ATPL Radio Navigation - Class 6: DME.

Summary

In this video, Grant explains Distance Measuring Equipment (DME) used in aviation to accurately measure the distance between an aircraft and a ground station. DME works by sending and receiving unique pulse pairs, enabling the aircraft to distinguish its own signal. By calculating the time it takes for these signals to travel, DME provides the aircraft’s slant range, crucial for precise navigation, especially during approaches and landings. The system is highly reliable, handling issues like signal loss, saturation, and reflections, and offering impressive accuracy, making it an indispensable tool for safe flight navigation.

Takeaways

• 😀 DME (Distance Measuring Equipment) is used in aviation to measure the exact distance between an aircraft and a ground station.
• 😀 DME operates as a form of secondary radar, requiring both aircraft and ground station to transmit and receive signals.
• 😀 The DME system uses two frequencies separated by 63 MHz, operating in the ultra-high frequency (UHF) band between 300 MHz and 3 GHz.
• 😀 Aircraft use a unique pulse pair signature with gaps between pulses to distinguish signals from other aircraft using DME.
• 😀 The DME receiver calculates slant range, not horizontal distance, based on the time it takes for the signal to travel to and from the ground station.
• 😀 Slant range measurements are designed to be accurate for most flight distances, but the effect of altitude difference becomes more significant at shorter ranges.
• 😀 Aircraft follow specific procedures like Standard Instrument Departures (SIDs) and Standard Terminal Arrivals (STARs), which rely on DME distances for accuracy during approach or departure.
• 😀 DME systems can offset the zero DME reading to display the correct distance, such as setting it to zero at the runway threshold.
• 😀 A DME system can lose signal temporarily but will continue to count down based on the aircraft's last known speed, with a memory system in place.
• 😀 Errors in DME systems can occur due to signal loss, saturation (too many aircraft), or ground reflections, but the system ensures accuracy by only using the first signal reply.

Q & A

What is the primary purpose of Distance Measuring Equipment (DME) in aviation?

The primary purpose of DME is to accurately determine the distance between an aircraft and a ground station, which is crucial for navigation, especially when combined with other navigational aids like VOR or NDB.

How does DME differ from primary radar systems?

DME is a form of secondary radar, which requires both the aircraft and the ground station to transmit and receive signals. In contrast, primary radar only requires one transmitter and receiver, typically like sonar used by bats.

What is the significance of the 63 MHz difference between the frequencies used by DME?

The 63 MHz difference ensures that the aircraft can distinguish its own signal from others and avoid receiving signals from reflections, ensuring the data comes from the intended DME station.

How does the DME system calculate the distance to the ground station?

The DME system calculates distance based on the time delay between sending out a signal and receiving the reply. Using the formula 'Speed = Distance / Time,' it calculates the slant range, which is halved to give the actual distance.

Why is DME distance referred to as 'slant range' and not horizontal distance?

Slant range includes the aircraft's altitude in its measurement, which means it's the straight-line distance from the aircraft to the ground station. It differs from horizontal distance, which measures only along the ground.

How does the change in aircraft altitude affect DME accuracy?

The effect of altitude on DME accuracy becomes more significant at closer distances. While the difference between slant range and horizontal distance is negligible at long ranges, it increases as the aircraft gets closer to the DME station.

What is the role of time delay in offsetting the zero DME distance?

Offsetting the zero DME distance involves reducing the time delay (e.g., from 50 microseconds to 25 microseconds), which manipulates the DME reading, allowing it to show zero when the aircraft is at a specific location, such as the runway threshold.

What happens if an aircraft loses the DME signal temporarily?

If the aircraft temporarily loses the DME signal, the system will continue to calculate the distance based on the aircraft’s speed and the last known distance until the signal is restored. If the signal is lost for more than 10 seconds, the system may display a failure flag.

What is DME saturation, and how is it prevented?

DME saturation occurs when too many aircraft try to use the same DME station, leading to incorrect or missed distance information. To prevent this, DME stations use an automatic shedding system, which prioritizes the strongest signals.

How does the DME system prevent errors caused by ground reflections?

To avoid errors from ground reflections, the DME system only listens for the first pulse response to each interrogation. This ensures that only the shortest, most accurate distance is recorded, even if the signal is reflected from obstacles like mountains.