Built for autonomous operation, the Rotating Universal Radiometer (RUR) is a robust, IP67 sealed, remote sensor, not requiring a liquid nitrogen target for calibration. It is suitable for operation under all weather conditions, including precipitation events. The frequency span between 6 and 220 GHz corresponds to wavelengths of 50 mm and 1.4 mm. Such a span of wavelengths can characterize cloud properties, precipitation, vertical temperature and humidity profile quite well. The combination of temperature retrieval and cloud-phase (ice/liquid) characterization enables detection of supercooled liquid clouds.

The capability to penetrate and characterize clouds is what differentiates microwave radiometers from other measurement methods, such as infrared or laser (optical) measurements.

Dual-polarization measurements might be seen as redundant; however, these measurements reduces observational noise, are useful for radio-frequency interference (RFI) detection, and support retrievals within an RFI-congested environment. They can also be used as a quality indicator of the radiometer receivers’ proper operation.

The plots of temperature and humidity retrievals up to 500 m and up to 6 km are shown for illustration of the RUR capabilities. The two hyperspectral RUR variation are shown, one operating up to 220 GHz and the second up to ~600 GHz, and their observational capabilities are compared to the commercially available radiometer, such as MP-3000A from Radiometrics Corporation, or HATPRO from Radiometer Physics, GmbH. These are functionally essentially the same ground-based radiometers. The retrieval up to 500 m shows an advantage of hyperspectral sensors. Both variants measure the temperature and humidity profiles with much better accuracy and precision than commercially available sensors. They also follow the inversion accurately.

Plot up to 6 km altitude above the sensor shows that the commercially available sensor does not have the capability to resolve the temperature and humidity profile accurately. The hyperspectral sensor with channels up to 600 GHz provides the most accurate retrieval for both the temperature and humidity profile.

These retrievals are for the clear air conditions. The advantage of the hyperspectral sensors is more pronounced in the complex atmosphere that includes clouds and precipitation.

With the availability of low-frequency observations (i.e., < 20 GHz) from the RUR, applications related to accurately monitoring of water fluxes in soil and vegetation are feasible. High-speed, multi-angle measurements make it possible to simultaneously differentiate between vegetation and soil water content and correct for atmospheric contributions. A recent study using aircraft observations over the United States revealed that soil moisture retrievals from multi-angle observations showed strong potential. In the plot below the well-established 9-km soil moisture product based on single-angle L-band observations from NASA’s dedicated Soil Moisture Active Passive (SMAP) mission was compared to multi-angle retrievals from Advanced Microwave Precipitation Radiometer (AMPR) observations from an aircraft. In the plot two flights in October 2024 over Texas and Oklahoma were analyzed and showed a strong similarity (R²=0.81). The higher variability in AMPR soil moisture is primarily due to the higher spatial resolution of this instrument.