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Lecture 8 : D istance M easuring E quipment ( DME )

Lecture 8 : D istance M easuring E quipment ( DME ). Outlines. Introduction The uses of a DME DME Equipment How DME work? Advantages of DME Disadvantages of DME. Introduction.

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Lecture 8 : D istance M easuring E quipment ( DME )

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  1. Lecture 8: DistanceMeasuring Equipment (DME)

  2. Outlines • Introduction • The uses of a DME • DME Equipment • How DME work? • Advantages of DME • Disadvantages of DME

  3. Introduction • Distance measuring equipment (DME) is a transponder-based radio navigation technology that measures slant range distance by timing the propagation delay of VHF or UHF radio signals. • Developed in Australia, it was invented by James Gerry Gerrand[1] under the supervision ofEdward George "Taffy" Bowen while employed as Chief of the Division of Radiophysics of theCommonwealth Scientific and Industrial Research Organisation (CSIRO).

  4. DME is similar to secondary radar, except in reverse. The system was a post-war development of the IFF (identification friend or foe) systems of World War II. To maintain compatibility, DME is functionally identical to the distance measuring component of TACAN.

  5. Definition • DME is stand for Distance Measuring Equipment. • DME is a type of en-route navigation system for aircraft. • DME often installed near VOR stations so as to provide combined bearing and distance. • When DME is installed with the VOR, it is referred to as a VOR/DME.

  6. The uses of DME • DME provides the physical distance from the aircraft to the ground DME transponder expressed in Nautical Miles (NM). • DME also calculates ground speed and the time needed to reach the station if the aircraft is fitted with appropriate computer.

  7. DME System Components: The DME system consists of three basic components which are: • DME antenna on the aircraft body • DME navigation display unit in aircraft cockpit • DME transmitter/receiver in the ground

  8. DME INDICATOR IN THE COCKPIT

  9. DME Indicator • DME enables aircraft to establish its range to the ground station: Distance in nautical miles, Ground speed in knots, Flying time to the station in minutes

  10. DME PRINCIPLE

  11. How DME works? • DME provides distance (slant range) from the aircraft to the ground DME. • DME operates on Ultra High Frequency (UHF) which is between 962 to 1213 MHz. • DME works based on pulse techniques, where pulse means a single vibration of electric current. • The aircraft’s antenna sends out paired pulses at specific spacing. • The ground DME station receives the pulses and then responds with paired pulses at the same spacing but a different frequency.

  12. How DME works? • The aircraft receiver measures the time taken to transmit and receive the signal which is transmitted into distance. • Beside that, the distance formula is also used by the DME receiver to calculate the distance from DME station in Nautical Miles.

  13. A radio signal takes approximately 12.36 microseconds to travel 1 nautical mile (1,852 m) to the target and back—also referred to as a radar-mile. The time difference between interrogation and reply, minus the 50 microsecond ground transponder delay, is measured by the interrogator's timing circuitry and converted to a distance measurement (slant range), in nautical miles, then displayed on the cockpit DME display.

  14. The distance formula, distance = rate * time, is used by the DME receiver to calculate its distance from the DME ground station. The rate in the calculation is the velocity of the radio pulse, which is the speed of light (roughly 300,000,000 m/s or 186,000 mi/s). The time in the calculation is (total time – 50µs)/2.

  15. DME IDENTITY • DME facilities identify themselves with a 1350 Hz morse code three letter identity. If collocated with a VOR or ILS, it will have the same identity code as the parent facility. Additionally, the DME will identify itself between those of the parent facility. The DME identity is 1350 Hz to differentiate itself from the 1020 Hz tone of the VOR or the ILS localizer.

  16. Advantages of DME • DME is extremely accurate: Provide continuous and accurate indication of the slant range distance (±0.1 nmi) . • Aircraft Handling Capability: The transponder equipment should be capable of handling 100 to 200 aircrafts. • Large coverage: DME facility provides coverage up to 200 NM.

  17. Disadvantages of DME • As VOR the DME is also restricted to line-of-sight transmission. For example, the aircraft at altitude below 10’000 ft is unable to detect the DME signal.

  18. Disadvantages of DME • Errors and abnormal indications: • Slant range • Speed and time calculation • Ground system saturation – 100 aircraft • System error

  19. Exercise • What is DME? Give two 2 uses of DME? (3 M) • How DME works? (10 M) • What the advantages of DME ( 2 M) • What are the disadvantages of DME (1 M) • What are 2 differences between VOR and DME? (2 M) • An aircraft flying at altitude about 3 N.M (18’000ft). The ground distance is about 4 N.M. What is the distance (slant range) of the aircraft in N.M calculate by DME? (2 M)

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