Airborne lidar reflectance measurements at 1.57 μm in support of the A-SCOPE mission for atmospheric CO<sub>2</sub>

Abstract

Abstract. The characteristics of the lidar reflectance of the Earth's surface is an important issue for the IPDA lidar technique (integrated path differential absorption lidar) which is the proposed method for the spaceborne measurement of atmospheric carbon dioxide within the framework of ESA's A-SCOPE project. Both, the absolute reflectance of the ground and its variations have an impact on the measurement sensitivity. The first aspect influences the instrument's signal to noise ratio, the second one can lead to retrieval errors, if the ground reflectance changes are strong on small scales. The investigation of the latter is the main purpose of this study. Airborne measurements of the lidar ground reflectance at 1.57 μm wavelength were performed in Central and Western Europe, including many typical land surface coverages as well as the open sea. The analyses of the data show, that the lidar ground reflectance is highly variable on a wide range of spatial scales. However, by means of the assumption of laser footprints on the order of several tens of meters, as planned for spaceborne systems, and by means of an averaging of the data it was shown, that this specific retrieval error is compatible with the sensitivity requirements of spaceborne CO2 measurements.

Simulation studies for a space-based CO₂ lidar mission

“…We include a 0.05% 'representation error' globally to account for the ability of the narrow (∼70 m wide) laser sample stripe to measure mean <CO 2 > in a grid sample volume of about (70 km) 2 (Miller et al, 2007;Corbin et al, 2008). Another 0.1% uncorrelated error is introduced over land surfaces to account for noise produced by changes in surface reflectance and non-simultaneity of the laser footprints at the pulsed laser sample wavelengths (Amediek et al, 2009) along with other pointing/timing errors (Ehret et al, 2008). We have not included spectroscopy, a priori and retrieval fitting, or other systematic instrument errors at this time.…”

Performance simulations for a spaceborne methane lidar mission

“…Occasional ambiguities by surface pressure uncertainties or the presence of near-surface clouds can be circumvented by accurate ranging (in the order of several meters), use of a high-resolution digital elevation model, and accurate pointing knowledge. Recent studies assessed the potential of spaceborne IPDA lidars: On board a low-polar orbit satellite, this method is able to provide column carbon dioxide or methane measurements with an accuracy (systematic uncertainty; bias) of better than 1%, a precision (random uncertainty; noise) of around 1%, and global coverage between 82°S and 82°N, largely independent of aerosol load or sunlight [Dufour and Bréon, 2003;Ehret and Kiemle, 2005;Ehret et al, 2008;Amediek et al, 2009;Kawa et al, 2010;Kiemle et al, 2011]. First airborne IPDA lidar deployments recently corroborated the studies' results [Abshire et al, 2014].…”