DME: Distance Measuring Equipment Basics Explained

DME, or Distance Measuring Equipment, is a short-range navigational aid that provides aircraft with the distance to a ground station. Ideally, we want to measure the separation between the aircraft and the DME station directly over the ground. However, the system actually measures slant distance (S). From this, the ground distance (D) is calculated, taking into account the aircraft’s altitude and potential error, as explained below.

Ground DME Features

• Frequency Band: Operates in the UHF frequency band from 960 to 1215 MHz, offering 100 channels.
• Capacity: Each ground station can handle up to 100 aircraft simultaneously.

Figure 1 (not provided here) depicts a typical DME ground station setup.

Aircraft DME Specifications

• Also referred to as an “Integrator.”
• Operates in the UHF band.
• Range: Operates within a range of 150 to 200 nautical miles.
• Display: Aircraft DME equipment displays distance in nautical miles (nm), ground speed (GS) in knots (kts), and estimated time to the ground station in minutes.

Figure 2 (not provided here) depicts a typical DME airborne system.

Functional Description

The DME system operates as follows:

1. Interrogation: The aircraft DME interrogator transmits a signal on a specific channel at a frequency (e.g., ‘f1’).
2. Response: The DME ground station receives the signal and transmits a reply back on a different frequency (f2), which is equal to f1 +/- 63 MHz, after a delay of 50 microseconds.
3. Calculation: The aircraft DME interrogator receives the reply signal and measures the total time the signal traveled. Using this time, it calculates the distance to the ground station.

Advantages

• DME systems are rarely affected by precipitation static or thunderstorms.

Disadvantages

• Slant Range Error: The system provides slant distance (S) instead of the actual ground distance (D). The error between D and S is minimized when the slant distance is significantly greater than the aircraft’s altitude.
• Line-of-Sight Limitation: DME systems are limited to line-of-sight due to their use of the UHF band.