Monitoring fog with cellular network infrastructure

In terms of economic consequences—including accidents, traffic delays, property loss, and injuries or casualties—fog can be as damaging as winter storms. Common fog-detection systems include satellites, visibility sensors, and human observations, but such methods can suffer from low spatial resolution, high cost, or low sensitivity near ground level, where the information may be particularly needed. A solution might be found, however, in cellular communication networks. Most microwave links in a network’s infrastructure currently operate at frequencies between roughly 6 GHz and 40 GHz. Although rainfall produces measurable attenuation of signals at those frequencies, dense fog has a relatively slight effect. But to satisfy the growing demand for higher data rates and wider bandwidth, many countries are turning to higher frequencies, 70–80 GHz. Noam David, Omry Sendik, Hagit Messer, and Pinhas Alpert from Tel Aviv University show that because transmission at those frequencies is much more sensitive to fog, the new links could provide wide-scale, high-resolution monitoring of fog densities. In a proof of principal, the team took a map of existing microwave links in Israel and, for simulated fog, calculated the minimum liquid water content that could be detected using signal attenuation data at 20, 38, and 80 GHz. At 80 GHz, even light fog, with a visibility of up to 750 meters, had a measurable effect. And when the researchers analyzed actual 38-GHz signal data for an evening that was foggy in Tel Aviv but clear in Jerusalem, the visibilities and fog densities they calculated were consistent with recorded observations. (N. David et al., Bull. Amer. Meteor. Soc., in press.)