Abstract
With three imaging grating spectrometers, the Orbiting Carbon Observatory-2 (OCO-2) measures high spectral resolution spectra ( λ / Δ λ ≈ 19,000) of reflected solar radiation within the molecular oxygen (O 2 ) A-band at 0.765 μ m and two carbon dioxide (CO 2 ) bands at 1.61 and 2.06 μ m. OCO-2 uses onboard lamps with a reflective diffuser, solar observations through a transmissive diffuser, lunar measurements, and surface targets for radiometric calibration and validation. Separating calibrator aging from instrument degradation poses a challenge to OCO-2. Here we present a methodology for trending the OCO-2 Build 8R radiometric calibration using OCO-2 nadir observations over eight desert sites and nearly simultaneous observations from Moderate Resolution Imaging Spectroradiometer (MODIS) with sensor viewing zenith angles of 15 ± 0.5 ∘ . For the O 2 A-band, this methodology is able to quantify a drift of −0.8 ± 0.1% per year and capture a small error in correcting the aging of the solar calibrator. For the other two OCO-2 bands, no measurable changes were seen, indicating less than 0.1% and less than 0.3% per year drift in the radiometric calibration of Band 2 and Band 3, respectively.
Highlights
The Orbiting Carbon Observatory 2 (OCO-2) satellite launched in 2014 and the Orbiting Carbon Observatory 3 (OCO-3) payload launched in 2019 are NASA Earth science missions designed to return estimates of CO2 with the precision, accuracy, resolution and coverage needed to identify sources and sinks of CO2 on regional scales [1,2,3,4]
In a follow-up paper, we will improve the accuracy of the slopes by normalizing the seasonal variations using a semi-empirical Bidirectional Reflectance Distribution Function (BRDF) model
The Moderate Resolution Imaging Spectroradiometer (MODIS) data are collocated into the OCO-2 geolocation grid using a circular region with a radius of 2 km around OCO-2 footprint
Summary
The Orbiting Carbon Observatory 2 (OCO-2) satellite launched in 2014 and the Orbiting Carbon Observatory 3 (OCO-3) payload launched in 2019 are NASA Earth science missions designed to return estimates of CO2 with the precision, accuracy, resolution and coverage needed to identify sources and sinks of CO2 on regional scales [1,2,3,4]. OCO-2 and OCO-3 use high resolution imaging spectrometers to measure solar radiance reflected by the Earth atmosphere and surface at high spectral resolution (λ/∆λ ≈ 19,000) After calibration, these measurements are analyzed to estimate the column-average dry air mole fraction of CO2 (XCO2) in the atmosphere [1]. To achieve the required 0.25% accuracy in the retrieved XCO2 estimations, the OCO missions require a 5% accuracy at 1σ confidence level on absolute radiometric calibration. The OCO-2 instruments were characterized and calibrated during preflight testing [5], but the radiometric performance must be tracked on orbit as various components degrade
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