The Earth Surface Mineral Dust Source Investigation (EMIT) is an Earth Ventures-Instrument (EVI-4) Mission aboard the International Space Station (ISS). EMIT’s prime mission was designed to map surface mineralogy of arid dust source regions using imaging spectroscopy in the visible and short-wave infrared (VSWIR). EMIT uses imaging spectroscopy to take mineralogical measurements of sunlit regions of interest between 52° N latitude and 52° S latitude. During the prime mission, the EMIT team focused on using surface reflectance to resolve absorption features of iron oxides, clays, sulfates, carbonates, and other dust-forming minerals in Earth's dust source regions. In the extended mission, EMIT’s core measurements remain the same, but acquisitions expand to sample all regions observable from the ISS orbit, supporting efforts across the atmosphere, biosphere, cryosphere, geosphere, hydrosphere, and more. An interactive map showing the regions being investigated, current and forecasted data coverage, and additional data resources can be found on the VSWIR Imaging Spectroscopy Interface for Open Science (VISIONS) EMIT Open Data Portal.

The EMIT Level 3 Gridded Surface Reflectance and Uncertainty (EMITL3RFL) Version 2 data product provides surface reflectance, uncertainty, atmospheric state, and observation data which are projected to a regular grid using World Geodetic System 1984 (WGS 84). The pixel size of the grid is 0.00055 degrees, corresponding to an approximate spatial resolution of 60 meters (m).

Each EMITL3RFL granule consists of three Network Common Data Format Version 4 (netCDF-4) files (gridded Reflectance, gridded Reflectance Uncertainty, gridded Observation), one sidecar JSON (JavaScript Object Notation) file, and one full-resolution browse image. The Reflectance file (EMIT_L3_RFL) contains surface reflectance maps of 285 bands with a spectral range of 381-2493 nanometers (nm) at a spectral resolution of ~7.5 nm. These are all contained within a single science dataset layer/variable. Aerosol optical thickness and water vapor values are also included within their own separate variables. The Reflectance Uncertainty file (EMIT_L3_RFLUNCERT) contains uncertainty reported as one standard deviation, per channel, with covariance ignored. The Observation file (EMIT_L3_OBS) contains variables with viewing and solar geometries, timing, aspect, slope, and other information related to the observation for each pixel. The spectral information provided within each granule is the same as EMIT L2A Surface Reflectance and Uncertainty 60 m V002 (EMITL2ARFL), that has been orthorectified and had the cloud mask applied in advance. Each netCDF-4 file is also chunked and compressed into five precision fragments, to provide more efficient file access, particularly for cloud-access interfaces. The JSON sidecar file (EMIT_L3_RFL*.sidecar.json) is an optional file that may decrease the latency for some cloud-access interfaces.

Each granule is approximately 75 kilometers (km) by 75 km, nominal at the equator, with some reaching approximately 150 km in length at the end of an orbit segment.

Known Issues:

• Data acquisition gap: From September 13, 2022, through January 6, 2023, a power issue on the ISS caused a pause in EMIT operations. Due to this shutdown, no data were acquired during that timeframe. Outside of this period, regular short-term gaps in coverage are expected depending on the solar and sensor geometry.