Lecture Notes: Distance Measuring Equipment (DME)

Introduction

• Presenter: Grant
• Series: Radio Navigation
• Topic: DME (Distance Measuring Equipment)
• Purpose: Measure distances in aviation accurately.

Key Concepts

What is DME?

• DME: A system used in aviation to determine the exact distance between an aircraft and a ground station.
• Secondary Radar: Needs both the aircraft and ground station to transmit and receive signals.
• Collocation: Ground station is usually located with a navigational aid like VOR, NDB, or ILS.

How DME Works

• DME Interrogator: Installed in an aircraft to send and receive radio signals.
• Pulse Pairs: Aircraft sends unique jittered pulse pairs to the ground station.
  • Frequency: Specific frequency with each pulse separated by 12 microseconds.
• Reply Signal: Ground station replies on a slightly different frequency (63 MHz difference).
• Slant Range Calculation: Uses time delay and speed to calculate distance.

Technical Details

Frequencies and Channels

• Frequency Band: UHF band (300 MHz - 3 GHz).
• Paired Frequencies: Two frequencies, 63 MHz apart, are paired into channels.
• VHF Linkage: Channels linked to VHF frequencies for automatic tuning.

Identifying DME

• DME Ident: Similar to radio beacons like NDB or VOR.
  • Pitch Variation: Sequence of tones to confirm DME tuning.

Distance Measurement

• Slant Range vs. Horizontal Distance: DME measures slant range, not horizontal distance.
• Impact of Height: Typically minimal difference in slant range for long distances.
• Standard Procedures: Procedures like SIDs and STARs use DME distances for navigation.

System Features

Offset Zero DME

• Offsetting: Adjusts zero point to useful locations, like a runway threshold.
• Time Delay Reduction: Alters delay to display accurate distance countdown.

DME System Errors

• Signal Loss: Temporary loss adapts using a memory function.
• Saturation: Manages multiple signals by prioritizing the 100 strongest.
• Ground Reflections: System picks the first signal to avoid reflections.

Accuracy

• Old Systems: Accurate within 0.25 nautical miles plus/minus 1.25%.
• New Systems: Accurate within 0.2 nautical miles 95% of the time.

Summary

• Effectiveness: DME provides accurate distance information for aviation.
• Reliability: Built-in mechanisms ensure high accuracy and usability in various conditions.