When a/c DME is switched on it sends series of pulses at random time in the pair of pulses at random time interval. The transmission is in the pair if pulses, which are 12 µsec apart at 150 PRF. The ground station replies to each pulse it receives after fixed delay of 50µsec. with another frequency 63 removed from it. Type of transmission is P0N (Papa Zero November). PRF during transmission During lock mode (i.e. Interruption) @ 150 PPS If no response from DME @60 PPS Ground transmitter is able to respond to max of 2700 PPS.
Q.1. No of a/c required, saturating of DME beacon ____________? a. 100. b. 126. c. cannot saturate. d. None of the above. Ans : - (a) 100 Q.2. No of a/c is required to saturate of DME beacon in lock on mode __________? a. 100 b. 126 c. cannot saturate. d. None of the above. Ans : - (a) 100 Note: - Unless Specified III search mode, If specified = 126 a/c. Identification Code: Ground VOR / DME station also transmits an identification signal approximately every 10 sec. If identification code comes after every 30 sec then one of the components is not working.
Q. An identification code is received every 30 sec. This indicates ____________? a. VOR b. DME c. Both is not working
Range of DME: - Memory Circuit: The equipment has a memory circuit which will continue to update the range upto 10 sec after equipment failures at the ground, after that search is resumed.
Q.1. an a/c is flying at 30000ft, over DME beacon the indicator will show___________? a. b. c. d.
zero 30000ft Approx 5nm. Approx 2nm.
Ans: - (c) approx 5nm
A DME never shows zero over a beacon DME will indicate height of a/c in nm. Difference between slant range and ground range is called slant range error. Slant range error is negligible if the height of a/c is less than 1000ft per nm from a ground station.
ILS DME: -
Only ILS DME shows zero over ILS threshold. It is co-located with glide slope equipment. If they are not co-located time delay is induced to give correct distance. Note: - An ILS DME shows correct indication if a/c is in localizer coverage area or approach funnel. In any other direction it will not show correct distance. Accuracy: - ± 0.5 or 3% of range which ever is greater.
Q. A/c is flying at indication DME of 50nm. The accuracy of indication will be _______? a. 0.5nm b. 0.15nm c. 1.5nm d. None of the above. Ans: - (c) Q. A/c is flying at indication DME of 5nm. The accuracy of indication will be ________? a. b. c. d.
0.5nm 0.15nm 1.5nm None of the above.
Ans: - (a)
Frequency Paired: DME and VOR is frequency paired. If one is selected, other is selected automatically. Both can be frequency paired only if co-located. They still can be frequency paired if not co-located but serving same station and are upto 7nm apart. In that case the code will have Z indication. TACAN (TACtical Air Navigation): It is a military navigation aid and is similar to VOR & DME co-located and therefore gives both range and azimuth (Bearing) information to suitable equipped a/c. But civil a/c can obtain only range information. Swamping: -
It is a phenomenon in which a high power transmitter from an aircraft push out a closer or older version fitted DME.
