L-band radiometers are used to estimate and analyze soil moisture and ocean salinity. They detect emitted radiation in the frequency range of 1-2 GHz and have a wavelength range of 30-15 cm. Current space-borne L-band radiometers offer spatial resolutions in the order of 40 km per pixel or 1 km with multi-sensor data fusion algorithms. The sensitivity depth of L-band radiometers offers a trade-off option between thermal imagery and ground penetrating radars for topsoil investigation purposes. Applications of L-band radiometers include investigations into soil moisture and soil variability, sea ice thickness, moisture measurement and leakage detection in structures, and road density changes.