How Distance Measuring Equipment (DME) Works In Aviation

‍An aircraft comes equipped with many instruments for various purposes. Distance Measuring Equipment or DME is one such measuring device.

However, understanding how DME works can be a complex task. It is usually coupled with a VOR and an ILS, which makes things even more complicated.

DME consists of a ground station and in-aircraft equipment. The aircraft sends a signal to the ground station, which then sends a response signal to the aircraft and times the signals to calculate the distance between the DME station and the aircraft, known as Slant Range Distance.

The in-aircraft equipment lets the pilot know about the aircraft's position relative to the DME station on the ground. Usually, the DME station is also paired up with a VOR (VHF Omnidirectional Range) to tell the pilot about the exact position the aircraft is on a VOR radial. In this article, we'll discuss how DME works, how it is limited by the aircraft's position relative to the ground station, and how it is displayed to the pilot.

As an experienced pilot and aviation enthusiast, I will discuss in detail the working of DME and whether it is still used in the world of aviation.

‍How Does DME Work?

A DME is part of the TACAN (Tactical Air Navigation), a military navigation system operating in frequencies ranging from 960 to 1215 MHz. A DME station on the ground, also known as a transponder, is almost always paired with a VOR station. Together they make up a VORTAC station.

Before the aircraft antenna can send a signal to the DME station, the DME needs to be tuned first. Because the DME transmitter is usually located on VOR stations, their frequencies are paired so the aircraft can pick up their signals.

The frequency at which the aircraft sends a signal has an encoded digital word that is meant for a specific receiver. This is usually in the form of random spacing between the pulses of the signal. The DME receives the signal and then decodes it. This way, one DME can decode multiple signals for multiple aircraft to provide them with distance info.

Your aircraft sends RF (radio frequency) pulses to the DME station, which waits for around 50 microseconds before sending pulses back to the aircraft. The delay helps prevent any incoordination in case the aircraft is flying close to the ground station.

After receiving the pulses from the ground station, the aircraft measures the time taken for the exchange of pulses minus the delay and calculates the straight-line distance between itself and the ground station. The pilot can see this distance displayed in front of them.

What is the Range of DME?

DME's range is limited and operates within the line of sight of the transponder. A physical barrier between the ground station and the aircraft can prevent the pulses from reaching their intended destination, which can interfere with calculating the correct slant range distance.

High air traffic density can also alter the DME range. The ground station or transponder can respond to only a specific number of interrogations at a particular time. If there are more aircraft sending signals to the ground station than it can handle, then the transponder will only respond to the higher frequency interrogations.

An aircraft can receive a DME signal up to 199 miles from the ground station if it is flying at a very high altitude and there is no physical barrier between it and the ground station. However, the range is smaller for small aircraft, up to 50 miles or even less if the aircraft is flying over mountainous terrain.

What is the Accuracy of a DME Station?

DME ground station has an accuracy of 185 m (±0.1 nautical miles), while the precision equipment used during landing has an accuracy of up to 30 m.

The DME station does have an error known as Slant Range Error. The slant range distance is calculated using the aircraft's altitude and the ground distance, which is the horizontal distance between the aircraft and the ground station.

So, if the aircraft is right above the transponder, for example, 6000 ft or 1 nautical mile above the ground station, the DME display would show 1 nm.

This error can be a problem when the aircraft is on top or close to the station. For this reason, pilots have concluded that when the aircraft is at least 1 nautical mile away from the ground station for every 1000 feet above the ground, they can ignore this error without causing any problem. So, if an aircraft is flying at 10,000 ft, the pilot can get an accurate DME reading if it is 10 nautical miles away from the DME station.

What is the Purpose of the Hold Function?

In front of the pilot are many indicators so they can keep track of the aircraft's heading, vertical speed, and altitude. This is known as the pilot's six-pack due to six different indicators. However, there is one more that tells the pilot whether the aircraft is on a specific VOR radial or not and whether it is moving away or to a VOR station.

If the pilot has turned into NAV 1, it shows two frequencies, one active and one on standby. Both frequencies correspond to two different VOR stations. So, if the pilot wants to switch between the two VOR stations, they should click the HOLD function, which holds on to the DME of the VOR that the pilot is currently on and which is active.

This holds on to the distance displayed on the indicator, and it doesn't change even when the pilot switches to another VOR station. The pilot will see the VOR needle moving, but the DME distance will remain the same.

What Information Can DME Display?

In the aircraft, a Frequency Selector allows the pilot to select different DME frequencies relating to specific VOR stations. As the DME frequencies are paired with VOR frequencies, if the pilot selects a specific VOR frequency, it will also tune the DME.

There are different indicator instruments to receive the DME. These include:

1. Standalone Instrument: This displays information pertaining only to DME, such as the slant range distance in nautical miles between the aircraft and the ground station, aircraft speed in knots, and the time required to reach the DME station. The standalone instrument is usually present in smaller aircraft.
2. RMI Instrument: This is an electronic navigation system called Radio Magnetic Indicator. It has a magnetic compass combined with a VOR. It shows the aircraft's magnetic heading and the distance in nautical miles to a DME station. The RMI instrument is present in larger aircraft, such as the airbus.
3. Electronic Navigation Instrument: This is an all-in-one instrument that can display VOR and the DME distance, weather, traffic, navigation fixes, and ILS with the paired DME distance.

What is the Purpose of a DME Arc?

A DME arc is a circle around the DME ground station at a specific distance from the station. It is a procedure that includes a series of maneuvers for the pilot to perform to transition to the instrument approach, i.e., to begin landing.

Pilots can intercept a DME arc while flying and then follow the arc to a specific point known as Lead Radial, after which they can safely head to the airport to land the aircraft.

As the DME arc is a complex procedure to perform and can only work for aircraft equipped with DME, The ACS (Airman Certification Standards) does not recommend the pilot to fly a DME arc. It only ensures they know how to fly a DME arc when it is available. The FAA also recommends flying the DME arc when RMI is available in the aircraft or when the pilot is highly proficient in using the HIS/OBS.

Is DME Still Used Today?

Although DME is still used today, it has become rare. While they were an essential part of air traffic control, they are no longer used as much as now; they have been replaced with GPS and WAAS, which are based on satellites.

With the advancement in technology, RNAV or area navigation systems had gained popularity as they allowed pilots to navigate directly to a referenced VOR radial instead of to a ground station.

GPS is responsible for helping pilots navigate from one point to another without relying on VOR stations. Also, GPS is more popular and commonly used than RNAV, and it has replaced a DME unit.

However, with that said, the FAA is still going to broaden the DME infrastructure to ensure that when a system fails, or GNSS (Global Navigation System) encounters disruptions, there is a level of service on board that can keep the aircraft on track. In conclusion, the FAA will not completely eliminate DMEs even though GPS is taking over as a legal alternative to keep them as a safety net if potential GPS system failures occur. Also, there are still some aircraft that rely on DME for navigation.

The FAA Strategic Plan for the years 2019 to 2022 gives multi-year strategies that focus on expanding DME infrastructure, such as the NextGen DME, along with the Very High-Frequency Omnidirectional Range (VOR) Minimum Operational Network (MON) to strengthen Performance-Based Navigation (PBN) operations.