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<p>Data from the US and German Gravity Recovery And Climate Experiment (GRACE) showed indications of pre-, co-, and post-seismic mass redistributions associated with earthquakes down to a magnitude of 8.3 Mw. These demonstrated state-of-the-art capabilities in obtaining high spatial resolution space-based gravimetry, and helped to improve understanding of mantle rheology, potentially even providing a route to developing early warning capabilities for future seismic events. We describe a new mission concept, GRAvity observations by Vertical Laser ranging (GRAVL), which aims to extend the earthquake detection limit down to magnitude 6.5 Mw, significantly increasing the number of observable events.</p><p>GRAVL directly measures the radial component of the acceleration vector via “high-low” inter-satellite laser ranging, increasing gravity field sensitivity. A constellation of Low-Earth Orbit (LEO) satellites act as test masses, equipped with reflectors and high precision accelerometers to account for non-gravitational forces. Two or more larger satellites are placed above these, in Geostationary or Medium Earth Orbit (GEO / MEO), and measure the distance to the LEO satellites via time-of-flight measurement of a laser pulse. To do this, the GEO/MEO spacecraft are each equipped with a laser, telescope and detector, and additionally require highly  accurate timing systems to enable ranging accuracy down to sub-micron precision. To detect co-seismic mass redistribution events of the desired magnitude, we determine a gravity field measurement requirement of order 0.1 µGal at a spatial resolution of approximately 100 km over a 3-day revisit interval. These are challenging requirements, and we will discuss possible approaches to achieving them.</p><p>The GRAVL mission concept was developed during the FFG/ESA Alpbach Summer School 2019 by a team of science and engineering students, and further refined using the Concurrent Engineering approach during the Post-Alpbach Summer School Event at ESA Academy's Training and Learning Facility at ESEC-Galaxia in Belgium.</p>

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<p>Carbon emissions related to fossil-fuel use are particularly localized, with urban areas being the dominant contributor responsible for more than 70% of global emissions. In the future, the share of the urban population is expected to continue to rise, leading to further increased focusing of fossil-fuel related emissions in urban areas. Cities are also the focal point of many political decisions on mitigating and stabilization of emissions, often setting more ambitious targets than national governments (e.g. C40 cities). For example, the Mayor of London has set the ambitious target for London to be a zero-carbon city by 2050. If we want to devise robust, well-informed climate change mitigation policies, we need a much better understanding of the carbon budget for cities and the nature of the diverse emission sources underpinned by new approaches that allow verifying and optimizing city carbon emissions and their trends.</p><p>New satellite observations of CO<sub>2 </sub>from missions such as OCO-3, MicroCarb and CO2M, especially when used in conjunction with ground-based sensors networks provide a powerful novel capability for evaluating and eventually improving existing CO2 emission inventories. We will set up a measurement network up-and downwind of London using portable greenhouse gas (CO<sub>2</sub>, CH<sub>4</sub>, CO) column sensors (Bruker EM27/SUN) together with UV/VIS DOAS spectrometers (NO<sub>2</sub>), which will be operated for extended time periods thanks to automatization of the sensors. The data acquired from the network will not only allow us to critically assess the quality of satellite observations over urban environments, but also to derive data-driven emission estimates using a measurement-modelling framework. In this presentation we will discuss the setup of the experiment, give a description of the sensors, and show some first observations obtained with the sensors.</p>