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Inbroadcast engineering, thereference distance is the distance which, under normal circumstances and flatterrain, aradio station would travel with a particular level ofsignal strength. This distance depends on two factors:effective radiated power (ERP) andheight above average terrain (HAAT). The actual distance a station's signal travels depends highly onweather: factors liketemperature inversions and heavyprecipitation have a strong and highly variable influence onradio propagation. However, for purposes ofbroadcast law such asconstruction permits andbroadcast licenses, fixed calculations calledpropagation curves are applied to determine the reference distances for all existing and proposed stations. These also take into accountbeam tilt,carrier frequency, and even theEarth's curvature at longer distances.[citation needed]
The reference distance is in turn used to define mostbroadcast classes forFM stations inNorth America. Each class (except D) is defined as having a maximum ERP and HAAT. If the HAAT of a station'sradio antenna exceeds that specified for the class, it must reduce ERP so that its signal does not exceed the reference distance. The signal strength used differs by class, but generally the value is 1.0 mV/m (millivolt per meter) or 60 dBu (decibels over one microvolt per meter) for most of theUnited States, and 0.5 mV/m or 54 dBu inCanada, and for some U.S. stations in parts of certain areas includingCalifornia, theGreat Lakes region, and theNortheast. This is considered theservice contour of a station by theFederal Communications Commission (FCC) and theCanadian Radio-television and Telecommunications Commission (CRTC), and byCOFETEL inMexico, according toNARBA and other international agreements among the three. The reference distances are in turn used to create mandatory minimum spacing distances amongco-channel stations, and certainadjacent channels as well.[citation needed]
Real-world calculations can also be done by including data from digitaltopographical maps, typically along 12 or 16radials, and including the specifications from theradiation pattern for adirectional antenna. However, this does not fit the definition of areference in this sense, even though it determines the actualbroadcast range of a station. This actual range is used in thereserved band in the U.S., while the reference distance is used forcommercial radio stations.[citation needed]