Aircraft are equipped with a wide range of instruments that serve various purposes, and Distance Measuring Equipment (DME) is just one example of a measuring device that aircraft take advantage of. However, understanding how DME works consists of familiarizing yourself with its complexities. For instance, DME is coupled with a VOR (VHF Omni-Directional Range) and an ILS (Instrument Landing System) which makes things even more complicated.
Apart from the VOR and ILS, DME consists of a ground station and in-aircraft equipment. The aircraft transmits a signal to the ground station, which then sends a response signal to the aircraft. The time it takes the signals to travel between the DME station and the aircraft is calculated, this value also known as Slant Range Distance (SRD). The in-aircraft equipment informs pilots about the aircraft’s position in relation to the DME station on the ground. Generally, the DME station will be paired up with a VOR to tell the pilot about the aircraft’s exact position on a VOR radial.
How Does DME Work?
A DME is a vital part of TACAN (Tactical Air Navigation), a military navigation system that operates in the 960 to 1215 MHz frequency range. A DME station on the ground, also referred to as a transponder, is paired with a VOR station, and together they make up what is known as a VORTAC station. Before the aircraft antenna is able to send a signal to the DME station, the DME must be tuned accordingly. Due to the fact that the DME transmitter is typically situated on VOR stations, their frequencies must be paired so that the aircraft can pick up their signals.
The frequency at which the aircraft transmits a signal has an encoded digital word meant for a specific receiver. In general, this is in the form of random lapses between the pulse of the signal, and once the DME receives the signal, it decodes it. Though, keep in mind that one DME can decode multiple signals for numerous aircraft to provide them with distance data. The aircraft usually sends RF (radio frequency) pulses to the DME station, which then waits for about 50 microseconds before transmitting pulses back to the aircraft. This tiny delay helps prevent any incoordination in case the aircraft is flying too close to the ground.
After receiving the pulses from the ground station, the aircraft measures how long it takes for the exchange of pulses, minus the delay, and calculates the straight-line distance between itself and the ground station. Generally, the pilot can see this distance displayed in front of them.
What Is the Range of the DME?
DME ranges are limited and operate within the line of sight of the transponder. Moreover, any physical barrier between the ground station and the aircraft can impede the pulses from reaching their intended destination. This can interfere with calculating the correct slant range distance.
It is worth noting that high air traffic density can also affect the DME range. In fact, the ground station or transponder can only respond to a certain number of interrogations at a particular time. If there are more aircraft transmitting signals to a single ground station than what it can handle, the transponder will exclusively respond to the higher frequency interrogations.
If an aircraft is flying at high altitudes and there are no physical barriers between it and the ground station, the aircraft can receive DME signals up to 199 miles from the ground station. This range is even smaller for lighter aircraft, with a range of up to 50 miles or less if the aircraft is flying over mountains.
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