FSSCAT is an innovative mission concept consisting of two federated 6U Cubesats in support of the Copernicus Land and Marine Environment services. The first CubeSat carries the Flexible Microwave Payload-2 (FMPL-2), a dual microwave payload (a GNSS-Reflectometer and an L-band radiometer with interference detection/mitigation), and the second one a hyper-spectral optical payload. The combined use of these payloads will allow to measure soil moisture, ice extent, and ice thickness, and to detect melting ponds over ice. FSSCAT also includes radio and optical inter-satellite links to test some of the techniques and technologies for the upcoming satellite federations. FSSCAT will be the precursor of a constellation of federated small satellites for Earth observation achieving high temporal resolution and moderate spatial resolution in a cost-effective manner.