An Agile Radar System for High-Resolution 3-D Surface Topography and Vegetation Structure Measurements from Stratospheric and Distributed Platforms

Interferometric synthetic aperture radar (InSAR) is a critical sensing modality for many Earth science investigations and has the capability of addressing key measurement gaps identified in NASA's Surface Topography and Vegetation (STV) Study Team Report (STR). When paired with the emerging category of stratospheric platforms (high-altitude pseudo satellites, or HAPS), a properly designed InSAR system provides a new measurement capability able to address the most stringent horizontal, vertical, and revisit aspirations for bare surface topography, water surface topography, and vegetation structure. For example, for bare surface topography, the critical values are 10 cm accuracy and 0.5 to 1.0 m resolution with repeat measurements as fast as one day. To achieve these aspirational requirements from space requires a large constellation of capable InSAR or LiDAR systems, an approach that is almost certainly too large in scope for a near-term STV mission. However, many of the most stringent requirements are driven by regional phenomena: volcanos, faults, and landslides among them. Suborbital HAPS can provide cost-effective broad regional coverage with tailored revisit times and reconfigurable formations, and when coupled with a compact X-band radar and innovative algorithms, can achieve these stringent measurement aspirations from a single platform or pair of platforms. Aloft Sensing, Inc. (Aloft) has developed an innovative Synthetic Aperture Radar (SAR) sensor small enough to be accommodated on the lightest of stratospheric HAPS vehicles, yet capable enough to meet the most demanding STV requirements. Funded by prior NASA ESTO IIP and DSI awards, Aloft's X-band Radar (AXR) system is enabled by Aloft's patented, ultra-precise Position, Navigation, and Timing (AloftPNT) algorithms, allowing SAR coherency to be maintained over long temporal and spatial apertures, including across multiple distributed platforms. The combination of our unique sensor architecture, AloftPNT, and emerging HAPS capabilities enables Aloft to provide perhaps the only realizable, single measurement methodology to simultaneously satisfy the STV accuracy, resolution, and revisit requirements. In this effort, Aloft proposes to extend our current stratospheric AXR SAR capabilities to include single-pass InSAR, enabling rapid repeat, aspirational-level accuracy that fills critical STV observation gaps. We start by establishing this capability from a single stratospheric vehicle, demonstrating a first-of-its-kind measurement system for bare earth topography measurements. We then propose to extend to multiple platforms, allowing longer baselines that can transform the measurement of surface water extent, and ultimately, through the use of three or more platforms, provide unique vegetation structure measurements currently unavailable from any existing measurement technique. The outcome of this effort is a new and innovative suborbital measurement system that significantly advances the state of the art. The resulting single-pass InSAR capability has immediate applicability to the most challenging STV domains, and particular relevance to regional studies of volcanism, pre- and post-hazard monitoring and assessment, surface water studies, and advanced vegetation structure characterization. The demonstrated performance has the additional benefit of informing future STV system architecture designs and positioning STV observation for low risk and high readiness in the next Decadal Survey. Period of Performance: A 36-month effort: 15 June 2025 to 14 June 2028. Entry TRL: 3, Exit TRL: 6. As a stand-alone SAR sensor, AXR currently exists at TRL-8; it is a fully functional SAR and repeat-pass InSAR sensor with laboratory, environmental, aircraft, and stratospheric flight testing. As a single-pass InSAR HAPS instrument, AXR is at TRL-3. We expect to reach system TRL-8 for the single-pass InSAR HAPS system and TRL-6 for the 3-D structure measurement system.

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