DMEs Flashcards
Echo Protection Circuit
Prevent errors arising from reflection from earth surface - installed on ac receiver to protect
DME operate in what frequency band
960-1215 MHz
Space waves
Decimetric
DME work by
Providing slant ranges - measures time interval between transmission from interrogator in ac to reply from transponder on the ground.
Slant range by pulse technique
Interrogator unit back to the transponder delay
50 microsecond delay
Jittering
Interrogation pulse randomised so identification can be made between ac and DME ground responder
Frequency used for receiving and transmission
1 for transmission/ 1 for receiving
UHF/VHF prevent own transmission being locked onto
+- 63MHz transponder reply frequency
What prevent DME receiver from locking onto other transmissions
Because time intervals between pulse pairs are unique to the AC
3 types of DME
DME normal
DME precision
Terminal DME - distance to define point on runway
Search Mode
150 pulses pairs per second for 100 seconds
60 pulse pairs per second after
Tracking Mode
27 pulse pairs per second after locking on
If signal lost due to masking of antenna will move back to search mode
Masking Mode
10 seconds after signal lost
Goes back to search mode
Beacon saturation
Ground transponder limited capacity to random jitters
2700 pulse pairs per seconds
100 aircraft limit
DME will respond to 100 strongest signals
Station identification
Beacon transmit 3 letter call sign every 40 seconds at a different frequency
Military TACAN
Able to be utilised for DME distance only
Slant Range Error when descending
Slant error reduces more accurate
Slant range error climbing
Slant error increases - less accurate
Slant range error - flying abeam
Groundspeed will be zero
VOR Speed measurement accuracy - flying away on radial/flying overhead/flying on radial different heading
Flying away/towards VOR on radial:
GS accurate more when flying away on radial
Flying overhead:
Groundspeed is zero
Flying on the radial:
Groundspeed less accurate/less value
Terminal VOR/DME
30NM max distance
En-route VOR/DME
Co-located 600nm
Theta Rho
VOR/DME
Theta = bearing
Rho = Range
Rho Rho
DME DME
Rho = Range
Theta Theta
VOR/VOR
Theta = Bearing (Angle)
VOR and DME is identified by
VOR/ILS is transmitted 3 times for every DME ident
DME has a higher tone frequency
What does it mean when the DME display counter rotates throughout there range
Airborne receiver is conducting a range search
Indicator rotate rapidly from zero nm through to max range
DME operates in and uses how many frequency’s?
UHF band and uses two frequencies
How many channels in DME
126
DME are more accurate the
Further away you get
Ident co located with a VOR time intervals
VOR = 3x per 40 second cycle
DME = 1x per 40 second cycle
10 sec intervals between ident codes (first three VOR/ILS and 4th DME (higher pitch)
Two DME ranges plotted on chart used to fix aircraft position what would be indicated on the chart?
Two circular position lines intersecting at two points, the distance from each transmitter being the slant range
What is a DME Line of position
A circle/arc with the DME transponder at the centre and the radius formed by the DME range
Accuracy of DME
0.2nm
