Airport beacons produce a visual effect of flashes to guide pilots to lighted airports at night. Differing rates of flash and color combinations may signify the classification of airport or heliport. In the United States a land airport is identified by projecting alternating flashes of aviation white and aviation green in two directions, 180 degrees apart, with a flash rate of 24-30 flashes per minute. The navigational aid is an integral part of an airfield lighting system. An airport beacon is normally operated from dusk until dawn. Operating the beacon between sunrise and sunset often indicates that the airport is operating under instrument flight rules.