Global Dynamic Topography : the next Frontier for Geosciences
Dewez, Thomas1; Z-Earth International Team, .2

The advent of free-of-charge global topographic data sets SRTM and Aster GDEM have enabled testing a host of geoscience hypotheses. This is because they revealed for the first time the relief of previously undescribed earth landscapes, enabled quantitative geomorphometric analyses across entire landscapes and improved the resolution of measurements. Availability of such data is now considered standard, and though resolved at 30-m to 90-m pixel size, which is amazing seeing where we come from, they are now regarded as mostly obsolete given the sub-meter imagery coming through web services like Google Earth. Two additional features would help meet the current data needs of the Geoscience communities: field-scale-compatible elevation datasets (i.e. meter-scale digital models and sub-meter elevation precision) and provision of regularly updated topography to retrieve earth surface changes, while retaining the key for success: data availability at no charge.

A new satellite instrument called Z-Earth has undergone phase 0 study at CNES, the French space agency to fulfil these aims. The scientific community backing this demand is that of natural hazards, glaciology and the biomass community. The system under study combines a native stereo imager and a lidar profiler. This combination provides spatially resolved elevation swaths together with absolute along-track elevation control point profiles. Data generated through this system, designed for revisit time better than a year, is intended to produce not only single acquisition digital surface models, colour orthoimages and small footprint full-wave-form lidar profiles to update existing topographic coverage, but also time series of them. This enables 3D change detection fields with centimetre-scale planimetric precision and metric vertical precision, in support of now traditional spectral change detection. Designed to meet the data requirements of natural hazard and glaciology communities, Z-Earth definition also supports other applications such as forest ecosystems monitoring and biomass assessment.

The purpose of this contribution is to present the mission concept and its philosophy and the scientific needs, including foreseen societal benefits, that justify the system development.