For which type of navigation system must an aircraft be DME-equipped?

The requirement for an aircraft to be equipped with Distance Measuring Equipment (DME) is most applicable when VOR (VHF Omnidirectional Range) navigational receivers are used. DME provides aircraft with distance information to a ground station and is often utilized alongside VOR for more precise navigation. When utilizing VOR navigational systems, DME enhances the accuracy of the navigational readout, allowing pilots to know not just their bearing but also their distance from the VOR station.

In the context of the other options, GPS navigation systems do not inherently require DME since they function via satellite, and precision approach systems, while they may use DME in some cases, do not explicitly necessitate it for all approaches. Additionally, the requirement for DME does not apply universally to routes flown in different countries, as regulations regarding equipment can vary by country but do not dictate a mandatory DME requirement.