The radio frequency bands used for many Earth observation satellites and other systems are now facing interference from the growing demands of non-meteorological applications.

Numerical weather prediction (NWP) relies on data from satellites, radiosondes, aircraft, radar and other observing systems as inputs. In turn, these observing systems rely on the allocation of radio frequency bands both for directly observing the Earth system and for exchanging observations through telecommunication networks. Many non-meteorological technologies, such as wifi, mobile phones and radios, also rely on radio frequencies, and many new applications are emerging.

It is vital to ensure continued allocation of the frequencies required for Earth observation, as well as protecting these frequencies from out-of-band emissions from neighbouring frequency bands. NWP users are already seeing evidence of radio frequency interference.

The allocation of radio frequency bands is agreed internationally at the annual International Telecommunication Union (ITU) World Radiocommunication Conference (WRC), held every four years. In the WRC, the demands of different application areas need to be weighed up, taking into account economic and societal benefits. The aim is not to hamper new technologies, but to make sure that their implementation does not interfere with life-saving applications, such as weather prediction.

The World Meteorological Organization (WMO), with strong support from space agencies such as EUMETSAT and ESA, as well as EUMETFREQ (a EUMETNET programme), coordinates the response of the international meteorological community and represents the community at WRC. For the meteorological case to be fairly heard, the best and most up-to-date information is required on the value of radio frequency bands to meteorology, in terms that can be compared to other application areas. ECMWF has been proactive in providing information and coordinating inputs from the wider meteorological community to support the WMO and the space agencies.

Many satellite observations for NWP use passive sensing techniques in radio frequency bands.

Passive sensing makes it possible to gain information on the current state of the Earth system by exploiting the absorption characteristics of the atmosphere. Such passive measurements are made by very sensitive instruments that measure the very low-power microwave radiances naturally emitted from the atmosphere and the Earth’s surface. Such passive microwave sensors are used to provide information on atmospheric temperature, moisture, clouds, precipitation and surface properties. These passive techniques are vulnerable to interference from new users of radio frequencies.

Furthermore, assessments of the impact of weather observations have found that microwave observations are presently the most important satellite observing system for global NWP, typically contributing around 30–40% of the overall improvement in forecast skill arising from the use of observations.

New applications outside the field of meteorology (e.g. 5G) are interested in the microwave frequency bands adjacent to 24 GHz and 50 GHz, which are crucial for obtaining accurate estimates of water vapour and temperature.

The use of radio frequencies in meteorology is not limited to these passive microwave observations. Systems such as weather radar also suffer from radio frequency interference. Radiosondes rely on a specific allocation for tracking and telecommunication. Command and download of data from all satellites needs specific frequency allocations.

ITU’s World Radiocommunication Conference in 2019 (WRC-19) in Sharm el-Sheikh, Egypt, agreed that 5G could operate at a number of frequency bands, including 24.25 to 27.5 GHz. To reduce interference with satellite observations, it was also agreed that 5G equipment would limit the strength of signals within the 24.25–27.5 GHz band. However, the agreement reached in Egypt falls far short of ensuring 5G applications do not interfere with weather observations. ECMWF continues to discuss with Member States and regulatory authorities to bring forward an agreement to strengthen protection before the main 5G rollout at 24.25–27.5 GHz.

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This is the first time we’ve seen a threat to what I’d call the crown jewels of our frequencies – the ones that we absolutely must defend come what may.

Stephen English, Head of Earth System Assimilation Section at ECMWF