What Is a Distance Measurement Equipment System and How Does It Work?
When navigating the skies, there are a lot of systems in place to aid pilots in determining their exact location. Given the fact that there are virtually no landmarks or reference points in the air to indicate an aircraft’s position, pilots rely on interactions with air traffic control (ATC) and other systems on the ground to determine their positions. One important system in place that pilots rely on is the distance measurement equipment (DME) system. For your better understanding of how such equipment is used in practice, this blog will cover the basics of distance measurement equipment.
As is the case with most aircraft navigation and communication systems, part of the distance measurement system is situated on the ground. The DME unit is located at a ground station, and the distance is calculated between this unit and the aircraft to offer the reading displayed on the flight deck for the pilot. On the aircraft, the DME system consists of an airborne DME transceiver, display unit, and antennae for the pilot. When a plane is flying to the station, it can also show the calculated aircraft speed and elapsed time for arrival. Given these capabilities, DME systems are often used in tandem with other on the ground aircraft systems.
Some popular systems that pair well with DME units are VHF omnidirectional range (VOR) navigation systems and instrument landing system (ILS) localizers. These pairings are referred to as VOR/DME and ILS/DME (also LOC/DME) respectively. These combinations are helpful for specific situations, such as when an aircraft approaches a runway during landing.
Similar to radio equipment and other such systems, DME relies on the sending and receiving signals in the UHF frequency range from 962 MHz to 1213 MHz. The aircraft sends a carrier wave from a small blade-type antenna on the fuselage; this signal is then modulated with a string of integration pulses. The ground DME unit receives the pulses and returns a signal to the aircraft. The time it takes for the signal to complete this round trip is then calculated and converted into nautical miles for display. The time to reach and the aircraft's speed are both computed and displayed on a dedicated DME display, as well as an EHSI, EADI, EFIS, or the primary flight display in a glass cockpit.
In practice, combined systems can be very helpful, allowing the pilot to identify the location of the plane using the bearing from the VOR on the ground and the distance to a known point, that of which is the DME antenna at the VOR to the distance from the aircraft antenna. Meanwhile, VOR/DME coupling works by automatically tuning the DME signal in with the VOR signal to aid in landing procedures. First, VOR identification signals are broadcast, and then the DME broadcast tones follow. The distance between the DME transmitter antenna and the aircraft, known as slant distance, is displayed on a typical DME.
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