For more than a decade, the remote sensing community has called for longwave spectral measurements of the Earth system to facilitate closing the radiation budget. In an effort to address this large gap in Earth-observing capability, NASA has supported the development and implementation of the Polar Radiant Energy in the Far-Infrared (FIR) Experiment (PREFIRE), which realizes two miniaturized thermal IR spectrometers (TIRSs), one on each of two polar orbiting small satellites with different ascending node crossing times. The spectral radiances measured by the TIRS instruments are utilized to extract cloud presence, atmospheric state, surface properties, and the associated top-of-atmosphere (TOA) spectral fluxes at both poles. This article describes the major system and algorithm components and introduces the geophysical products. PREFIRE data will be used to inform and improve ice-sheet and coupled-Earth system models to better predict the future state of the Earth’s poles.

The Orbiting Carbon Observatory-3 (OCO-3) instrument, operated from the International Space Station, was launched on May 4, 2019. Its primary objective is to provide high-precision estimates of the column-averaged dry-air mole fraction of carbon dioxide (XCO2) and solar-induced chlorophyll fluorescence. It consists of three long-slit imaging spectrometers that provide high-resolution spectra in three infrared bands. The shortest wavelength one, named A-band, is centered at$0.765~\mu $m and characterized by numerous absorption lines of molecular oxygen. Throughout the mission, this band has undergone important changes in instrument response related to contamination of the focal plane array (FPA). One way contamination buildup affects OCO-3 A-band data is by imparting an S-like shape to the observed spectra, which cannot be characterized and corrected through analysis of data from the onboard calibrator alone. In Version 10, this artifact manifested itself as a slow drift of XCO2, from February 2020 to January 2021, and required an additional postretrieval bias correction. In the latest collection, Version 11, OCO-3’s inflight calibration algorithm for the A-band was augmented to correct this artifact using clear ocean scenes to derive the spectral shape of changes in instrument response. This drastically improved the shape of spectral residuals between observed and forward modeled radiances and also significantly reduced the slow XCO2 drift observed in Version 10.

The Orbiting Carbon Observatory-3 (OCO-3) measures carbon dioxide and solar-induced fluorescence from the International Space Station (ISS). It uses the flight spare spectrometers from its predecessor, OCO-2, and produces spectral images in three near-infrared channels. Its preflight radiometric calibration was performed at NASA’s Jet Propulsion Laboratory and is constantly updated inflight to account for both gradual and abrupt changes in instrument response that display a wavelength dependency within the bands. Some of these are caused by the accumulation of contaminants and their subsequent removal after scheduled decontamination events, but sudden changes of the overall gain state of the two longest wavelength bands, unrelated to contamination, are also observed. They were found to be triggered by instrument resets as well as by decontamination events. OCO-3 cannot perform solar calibration due to its position on the ISS and the inflight updates to its radiometric calibration depend solely on its on-board calibrator, which consists of three lamps and a reflective diffuser. The lamps are observed with different cadences and thus degrade at different rates. Information from all three lamps is combined to provide high temporal resolution and minimize the impact of lamp aging on the gain degradation coefficients that describe the changes in the radiometric response of the instrument inflight. We use thousands of soundings from ocean scenes to assess the relative calibration within the two shortest wavelength bands. Here we present the algorithm developed for the inflight relative radiometric calibration of OCO-3 Version 10 products and discuss the state of the calibration.