DOI: 10.48577/jpl.bhu1l3
title: Compact-Fire Infrared Radiance Spectral Tracker (c-FIRST) for Small Satellite Platforms

description: 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 (&gt;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 (&gt;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 &gt;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 ~4.5m 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.