Passive Microwave Technology

Passive microwave radiometers function as highly sensitive temperature sensors that detect the Earth's natural microwave emissions. Unlike active systems such as radar, these instruments do not emit signals but instead measure the microwave energy naturally radiated from the surface. This emitted radiation correlates with the physical temperature and the material's emissivity, which varies based on its properties. This variability enables passive microwave techniques to retrieve numerous environmental parameters, including soil moisture levels, surface roughness, water content in plants, snow and ice characteristics, atmospheric water vapor, temperature profiles, wind speed, salinity, as well as precipitation and evaporation rates.

The specific data obtained depends on factors like frequency, polarization, viewing angle, and instrument setup.

At Radio Earth, we have our own ground-based passive microwave instruments,
designed specifically for land applications. Our instruments measure microwave radiation in numerous frequency bands between 6 and 18 GHz, from multiple viewing angles and at high sampling rates. Their unique rotating configuration allows us to observe both the Earth’s surface and the atmosphere, providing a more complete picture of environmental conditions. These capabilities make the instruments valuable for a wide range of applications. They can be used to validate satellite observations, while their high spatial and temporal resolution also makes them particularly powerful for studying local variations and changes at the land surface. With their unique combination of multiple frequencies, viewing angles and rapid measurements, our instruments offer a flexible platform for research into soil, vegetation, surface conditions, and atmospheric processes.

Passive microwave instruments have a long and proven history in Earth observation. Since the 1970s, they have been providing unique and continuous records of the Earth's environment, supporting research and applications in many different fields. At Radio Earth, we build on this rich heritage by developing advanced algorithms for data from different microwave platforms. By combining observation across platforms and spatial scales, we unlock new perspectives on moisture, temperture and environmental change.