development areas is synthetic aperture radar (SAR) antennas, where expected progress will be to increase the operating bandwidth - requiring, for instance wideband true-time delay (TTD) beamformers - and miniaturization, drastically reducing the mass and volume compared to current implementations.
In this scenario, the use of photonic integrated circuits (PIC) technology in the beamforming network, in combination with an optical fibre harness, are obvious key enabling technologies for future SAR instruments. Optically implemented TTD beamforming structures achieve orders-of-magnitude improvements in size and mass. Photonic technology also brings easy routing thanks to wavelength-division multiplexing, antenna and RF system integration due to the EMI-free characteristic of the optical fibre and a reduction of the risks associated with the in-orbit antenna deployment. Additionally, the inherent broadband characteristic of photonic technology, related to the transport and processing of RF signals, simplifies the beamforming network and signal distribution design for different frequencies, applications and missions. This is because the same physical media can be used, as opposed to traditional RF implementations in which different substrates, waveguides and components must be used for different frequencies of operation.
RETINA will perform the necessary design and research to enable the development of an advanced reconfigurable multi-beam photonic beamformer with centralised processing. In particular, a very innovative SAR approach based on photonic technologies whose main impact is to increase the EU competitiveness in payloads for advanced SAR missions by bringing to TRL5-6 previous developments in photonic enabled payloads for SAR applications starting from the previous FP7 project GAIA. The implementation of compact, suitable for space, broadband frequency operation multi-beam photonic beamformer network (BFN) with centralized processing for Next Generation SAR solving the limitations found in terms in power consumption and switching speed.

The RETINA objectives have been clearly identified, individualized and described in a measurable manner:
Objective 1: Development of a multi carrier laser with space-grade specifications reducing the power consumption associated to the thermal cooling by a factor of 3 per laser.
Objective 2: The design of photonic integrated circuits (PIC) designs in silicon-nitride technology implementing a true-time delay (TTD) BFN compatible with large antennas requirements.
Objective 3: Array antenna element (AE) design at X-band, amplification section and opto/electronic converters for integration in the AE.
Objective 4: Design and manufacturing of a TRL6 small footprint broadband beam optical transmitter module up to Ka-band including RF and optical amplification and modulation.
Objective 5: Design and manufacturing of a TRL6 compact antenna optical receiver in X-band with a co-package of amplifiers, optical receiver and AE.
Objective 6: Design, manufacturing and supply of photoreceivers in the X band with optimized footprint and self-stabilized gain vs temperature. The photodiode within the photoreceiver will cover up to Ka-band to address the objective of broadband frequency operation.
Objective 7: Flexible beam-shaping and beam-switching in a PIC-based optical BFN, through multi-orthogonal beam synthesis.
During the first period the main activities have been focused on the identification of the application scenarios (suitable missions) and its top level requirements, the system definition and the design of the subsystems integrating the multi-beam photonic beamformer.

Different application scenarios have been identified during the first period which are suitable to apply the photonic technology to be developed in RETINA for SAR antenna beamforming; the baseline scenarios include one related to big platforms, one to small platforms and one to satellites in formation, in order to cover as many applications as possible.. Several high-level requirements have been included covering such scenarios, most of them applicable not only to the Antenna system, but also at full payload level. Moreover, it has been defined the antenna conception incorporating an advanced broadband reconfigurable multi-beam photonic beam-former.
According with the above beamformer requirements, the design of the different subsystems has been initiated in this period, which included important parts of the system as the photonic integrated circuit (PIC) as part of the Beam Former network (BFN), the photoreceivers & the array antenna element as part of the Antenna optical X-band Receiver (AOR), a laser array module and the Beam optical X-band Transmitter (BOT). In parallel to the PIC design, important advances in the photonic manufacturing process have been made as well as in the photonic packaging design.
The evolution of future generation of SAR has shown a clear trend towards systems with higher performance resulting on higher complexity (larger antennas, operating bandwidth and/or different frequencies) at lower cost, less mass, size and power consumption. This trend imposes strong requirements in the today’s RF and antenna technology since larger antennas means complex, bulky, difficult to route RF harness to transport the signal from/to the beamforming to/from antenna, and strong mechanical and thermal requirements, especially when in-orbit deployable antenna structures are required. On the other hand, larger bandwidths associated with larger antennas and scanning angles requires True-Time-Delay beamforming, resulting in bulky and complex solutions (and very limited for lower frequencies – larger delays) impacting directly in the size, mass and integration cost. After FP7 GAIA project completion in September 2015, it was demonstrated that significant progress beyond the State-of-the-Art in antenna technology for SAR applications can be achieved by the use of photonic beamforming and PIC technology, but that further improvement of the SWaP and switch speed will be required to develop a competitive photonic enabled SAR technology for future Earth observation missions which will contribute to strength the European leading in Earth Observation in line to Copernicus objectives.
The main expected impact of RETINA is to increase the EU competitiveness in payloads for advanced SAR missions by bringing to TRL5 previous developments in photonic enabled payloads for SAR applications starting from the previous FP7 project GAIA, solving the limitations found in terms of power consumption and switching speed and incorporating new features such as centralised signal processing and a truly broadband frequency operation approach thanks to the multi-beam TTD reconfigurable beamforming architecture.
RETINA building blocks chip