RainCube, a Constellation of Precipitation Profiling Radars in CubeSat

Precipitation profiling capabilities pioneered by TRMM's PR (Tropical Rainfall Measurement Mission Precipitation Radar) and further advanced by CloudSat's CPR (Cloud Profiling Radar) and GPM's DPR (Global Precipitation Measurement mission, Dual frequency Precipitation Radar) are currently limited to few (i.e., 3) instruments deployed in LEO. As such, their high quality observations are sparse in time with respect to the typical time-scale of weather phenomena (tens of seconds to hours). These missions therefore are generally unable to observe the short-time evolution of weather processes, which is necessary to validate and improve the current assumptions and skills of numerical weather models. The synergy with larger numbers of wide-swath microwave radiometers and other passive sensors mitigated this observational gap, but only to the extent of coarse vertical profiling and increased uncertainties (especially over land and ice) which do not allow to actually observe large part of the processes driving the evolution of many types of weather systems. Projects to advance the technologies enabling the deployment of precipitation radars in GEO have been successful on several fronts, but the technological state of the art for very large lightweight deployable antennas and related cost considerations don't enable this approach quite yet. The alternative is to deploy several radars in LEO (as convoy or constellation). This has not been realistically affordable for decades until the arrival of the SmallSat and CubeSat platforms, at which time the challenge moved to the capability to simultaneously miniaturize, reduce cost and preserve fundamental performance requirements for this type of radars.

A novel architecture compatible with the 6U class (or larger) has been developed at JPL. The key lies in the simplification and miniaturization of the radar subsystems. The RainCube architecture would reduce the number of components, power consumption and mass by over one order of magnitude with respect to the existing spaceborne radars. We can now actually consider deploying a constellation of identical copies of the same instrument in various relative positions in LEO to address specific observational gaps left open by the current missions. 

The baseline instrument configuration for the RainCube concept is a fixed nadir-pointing profiler (non-scanning as CPR) at Ka-band (same as DPR high-sensitivity channel, and as the low sensitivity channel of the Aerosol Clouds and Ecosystem, ACE, mission concept) with a minimum detectable reflectivity factor better than +10 dBZ (with a goal of 0dBZ) at 250m range resolution (DPR's high sensitivity channel achieves +12 dBZ at a range resolution of 500 m). The footprint size (i.e., horizontal resolution) would be determined by the antenna size. For a nominal orbital altitude of 400 km, a 0.75 m antenna results in approximately 5 km footprint (same as DPR). This is the current standard for precipitation mapping, and sufficient to resolve large convective systems and stratiform precipitation.

The performance of the radar instrument has been demonstrated on the DC-8 for the first airborne test during the PECAN (Plains Elevated Convection at Night) campaign. The picture below shows a typical measurement obtained during the airborne experiment.

In order to satisfy this set of requirements the RainCube architecture would use range compression to achieve the 250 m resolution with excellent range side lobe. The radar electronics would occupy a 2U volume and consume less than 30W. A highly constrained high gain deployable antenna is currently under development that occupies a 2.5U volume.

Many scenarios relevant to NASA's Weather focus area are enabled by the proposed architecture, as described in this section. A first mission science goal would be the reconstruction of the statistics of precipitation as they vary during the diurnal and seasonal cycles across all latitudes from the Arctic to Antarctica.  A constellation of four CubeSats deployed by means of launches of opportunity over four sun-synchronous orbits with different equatorial crossing times, more or less equally spaced across the day, would populate the statistics of precipitation in a distributed fashion across the globe and across the times of day.

Narrow time separations between CubeSats would allow the study of the evolution of convective systems at the convective time scale: profiles acquired systematically a few minutes apart in each region of interest would reveal the dominant modes of evolution of each corresponding climatological regime. Furthermore, if additional capabilities are included either in the platform (fine attitude control) or in the instrument (limited scanning) then it would be possible to obtain collocated profiles separated by anything between 15 seconds and, say, 5 minutes, as done for example by CloudSat and CALIPSO, to enable the direct observation of the evolution of the vertical profile of reflectivity at the time-scale that is critical to the convective processes. This last scientific goal specifically does not require also the deployment of CubeSats on other orbital planes, and it would be a breakthrough in its own.

A constellation of CubeSats would also be a natural complement to other resources aiming at monitoring the evolution of weather systems, for example the Geostationary IR/VIS imagers, the NEXRAD network, and the GPM constellation. By providing systematically high resolution profiles of reflectivity, the RaInCubes could validate and improve estimates of convective activity estimated from the IR/VIS imagery. "List_Numbered_RS09" style. Type your text here.

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