What is DME and How Does It Work?

Aircraft instruments are crucial for enacting safe flight, often being used to provide a pilot with information regarding their altitude, airspeed, orientation, and more. Distance measuring equipment (DME) is no different, allowing for distances between ground stations to be determined for the means of navigation. As a common tool that is often compared to GPS, it can be beneficial to understand DME and how it may be used in the case that you must rely on it for navigation during flight.

In order for DME to function as intended, it relies on ground-based and in-aircraft equipment. Generally, DME equipment will be placed alongside a VOR or ILS/LOC within a ground based station. While many NAVAIDS take advantage of VHF for the transmission of their signals, DME utilizes UFH. To ensure that all devices can function together, the FAA has matched standard VHF NAVAID frequencies to UHF DME frequencies. As such, VORs transmitting signals over a specific frequency will all utilize the same DME frequency. Additionally, all NAVAIDs are placed far enough from one another that they have optimal spacing for the benefit of the aircraft receiver.

In order for DME to determine distance, they calculate the timing between the initial signal pulse from the transmitter to receiving the reply signal from the receiver. The readings that are attained by the DME are provided in nautical miles, and it is important to understand that the value is considered to be a slant-range distance. This means that the distance is affected by the altitude of the aircraft, resulting in a slight error in the measurement. Typically, slant error is minimal when the distance between the aircraft and the ground station follows a ratio of 1NM per 1,000’ AGL.

As DME ground-based equipment relies on sending signals to aircraft for distance measurement, they must have a direct line of sight to the aircraft itself. Because of this, a DME behind obstructions such as a mountain would not be able to provide an aircraft on the other side with any data. Nevertheless, a single transmitter can handle upwards of 100 aircraft at one time, allowing for traffic to be handled optimally. Once the amount of aircraft present reaches over 100, the particular aircraft that have the most distance from the DME ground-station will most likely lose the ability to receive signals.

With the rise of GPS equipment since their original installation on aircraft, many pilots have begun moving away from traditional DME. This is especially true when conducting IFR flight, as an approved GPS can be used as a substitution for DME. The rise of GPS is partly due to the accuracy of such systems as they often do not have to consider slant error that comes as a result of using ground stations.

While GPS equipment has begun to dominate glass panel aircraft and a majority of general aviation aircraft, DME radios are still present within many areas of the industry. As an example, a large number of transport aircraft still feature DME, alongside some older IFR-certified general aviation aircraft. As FAR 91.205(d)(2) dictates that all aircraft certified to fly IFR about FL240 must have an approved DME or a suitable RNAV system, such equipment continues to be of service to many pilots.

If you are a pilot operating an aircraft with distance measuring equipment (DME) and require various components for maintenance, repair, and overhaul, let the experts at ASAP Fulfillment help you secure everything you require with competitive pricing and rapid lead-times. ASAP Fulfillment is a website owned and operated by ASAP Semiconductor, and we are your sourcing solution for aircraft equipment related to DME, GPS, avionics, and much more. With AS9120B, ISO 9001:2015, and FAA AC 00-56B certification and accreditation, we go above and beyond industry standards in terms of quality assurance. If you are ready to experience the future of part procurement, get in touch with one of our representatives at your earliest convenience. 


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