# Distance Measuring Equipment (DME): Slant Range and Use

> Understand DME principles, slant range, and operational use with VOR and ILS.

- **Subject:** 062 — Radio Navigation
- **Section:** 062-02 — Radio Aids
- **Topic code:** 062-02-04
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/radio-navigation/radio-aids/distance-measuring-equipment-dme/

---

**DME (Distance Measuring Equipment)** is a secondary radar system that provides the distance between an aircraft and a ground station. It operates in the **UHF band (962 - 1213 MHz)**.

## Operating Principle

It is based on measuring the travel time of radio pulses:
1.  The **interrogator** (airborne) sends pairs of pseudo-random pulses.
2.  The **transponder** (ground) receives the pulses, waits **50 microseconds**, and retransmits them on a frequency offset by **63 MHz**.
3.  The airborne equipment measures the total time, subtracts the 50 $\mu$s delay, and calculates the distance.

## Slant Range

![DME Slant Range Diagram](https://upload.wikimedia.org/wikipedia/commons/7/76/DME_Slant_and_Plan_range.jpg)

DME measures the direct distance between the aircraft antenna and the station, not the horizontal distance over the ground. This is known as **Slant Range**.
- The error is maximum when flying directly over the station, where the DME will indicate the **aircraft's altitude** (in NM).
- 1 NM $\approx$ 6080 feet.

## Operating Modes

- **SEARCH:** The equipment sends up to **150 pulse pairs per second (ppps)** to lock onto the station.
- **TRACK:** Once locked, the pulse rate drops (typically to **24-30 ppps**) to maintain tracking.
- **MEMORY:** If the signal is momentarily lost, the equipment maintains the last rate of change of distance for **8-10 seconds** before returning to search mode.

## Association and Frequencies

- The DME is usually **associated** with a VOR or ILS. The pilot tunes the VOR/ILS VHF frequency, and the equipment automatically tunes the paired DME UHF frequency.
- If associated, the DME Morse identifier is heard every 30-40 seconds (with a higher pitch) interleaved with the VOR identifier.

## Beacon Saturation

A DME beacon can serve a maximum of approximately **100 aircraft** simultaneously (about 2700 ppps). If this limit is exceeded, the beacon becomes **saturated** and prioritizes stronger signals (usually from closer aircraft).

## Derived Calculations

- **Groundspeed (GS):** The equipment calculates groundspeed based on the **rate of change of distance**. It is only accurate if flying directly towards or away from the station.
- **DME Arc:** Flying maintaining a constant distance. In this case, the GS indicated by the DME will be **zero**.

---

Free study notes from pilotsyllabus.com. Not a substitute for training at an approved ATO.