Chapter 5 DME

Question 1

What is DME a type of?

Answer 1

Secondary radar system (SSR)

Question 2

What distance does DME give?

Answer 2

Slant range in nautical miles

Question 3

How does DME calculate the range?

Answer 3

Based on time taken for signals to travel between the interrogator (airbourne) and transponder (ground based)

Question 4

Describe the signals sent out by the aircraft to DME?

Answer 4

Jittered pulse pairs

Question 5

How much is the delay between interrogation and response?

Answer 5

50 micro seconds

Question 6

What is the frequency of the reflected signal?

Answer 6

+/- 63MHz compared to the interrogator signal

Question 7

Where are DME/Ps used?

Answer 7

In MLS

Question 8

What is the pulses per second (pps) of the ground station?

Answer 8

2700pps

Question 9

What is the pps of each aircraft?

Answer 9

27pps

Question 10

What is usual number of max aircraft per DME?

Answer 10

100 (2700/27)

Question 11

What signals are dropped first when a DME becomes saturated?

Answer 11

The weakest signals

Question 12

Describe search mode

Answer 12

150pps for 10 seconds, then 60pps for 30 seconds and then 30pps thereafter

Question 13

Describe tracking mode

Answer 13

25pps

Question 14

Describe memory mode

Answer 14

For seconds after signal loss, DME freezes and returns to search mode

Question 15

Describe the identification of a DME

Answer 15

Station transmits 3 letter idents every 40 seconds

Question 16

Describe the identification of a DME when co-located with a VOR?

Answer 16

• VOR ident x3 at low frequency

- DME ident x1 at high frequency

Question 17

What DME range limited to?

Answer 17

Line of sight

Question 18

What is the accuracy of a DME before 1st Jan 1989?

Answer 18

+/- 0.25nm and 1.25% of slant range on 95% of occasions or more

Question 19

What is the accuracy of a DME after 1st Jan 1989?

Answer 19

Max error of 0.2nm

Question 20

When is the only time groundspeed is correct when using DME?

Answer 20

When tracking directly to or from the DME station

Question 21

What is read by the DME if directly overhead the ground station?

Answer 21

Altitude