Distance Measuring Equipment (DME) is used by pilots to determine their distance from ground stations, using propagation delay. It has limitations, such as measuring distance in a straight line and only supporting a certain number of aircraft, but is still useful for navigation. It is being replaced by GPS.
Distance Measuring Equipment (DME) is a type of navigation technology used by aircraft pilots for the purpose of determining distance from navigation solutions. Found connected to many very high frequency omnidirectional ranging (VOR) or locator stations, this technology establishes a connection between a ground transmitter and an airborne receiver box. Once the connection is established, pilots can determine their distance from the ground station. A pilot can combine information collected from ranging equipment with the relative position of one or more VOR stations to determine the exact location of the aircraft.
To use the ranging equipment, the pilot tunes his receiver to the specific frequency assigned to the equipment on the ground. There are only 200 possible frequencies, so some stations may share the same frequency. The use of distance measuring equipment requires that the space between the aircraft and the ground station is unobstructed. This requirement allows widely separated stations to operate on the same frequency without interference.
Distance measuring equipment works on the concept of propagation delay. After the pilot tunes the receiver, an interrogation signal is sent to the ground station. Precisely 50 milliseconds after the station receives the signal, it will send a signal back to the aircraft. The airborne receiver measures the time it takes for the signal to be returned and subtracts the 50 milliseconds. It then takes that number and multiplies it by the speed at which the radio wave travels, resulting in the distance between the plane and the station.
An important consideration when using ranging equipment is that it measures the distance in a straight line between the aircraft and the station, not the distance above the earth’s surface. In reality, the distance shown to the pilot will be a bit further. The higher the altitude of the plane, the greater this margin of error. Another limitation of ranging equipment is that ground stations are only capable of supporting a predetermined number of aircraft. The transmitters will evaluate each interrogation signal based on its strength, giving priority to the closest aircraft.
As long as its limitations are understood, ranging equipment is an effective tool for improving pilot navigation and increasing situational awareness. This technology allows pilots to determine exact locations while en route and identify drop points on an instrument approach. This aging technology is gradually being replaced by the more sophisticated Global Positioning System (GPS), but it still plays a critical role in modern aviation.
Protect your devices with Threat Protection by NordVPN
