The CarbonSat candidate mission: imaging greenhouse gas concentrations from space

CarbonSat is a proposed Earth observation mission, which was selected in 2010 as one of two candidates for becoming the European Space Agency's (ESA) eighth Earth Explorer (EE8). It is currently undergoing parallel feasibility studies (phase A) performed by two industrial consortia. CarbonSat aims at a better understanding of the natural and anthropogenic sources and sinks of the two most important anthropogenic greenhouse gases CO2 and CH4, which will contribute to a better understanding of climate feedback and forcing mechanisms. To achieve these objectives the instrument will quantify and monitor the spatial distribution of carbon dioxide (CO2) and methane (CH4). It will deliver global data sets of dry air column-averaged mixing ratios of these gases with high precision (1 - 3 ppm for CO2 and 6 - 12 ppb for CH4) and accuracy (0.5 ppm for CO2 and 5 ppb for CH4). The measurements will provide global coverage every 12 days above 40 degrees latitude at a spatial resolution of 2 x 3 km(2). The retrieval products are inferred from observations of Earth radiance and solar irradiance at high to medium spectral resolution (0.1-0.55 nm) in the Near Infrared (747-773 nm) and Short Wave Infrared (1590-1675 nm and 1925-2095 nm) spectral regions. The combination of high spatial resolution and global coverage requires a swath width larger than 180 km for three spatially co-aligned push-broom imaging spectrometers. The targeted product accuracy translates into stringent radiometric, spectral and geometric requirements for the instrument. This paper presents the system requirements derived from the demanding mission objectives and reports preliminary results of the feasibility studies. It highlights the key components of the instrument, focusing on the optical conceptual design, and addresses the identified critical performance aspects.