DME Flashcards
What is a DME?
Distance measuring equip.
As the name implies, it measures you’re distance from the station.
It can also measure groundspeed but this is less commonly used as accuracy is limited to the A/C going directly to or from the ground station.
How does a DME work?
The interrogator (airborne DME) sends out a signal to the ground station DME, and measures the time it takes for the station DME to reply.
This is called propagation delay.
What are the 2 main DMEs?
DME/N : DME for normal Nav aid use i.e. Paired with VOR
DME/P: DME precision which is usually paired with MLS i.e. For ILS approach
DMEs are usually paired with other Nav aids.
What is the DME freq band?
962 to 1213 MHz
UHF (line of sight)
What is PRF and jittered PRF?
A signal, in the form of pulse pairs, are sent out to the ground DME at random intervals.
While the no of pulse pairs sent out in a second is called PRF, the random interval between the sent pulse pairs is called jittered PRF.
These jittered PRF is how DMEs are able to differentiate from the different signals.
DME: What is search mode?
Search mode is the first search and tracking mode for a DME.
This is where the A/C DME sends out 1500ppps in an attempt to track a ground DME.
If there is no reply after 100 secs (or 15,00pp) the transmit will slow the signal to 60ppps.
After 30 secs if there is no reply then the transmit slows to 30ppps
The DME display on the A/C will be blank of show a series of dashes during search mode.
DME: if a ground station picks up on a transmit, how does it respond?
The station will respond
• after a 50 µs (normally) delay
• using the same jittered PRF
• but on a different reply freq of +/- 63mHz (this is intentional)
What is tracking mode for a DME?
This is the 2nd mode when an A/C is searching for a ground DME.
Once the A/C DME receives a signal with the same jittered PRF and freq +/-63mHz, the A/C DME locks onto the ground DME.
What is memory mode for a DME?
This is the 3nd mode when an A/C is searching for a ground DME.
Once the A/C DME is on tracking mode, and the signal with the station has been lost for > 2secs, the A/C DME will go into memory mode where it has stored and is using the last received rate.
If >10secs and an interrogation has not been reestabilished, the A/C DME will revert back to search mode and start the process again.
What is propagation delay?
When the A/C DME transmit, it also starts a timer, only stopping when it receives the correct reply signal.
This time delay, excluding the 50 µs which the ground DME takes to prepare a response, this is the propagation delay.
The distance calc between the A/C is and the ground DME is the slant range.
What is the formula for distance from propagation delay?
Distance (nm) = (propagation delay - 50 (µs))
÷ 12.36 (µs)
12.36 µs = radar mile.
That is 1nm to the station and 1nm back. It is stated that it takes 12.36 microsec (µs) for this to take place.
Ground DME beacons can become saturated, how?
DME ground beacons has a capacity of 2700ppps. If the beacon is under capacity it will continue to transmit squitter signals to maintain continuous transmission.
2700pps averages approx 100 A/C 95% on tracking mode and 5% on search mode, therefore if over capacity the beacon will respond to A/C closer to the beacon and/or those with the strongest signals.
DME ground beacon must have an Ident and what other requirement(s)?
Beacons must have a 3 letter ident. Those paired with ILS must have a prefixed ‘I’ in front of the ident e.g. I-CDF
It is also an ICAO requirement that the beacon must transmit it’s ident (in morse code) no less than 40 secs
DME: What is a single facility ident?
When a DME is paired with a VOR or ILS, this is called single facility. The ident of both must be transmit within a 40sec time frame. The 40secs are split into:
- VOR/ILS x 3 at lower freq (lower tone)
- DME X1 at higher freq (higher tone)
Where it is a single facility, what are the max distances allowed between the 2 antennas?
Instrument approaches (ILS or VOR/DME):
CVOR: 30m
DVOR: 30-80m
Enroute: (VOR/DME): 600m
