07 DME Flashcards
DME equipment?
DME equipment includes an airborne based interrogator and a ground based transponder.
DME Frequency?
DME operates in the UHF frequency band, using different UHF frequencies for interrogation and reply.
960MHz - 1215MHz
P0N - Emission code.
DME UHF frequencies are paired with VOR and ILS VHF frequencies, allowing two items of navigation information from one frequency.
Paired top decrease pilot workload
DME is classed as secondary radar.
DME operating principle?
Aircraft sends out a series of pulse pairs at a 12us seperation as a request for infomation. These pulse pairs are are seperated by a random jittered period. (PRF).
Signal is transmitted on a set frequency, this is then delayed 50us, and responds on a different frequency 63MHz lower. with the same jitters.
(Pulse-pairs that are not coincident with the individual aircraft’s interrogation pattern e.g. not synchronous, are referred to as filler pulse-pairs, or squitter.
Why is the returning frequency different?
To allow the transponder to be able to distiguish between deflections.
How do you know if returning frequency is increased or decreased.
Denoted with a X or a Y:
X + 63MHz
Y - 63MHz
VOR frequency ending in ._0 is a X.
VOR frequeuncy enging in ._5 is a Y.
DME Ground range?
Ground range is calculated from slant range (and vice versa) using the formula: Ground Range² = Slant Range² - Altitude²
Most accurate when far and low altitude.
When overhead should give the height of the AC.
TACAN usage?
Military UHF TACANs may be used by civilian DME equipment to obtain range only.
DME time and speed?
DME systems can also display groundspeed and time to station.
Ground speed will reduce as you approach the beacon.
DME Arc?
DME can be used to fly a constant range arc around an aerodrome, onto a final approach track. Required to maintain +/- 1nm to the intermediate approach fix. Groundspeed readout would be 0kts.
DME groundspeed?
DME can be used to fly a constant range arc around an aerodrome, onto a final approach track.
Double DME - Rho/Rho
Can be ambiguous as tjere are 2 intersections, but should show as incorrect given rough location and geometery.
Position can be determined by checking whether distances are increasign or decreasing.
DME capacity?
DME transponders generally reply to a maximum of 100 aircraft, those having the strongest signals.
Search Mode?
- 150 interrogation pulse-pairs per second (PPPS), for 100 seconds to find a lock on.
- If no lock on, it will reduce the number to 60ppps until there is a lock on.
- Once locked this goes to 24ppps
Beacon saturation?
- Average maximum interrogation range would be enough time for 27PPs
- Each beacon has a limitation of 2700PPPS
- Equates to approx 100 strongest aircraft signals (not nessesarly closest)
- If more then 2700PPS - reciicve reduces it gain to only alow strongest signals.
Track Mode?
24 interrogation Pulse-pairs per second, as the average number of pulses in SEARCH and TRACK is limited to max 30 pulse pairs per second. The aircraft interrogator locks on to the DME ground station once it recognizes a particular reply pulse sequence has the same spacing as the original interrogation sequence. Once the receiver is locked on, it has a narrower window in which to look for the echoes and can retain lock.
