Compact-Fire Infrared Radiance Spectral Tracker (c-FIRST)

Remote sensing of wildland fires is required for many cross-disciplinary science investigations including wildland fire impacts on ecology. For decades this research has been hindered by insufficient spatial resolution and detector saturation at short and mid-infrared wavelengths where the spectral radiance from high temperature (>800 K) surfaces is most significant. To address this, we’re developing a compact high dynamic range (HDR) multispectral imager. The Compact Fire Infrared Radiance Spectral Tracker (c-FIRST), which leverages digital focal plane array (DFPA). The DFPA is hybridized from a state-of-the-art high operating temperature barrier infrared detector (HOT-BIRD) and a digital readout integrated circuit (D-ROIC), which features an in-pixel digital counter to prevent current saturation, and thereby provides dynamic range (>100 dB). The DFPA will thus enable unsaturated, high-resolution imaging and quantitative retrievals of targets with a large variation in temperatures, ranging from 300 K to >1600 K (flaming fires). With the resolution to resolve 50 m-scale thermal features on the Earth’s surface from a nominal orbital altitude of 500 km, the full temperature and area of wildland fires and the cool background are captured in a single observation, increasing science content per returned byte. The use of a non-saturating FPA is novel, overcomes previous problems where high radiance values saturate FPA pixels (which diminishes the science content), and demonstrates a breakthrough capability in remote sensing. Thus, c-FIRST is suitable for quantifying emissions from wildland fires, which is critical for establishing their impact on ecosystems at global scales. The FPA for the c-FIRST was fabricated using InAs/InAsSb HOT-BIRD epitaxial material into 20m pixel pitch, 1280x480 format detector arrays and hybridized to analog DROIC. The 50% cutoff of the DFPA is at ~4.5um and the measured external QE~50% across the full QE spectrum at 140K operating temperature. We fix the integration time at 6 ms in order to obtain good sensitivity in the MWIR bands when observing normal 300K background scene at 150 Hz frame rate. For a standard analog ROIC, the detector pixels are easily saturated at target temperatures ~700 K. With the D-ROIC operating in the 16-bit mode, we can increase the saturation temperatures significantly to ~1100 K. With the D-ROIC operating in the ultra-HDR 32-bit mode (28 trillion e- well depth), the detectors do not come near to saturation even for 1600 K targets. A critical metric for remote sensing of fires is the minimum detectable target size. The c-FIRST would provide an order of magnitude improvement in the minimum size of a detectable fire, primarily due to the spatial resolution of the non-saturating detector than the current servicing instruments such as Advance Baseline Imager on GOES, etc. with reduced power, size, and weight. c-FIRST airborne flights for instrument test and validation scheduled for 2024 fire season. We’re expecting c-FIRST space validation based on a space technology validation opportunity in 2026 or later.

野火遥感是诸多跨学科科学研究的必要支撑，其中包括野火对生态系统的影响研究。数十年来，该领域的研究一直受限于空间分辨率不足，以及短红外与中红外波段的探测器饱和问题——而高温（>800 K）表面的光谱辐射亮度在该波段最为显著。为解决这一难题，我们正在研发一款紧凑型高动态范围（HDR）多光谱成像仪：紧凑型野火红外辐射光谱追踪仪（c-FIRST），其搭载数字焦平面阵列（DFPA）。该DFPA由当前最先进的高温势垒红外探测器（HOT-BIRD）与数字读出集成电路（D-ROIC）混合集成而成；D-ROIC内置像素内数字计数器，可防止电流饱和，因此可实现大于100 dB的动态范围。借此，DFPA可实现对温度跨度极大（300 K至>1600 K，即明火）的目标进行无饱和高分辨率成像与定量反演。该设备在500 km的标称轨道高度下，可分辨地表50米级的热特征，单次观测即可完整捕获野火的温度、面积以及低温背景信息，从而提升每回传字节所承载的科学价值。采用非饱和焦平面阵列的设计极具创新性，攻克了此前高辐射亮度导致FPA像素饱和（进而降低科学数据价值）的难题，展现了遥感领域的突破性技术能力。因此，c-FIRST可用于量化野火的排放物，这对于在全球尺度上评估野火对生态系统的影响至关重要。c-FIRST所用的FPA采用InAs/InAsSb型HOT-BIRD外延材料制备，像素间距为20 μm，探测器阵列格式为1280×480，并与模拟型D-ROIC实现混合集成。该DFPA的50%截止波长约为4.5 μm，在140 K的工作温度下，其全量子效率（QE）波段的外部量子效率约为50%。为在150 Hz的帧速率下观测300 K的标准背景场景时获得良好的中波红外（MWIR）波段灵敏度，我们将积分时间固定为6 ms。对于标准模拟ROIC而言，当目标温度约为700 K时，探测器像素便极易出现饱和。当D-ROIC工作于16位模式时，饱和温度可大幅提升至约1100 K。当D-ROIC工作于超高HDR的32位模式（阱深达28万亿电子）时，即便目标温度达1600 K，探测器也不会出现饱和。野火遥感的一项关键指标是可探测目标的最小尺寸。相较于当前在轨服务的仪器（如GOES卫星上的先进基线成像仪等），c-FIRST可将可探测野火的最小尺寸提升一个数量级，同时还具备更低的功耗、更小的体积与更轻的重量。c-FIRST的机载飞行测试与验证任务已计划于2024年野火季开展；我们预计将在2026年及以后的空间技术验证任务中完成c-FIRST的在轨验证。