The team created rainfall maps for the whole of the Netherlands by using data from a mobile network in the country. This was based on measurements of the attenuation of microwave signal levels across 2400 network links over a four-month period. The resulting maps had a strong correlation with the same measurements taken by the conventional techniques of weather radar and rain gauges.The research was carried out by Aart Overeem and colleagues, from both Wageningen University and the Royal Netherlands Meteorological Institute. "Microwave links are used in telecommunication networks to transmit signals from the antenna of one telephone tower to the antenna of another telephone tower," said Overeem, who led the study. "When it rains the signal is attenuated, which is noticed as a decrease of the received power measured by these telephone towers. The average rainfall intensity...can be computed from the decrease in power during rainy periods with respect to the power during dry periods."
The team looked at the minimum and maximum received signal power at each telephone tower over 15 minute periods. Passing through falling raindrops absorbs part of the incident microwave transmission and causes minor beam scattering, lowering the power that ultimately reaches the receiving tower. The more raindrops in the beam's path – or the larger the drops are – the more signal power is lost.
By comparing received powers for each network link with reference values for known dry periods – and factoring in accounts for humidity and the water films that can develop on the communications antennae – rainfall densities along each path can be calculated. These values are then treated as point measurements at the centre of each network link and used to extrapolate the larger rain distribution maps. In the frequencies employed in these links, attenuation caused by raindrops is the main source of power reductions, beyond free space losses. Typically, the microwave network links operate at least 10 m off the ground and use frequencies between 13 and 40 GHz.