Microwave radiometers measure the radiant flux of microwaves. They are commonly used in meteorological applications to provide temperature, humidity, and liquid profiles. When used for immediate or short-term weather forecasting, microwave radiometers can help monitor rain rate, cloud liquid, and precipitation; convection and lightning; and turbulence and fog. Microwave radiometers are also an important part of aviation weather decision support systems, where they are used for monitoring the potential for surface and aircraft icing. Additional applications for microwave radiometers include measuring urban pollution and airborne dispersion rates, rain effect mitigation, and cloud seeding.
There are four basic types of microwave radiometers: products for temperature, humidity and liquid profiles; products for temperature profiles and integrated liquid; products for water vapor profiles and integrated liquid; and products for dry climate humidity profiles. Polarimetric radiometers are also available. These microwave radiometers are designed for snow, ice, soil moisture, sea salinity, and wind sensing from either fixed or mobile platforms. They are typically ruggedized for extreme environments, and may be portable for ease of transportation and movement. Modular microwave radiometers with field-replaceable components should afford accuracy and long-term reliability.
Selecting microwave radiometers requires an analysis of product specifications. These parameters include temperature accuracy and long-term stability, resolution as a function of integration time, brightness temperature range, and antenna system optical resolution and side lobes. The frequency-agile tuning range for microwave radiometers is divided into water vapor bands, an oxygen band, and minimum frequency step size. There are also multiple factory-calibrated for these Gigahertz (GHz) bands. Typically, the number of standard channels ranges from 15 to 21. Microwave radiometers also differ in terms of pre-detection channel bandwidth and surface sensor accuracy, which has parameters for temperature, relative humidity, and barometric pressure.
Microwave radiometers for dry climate humidity profiles provide high moisture sensitivity for use in high altitude, desert, and polar environments. Their temperature and humidity profiles are typically comparable or superior to co-temporal radiosondes, and are more accurate than radiosondes with 12-hour latency. Microwave radiometers for dry climates also provide liquid water profiles that meteorologists can use for precipitation forecasting and climate modeling. They may also be equipped with integral software for processing thermodynamic data, forecasting the weather, and issuing weather alerts.