NACHOS: A High-Resolution Hyperspectral Imagerfor Atmospheric Trace Gas Monitoring in a CubeSat Configuration

Satellite-based hyperspectral imaging (HSI) has proven to be an essential tool for missions aimed at imaging and quantifying trace gasses in the atmosphere - a capability highly relevant to many topics in earth sciences. Space-based HSI gas imaging has traditionally entailed large and expensive satellites producing massive amounts of data, which must be downlinked for processing and analysis. The high cost limits the number of satellites that can be deployed, while the data downloading and subsequent processing, delays the extraction of relevant and actionable information. An alternative paradigm would be to deploy constellations of small, inexpensive satellites to measure selected targets with the advantageous combination of frequent revisit times and high spatial resolution. With this long-term goal in mind, we have designed and constructed a high-throughput f/2.9, Offner-type imaging spectrometer spanning the spectral region from 250-500 nm with optical spectral resolution of 1.0 nm, 0.6 nm sampling, and 350 across-track spatial pixels at a resolution of 0.3 km/pixel from 400 km altitude, all housed in a 3U+ CubeSat package. Our instrument, named NACHOS, is spectroscopically comparable to NASA's Ozone Monitoring Instrument (OMI), but aimed at high spatial resolution measurements of localized targets to enable imaging of NO₂ at individual power plants and within urban areas, low-level SO₂ degassing at awakening volcanoes, as well as measurements of formaldehyde, ozone, and aerosols. The NACHOS satellite employs streamlined gas retrieval algorithms to achieve rapid on-board data processing despite the limitations of our CubeSat CPU. These algorithms distill the extremely large raw hyperspectral data cubes down to grayscale detection maps of a few selected gasses, reducing data volume to a level easily handled by the CubeSat's limited downlink bandwidth while still providing equivalent sensitivity to traditional, more computationally intensive methods. We will discuss the design and construction of the NACHOS instrument, its on-board software, and recent results from the operational instrument. These include laboratory measurements characterizing its spectral and spatial resolution, spectra of our NO₂ and SO₂ target gases, ground-based outdoor hyperspectral images exhibiting high-quality solar and atmospheric spectra, and results of environmental testing performed in anticipation of a late-2021 launch via the International Space Station under NASA's CubeSat Launch Initiative. Together, these results verify the instrument's capabilities for sensitive gas monitoring, and the resilience of its highly compact design to the rigors of launch and orbit. With this system, we hope to demonstrate the first step in an overall paradigm shift from expensive large-platform satellites to agile, inexpensive constellations of small satellites that will eventually enable unprecedented levels of temporal and spatial detail in trace-gas monitoring.