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Pilots like to know where they are in three dimensions: height, track, and distance from their destination. That may seem self-evident, but knowing where you are is not always so obvious. Among the tools developed to aid in navigation is distance measuring equipment (DME).
Bendix/King DME in the Cessna 172 radio stack. |
DME defined
Technically defined, DME is "a pulse-type electronic navigation system that shows the pilot, via an instrument-panel indicator, the number of nautical miles between the aircraft and a ground station" (Dictionary of Aeronautical Terms, 3rd ed.) In other words, the DME radio exchanges signals with VORTAC stations to calculate speed and distance.
VORTACs, which combine very-high-frequency omnidirectional range (VOR) and tactical air navigation (TACAN) transmitters, are ground-based stations that beam directional information to aircraft VOR receivers. Airborne DME receivers calculate distance and speed by measuring the time it takes for an interrogating signal to travel from the receiver to a VORTAC and for an answering pulse to return. A display on the DME indicates the aircraft's distance from the station in nautical miles (nm) and its groundspeed in knots. VOR/DME systems are limited in range to 141 nautical miles (nm) or 260 kilometers (km).
DME can pinpoint aircraft location using the radial and the distance information from a VORTAC. There is only one point in space that is 27.5 miles (44.25 km) out from a specific station's 090 degree radial. If you know which VORTAC the radio is tuned to and can locate that station on a chart, you know where you are. For that reason, DME is also useful for establishing intersection and holding pattern locations. Without DME you must use radials from two stations to calculate these points in space.
DME can also be combined with instrument landing systems (ILS) and localizers. Some of these combinations have approach segments called DME arcs, which describe a semi circle leading from an initial approach point to the final approach course. The pilot flies the arc by using the DME to maintain a constant distance from the transmitter to the point that intersects with the localizer signal. When paired with an approach system, the DME has a range of about 40 nm (74 km).
Which stations?
How do you tell which VOR stations have DME signals? VORs, VOR-DMEs, and stand-alone DMEs have different symbols on the Flight Simulator map as well as on real-world aeronautical charts.
Flight Simulator map. |
Different stations, different symbols. |
Slant range
Imagine a taut string extending from your shoulder to the floor 10 feet (8.5 meters) away from you. If you move the string from your shoulder to the floor, keeping it taut, it will touch the floor somewhere behind your foot. DME measurement is similar to that slanted string.
Slant range distance is measured from an aircraft to the station. |
The distance from the VORTAC to the DME receiver in the airplane is called the slant range. The difference between slant range and the distance from a point on the ground to the station is negligible at long distances. As the aircraft moves closer to the station, the slant range error becomes more critical, especially when flying within one mile of the station and the error is greater the higher you are as well. Be aware of errors when close to the station.
How fast?
Groundspeed is the speed an aircraft is moving across the ground. DMEs display groundspeed in knots. The groundspeed calculation is only accurate if the aircraft is flying directly to or from the station. This is useful for determining accurate time estimates for arrival at checkpoints or destinations.
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DME also registers ground speed. |
The preceding image depicts a Bendix/King DME receiver that indicates that the aircraft is 27.5 nautical miles from the station and traveling at 98 knots. The information in this scenario is calculated relative to the station to which VOR1 is tuned. Use the R1/R2 (Radio 1/Radio 2) switch on the DME receiver to select the VOR from which you want the DME to measure.
DME provides easy-to-obtain answers to the questions of "how far, how fast."