DME Flashcards
Information you can get from a DME
Groundspeed, distance from the beacon and time to the beacon
Freq band and type of wave for a DME
UHF
Space wave
How does the DME work
A/c sends out jittered pulse pairs
DME picks them up and sends the signal back after holding it for 50 micro secs and adds 63MHz
Search mode - DME
Sends 15,000 at an average of 150pps then drops to 60pps
When found a station goes into tracking mode
- after lock on the PRF drops to 25pps
Memory mode - DME
Enters this for 10secs if the response is interrupted
After 10sec it goes back into search or tracking mode
DME distance
Slant range not ground range
Directly above the DME it will indicate
Height above the DME in NM
Radar mile
1NM = 12.34 micro seconds - time it takes for a signal to go backwards and forwards in 1NM
DME freq
960-1250 MHz
Do not tune into these as they are freq paired with a VOR/ILS freq
No of DME channels
352 between the freq band
TACAN
Military use of VOR DME
Airline pilots can only use the DME part
DME ident
Once every 40 secs with a slightly higher tone
3 letter callsign every 30secs
VOR ident is 3 times in the remaining 30secs with a slightly lower tone
Accuracy formula of a DME
Line of sight formula and
+0.25nm + 1.25% of the slant range
System error
Installations after 1989 - total system error should be no less than 0.2nm DME
Groundspeed error - DME
When flying directly towards/away
Closer you get to the station, the rate of change of distance gets lower so the GS gets lower
Perfect circle around the DME = indicate 0 GS due to no rate of change of distance recorded
