TANGO and the Synergistic Exploitation of Data using Polarimetric Observations

TANGO (Twin Anthropogenic Greenhouse gas Observers) is a CubeSat mission within the ESA SCOUT programme, scheduled for launch in 2028. The mission comprises two CubeSats flying in close formation—TANGO-Carbon and TANGO-Nitro. TANGO-Carbon is dedicated to quantifying anthropogenic greenhouse gas emissions of carbon dioxide (CO₂) and methane (CH₄), whereas TANGO-Nitro performs measurements of nitrogen dioxide (NO₂) to support the detection and delineation of emission plumes. Owing to highly agile attitude control and a ground sampling distance of approximately 300 × 300 m², the satellites are capable of resolving and characterizing individual emission sources. The TANGO mission will retrieve the dry-air column-averaged mole fractions of carbon dioxide (XCO₂) and methane (XCH₄) using the proxy retrieval methodology described in Frankenberg (2005). In this approach, a known column abundance of a proxy gas species (e.g. CH₄) is used to infer the total light-path modification, which is subsequently employed to correct a non-scattering retrieval of the mixing ratio of the target gas (e.g. CO₂). The primary scientific objective of TANGO is the accurate quantification of emissions in situations where the proxy gas is not co-emitted with the target gas, a condition that is characteristic of most anthropogenic emissions from the energy sector. In the present study, we assess the feasibility of implementing a full-physics retrieval framework that synergistically combines TANGO observations with collocated aerosol measurements, with the objective of disentangling and independently constraining the information content on CO₂ and CH₄ for optimized data exploitation. Specifically, we analyse a sequential data-processing strategy in which aerosol properties are first retrieved from measurements of a multi-angle polarimeter and then assimilated as prior information into a TANGO full-physics retrieval scheme. The TANGO satellites will exhibit overlapping spatial coverage with the 3MI instrument onboard MetOp-SG and with CO2M. The nominal local overpass times are 09:30 for 3MI, 11:00 for TANGO, and 12:00 for CO2M, providing aerosol observations with an approximate one-hour temporal offset relative to TANGO. This orbital configuration enables synergistic exploitation of data from the three missions. Although CO2M and 3MI provide measurements at a substantially coarser spatial resolution of approximately 4 km², the proposed sequential approach is expected to yield aerosol constraints of sufficient accuracy to mitigate aerosol-induced errors in TANGO data products. Furthermore, the synergy among the missions has the potential to lower the detection limits of all three systems by improving the precision of XCH₄ and XCO₂ retrievals and may facilitate observations over complex source regions with mixed emission types, such as large industrial agglomerations. To demonstrate the viability of the proposed approach, we will quantify the performance gains for the TANGO products in the joint retrieval of CO₂, CH₄, and aerosol properties from collocated CO2M and TANGO observations for individual overpasses over TANGO target areas, explicitly accounting for the differing spatial resolutions of the two missions.