The water cycle is fundamental to human society and to life on Earth. Despite its importance, there are still aspects which are poorly understood and for which we only have limited measurements. It is increasingly recognised that improved temporal sampling is needed for future space missions, so that processes on timescales of minutes to hours can be observed directly. From space, this could be achieved using a large constellation of satellites in conventional low orbits, or a few satellites in geosynchronous orbit (GEO). A single GEO satellite can view Europe and Africa continuously, and is the concept for the G-CLASS proposal. Of the imaging bands available, microwave imagers (radar) are particularly good for water both at the surface (radar backscatter measurements) and in the atmosphere (using radar interferometry), and can provide all-weather imaging 24 hours per day. Using synthetic aperture radar it is possible to achieve resolution as good as 20 m from GEO. To improve understanding of the diurnal water cycle and related societal concerns such as floods, landslides and water resources, we therefore propose a GEO synthetic aperture radar mission: G-CLASS.
Specific water cycle science objectives for the mission are:
- Improve prediction capability for intense storms and related impacts (flood, landslides),
- Improve understanding of the diurnal water cycle, especially soil moisture in dry regions and snow-melt / re-freeze in mountain areas,
As weather prediction capability improves we need commensurate high resolution observations (in space and time) to initialise and validate the predictions. A GEO radar with its continuous viewing capability can potentially provide these. Diurnal forcing of soil moisture and snow state through the day are important for hydrology, and affect water resource management and agriculture. The radar measurements also enable general ground motion to be monitored, thus enabling a third science objective at no extra cost to the mission:
- Enable near-real-time prediction of ground motion and response management (landslides, earthquakes, volcanoes).
The baseline mission concept uses a Vega-C launch followed by an orbit-raising phase to reach GEO. A standard small-GEO satellite (mass around 2 tonne, few kW electrical power) is suitable, with a C-band radar (300-400 W RF), compact polarimetry and a 7 m diameter deployable reflector. Beam-steering is achieved by slewing the satellite: the whole Earth disk is only 16° across so small manoeuvres are used and require no propellant. Coverage is controlled by pointing and is therefore almost independent of the orbit.
G-CLASS will be the first mission to provide continuous observations of key diurnal processes. It will simultaneously observe the land surface and overlying atmosphere, and powerfully complements existing EO missions. The temporal resolution opens up significant new opportunities – only some of which we can currently predict. G-CLASS has great potential for new science, especially of the diurnal water cycle, and enables applications with important societal impact.