05 Distance Measuring Equipment (DME)

Question 1

What frequency does a DME operate in?

Answer 1

960 to 1215 MHz

Question 2

Describe the general operating principle of a DME

Answer 2

• An AC sends out unique omnidirectional jittered pulses
• A beacon picks up the signal and responds with the same signal
• The response is altered by +- 63MHz so there is no confusion with reflected signals
• AC then calculates the distance from the time between send and receive (minus a 50 microsecond delay from the beacon)

Question 3

What is the maximum capacity of pulses a DEM ground beacon can handle per second?

Answer 3

2700 pps (pulses per second)

Question 4

About what number of AC can a DME ground station can support at any one time?

Answer 4

100

Question 5

What are the different modes an AC will cycle through for a DME and how many pps are associated with each?

Answer 5

• Search Mode: 150 pps for the first 15,000pp (100 sec) then 60 pps after
• Tracking Mode: 25 pps
• Freeze Mode: Signal lost, freeze display for 10 sec then switch to Search Mode after

Question 6

What is the required accuracy of a DME with given range?

Answer 6

Required accuracy (nm) = +- 0.25nm + 1.25% of Range (nm)

Question 7

What is the equation for the ground distance from an AC to the DME?

Answer 7

Ground Distance^2 = Slant Distance ^2 - AC Height (nm) ^2

Question 8

When is the only time a ground speed indication for a DME accurate and which direction is most accurate?

Answer 8

• When the AC is heading directly to or from the DME ground station
• Most accurate when heading away from the DME

Question 9

What Greek letter is assigned to a DME and to a VOR?

Answer 9

DME = Rho
VOR = Theta

Question 10

What type of location identification is most accurate (regarding Rho and Theta)?

Answer 10

• Rho Rho location

Question 11

For a DME, what is the frequency difference between the interrogation signal and the reply?

Answer 11

+- 63 MHz

Question 12

What happens to the ground speed reading given by a DME as an AC approaches the overhead of the ground station?

Answer 12

The ground speed will under read

Question 13

What is the principle of operation of a jittered pulse pair?

Answer 13

• A jittered pulse pair is a sequence of two pulses
• The time delay between these two pulses is randomly unique to an AC
• This unique timing between pulse pairs ensures that an AC only accepts the correct responses to it’s own interrogations

Question 14

What is the maximum distance between a VOR and DME ground installations if they are to share the same morse code identifier?

Answer 14

600m

Question 15

What is the built in time delay of a DME from receiving an interrogation signal to sending a response signal?

Answer 15

50 microseconds

Question 16

What is a Radar Mile and what is it’s value?

Answer 16

• A Radar mile is the factored time it takes for a signal to travel 1nm and back
• 12.36 microseconds

Question 17

What is the formula for calculating the distance to a DME given a propagation delay?

Answer 17

Distance (nm) = (Propagation delay (microseconds) - 50) / 12.36

Question 18

What happens to the accuracy of the Ground Speed given by a DME as the AC travels:

• Directly towards the DME
• Directly away from the DME

Answer 18

• Directly towards the DME: accuracy decreases

- Directly away from the DME: accuracy increases

Question 19

If a DME and a VOR are co-located, how does the morse ident of the station sound?

Answer 19

Every 40sec period is split into 4 sections:

• VOR ident will sound for 3 periods at a lower frequency
• DME ident will sound once at a higher frequency

Question 20

What is the rule of thumb used when determining when inaccuracies will begin to occur when measuring ground speed from a DME?

Answer 20

1 nm from the station for every 1000ft altitude