Flexible Configuration Distributed Synthetic Aperture Digital Beamforming Radar (FlexSAR)

We propose to develop technologies for a flexible, scalable, and adaptive distributed architecture for implementing synthetic aperture radar (SAR) systems, hereinafter referred to as "FlexSAR," to address a number of targeted observables (TOs) identified by the 2017-2027 Decadal Survey. Considering the diversity of products needed to accommodate the Surface Topography and Vegetation (STV) TOs, and the resulting observational frequencies and modalities, a fundamentally new approach to realizing this vision must be developed. We therefore propose the FlexSAR technology such that multiple observational needs can be realized within the same unified architecture. The proposed technology hinges on digital beamforming electronics that are (1) flexible and scalable across a large frequency range including P-band, L-band, and beyond, (2) reconfigurable for different imaging modalities such as polarimetry, interferometry, ScanSAR, and spotlight mode SAR, (3) reconfigurable for resolution and spatial coverage. This architecture will be suitable for implementation on low cost distributed platforms such as CubeSats, achieving a large synthesized aperture via docking of multiple CubeSats that provides high signal-to-noise ratio, high effective spatial resolution, reduced overall system risks and potentially overall system costs, unprecedented flexibility and reconfigurability, and increased resilience compared to traditional one-off SAR systems. We will demonstrate the FlexSAR architecture with L-band and P-band SAR. L-band is chosen due to its commonality amongst multiple STV and Surface Deformation and Change (SDC) observables. P-band is chosen due to its ability to penetrate dense vegetation, an advantage in solid Earth and vegetation structure observations as the majority of the land surface is covered by vegetation. Spaceborne P-band radar concepts have long been hindered by the lack of spectrum allocation for Earth exploration and the need for very large antennas. We plan to employ spread spectrum techniques to work around restricted bands, while this distributed aperture approach eliminates the need of very large deployable antenna structures. New technologies are needed to enable digital beamforming across multi-platform elements of the distributed aperture. We propose to (i) design a distributed aperture architecture that will minimize grating lobes of the sparse array while optimizing radar performance, (ii) develop clock synchronization and calibration scheme for cross-platform antenna elements necessary to facilitate digital beamforming across the distributed aperture, (iii) conduct multi-frequency and imaging trade study, an incubation activity highlighted in the STV Incubation Study Report, to guide the FlexSAR design optimization and architecture, (iv) develop novel synthetic wideband waveforms to address fragmented spectrum availability in P-band, (v) develop a flexible simulation environment for multi-frequency radar retrieval of STV/SDC observables, followed by proof-of-concept science product development. To validate the multi-platform distributed aperture technology, we propose to develop Software Defined Radar electronics with clock synchronization, internal calibration, digital beamforming, and synthetic wideband waveform generation capabilities in a compact form factor. We will utilize UAVSAR's L-band active array antenna front-end to conduct airborne demonstration to further test the proposed technologies in a relevant environment. By modifying UAVSAR, NASA/JPL's airborne SAR testbed, we will simultaneously demonstrate FlexSAR's feasibility and prepare UAVSAR to help mature technology and science algorithms in support of STV studies. The proposed development is a 3-year effort. We will enter at TRL 2 and plan to exit at TRL 5.

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