CHAPS: A New, Compact Hyperspectral Imager for Air Pollution Remote Sensing

Current and planned low Earth orbit and geostationary satellite instruments have long provided global surveys, revealing air pollution characteristics and trends. Targeted pollution observations with even finer spatial and temporal resolution would better characterize, quantify, and monitor emissions from urban areas, power plants, and other anthropogenic activities, with both scientific and societal benefits. The Compact Hyperspectral Air Pollution Sensor (CHAPS) is an imaging spectrometer in a CubeSat form factor, made possible by the use of freeform optics and additive manufacturing. CHAPS has the potential to complement global surveyors and provide targeted observations valuable for understanding air quality at urban scales. The instrument is designed to make measurements of atmospheric composition at 300–500 nm (@ 0.6-nm spectral resolution) at unprecedented spatial resolution from low Earth orbit (1 x 1 km²). The NASA Earth Science Technology Office has funded the development of a CHAPS–Demonstrator (CHAPS-D), which will result in an airborne demonstration of a CHAPS prototype instrument. The CHAPS-D project is a joint collaboration of JHU/APL (USA) and TNO (The Netherlands). CHAPS-D freeform optics derive heritage from the Sentinel-5 Precursor (TROPOMI) mission. Freeform optics has potentially huge advantages over traditional optical designs, including fewer optical surfaces and lower mass and volume, while maintaining optical performance, and CHAPS-D will fit within the design constraints of a 6U CubeSat. The CHAPS-D mechanical structure and some optical elements will be fabricated using additive manufacturing, using a next-generation aluminum alloy. This approach simplifies the construction of the instrument and allows for integral stray light baffling features not possible using traditional fabrication approaches. The compact size and relatively lower cost of CHAPS makes a constellation feasible for the first time, with unprecedented spatiotemporal sampling of global point pollution sources. The project will culminate in an airborne demonstration of CHAPS-D, with 30-m spatial resolution. We will retrieve NO₂, SO₂, HCHO, ozone, and other trace species relevant to air quality from solar backscatter measurements. We present the science context, measurement requirements, and preliminary design of CHAPS-D, as well as results from breadboard testing.