Difference between DME and TACAN Air Navigation Systems
This page compares DME and TACAN used in Air Navigation. It mentions basics of these systems and provide difference between DME and TACAN. Both these systems are used in Aeronautical communication between Aircrafts and Ground stations. Following sections describes basics of these systems including channel frequencies, components and specifications.
DME-Distance Measuring Equipment
DME stands for Distance Measuring Equipment. It is used to obtain range measurements from an aircraft station to a ground station. It operates in the frequency range from 960 to 1215 MHz band. DME systemis made of two parts aircraft based interrogator and a ground beacon. DME Interrogator transmits pair of pulses and waits for the reply from the ground beacon to calculate the distance. The Aircraft uses omnidirectional antenna to transmit these pair of pulses. Hence the system is referred as pulsed ranging system.
DME equipment or TACAN equipment is housed in aircraft cockpit which measures bearing information and range as described below.
TACAN-Tactical Air Navigation
As mentioned TACAN stands for Tactical Air Navigation. This system was developed to obtain both the range measurement as well as azimuth measurements of the aircraft. Azimuth information is obtained with the help of antenna rotating with the speed of about 900 RPM(Revolutions per minute). TACAN also has two parts interrogator and ground beacon and works similar to the DME. These have been described below.
Both DME and TACAN operates in the following four modes viz.X-mode, Y-mode, W-mode and Z-mode. Following table mentions frequencies used by the interrogator and ground beacon in these four modes.
Both the DME and TACAN pulse pair is of gaussian shapewith duration of about 3.5µS and separation between two pulses is about 12 µS for X-mode and 36 µS for Y-mode.
DME/TACAN Channel Frequencies
| Mode type | Interrogation frequencies from Aircraft to Ground | Reply Frequencies from Ground to Aircraft |
|---|---|---|
| X-mode | Between 1025 to 1088 MHz Between 1088 to 1151 MHz |
962 to 1025 MHz 1151 to 1213 MHz |
| Y-mode | Between 1025 to 1088 MHz Between 1088 to 1151 MHz |
Between 1088 to 1151 MHz Between 1025 to 1088 MHz |
| W-mode | Between 1025 and 1088 MHz | Between 962 and 1025 MHz |
| Z-mode | Between 1025 and 1088 MHz Between 1088 and 1151 MHz |
Between 1088 and 1151 MHz Between 1025 and 1088 MHz |
Interrogator
Pulse pairs are transmitted by DME/TACAN interrogator on one of the 126 frequencies in the band 1025 to 1150 MHz. All the frequencies are spaced at 1MHz apart. The reflected pulses are captured at the aircraft.The round trip time of these pulses is less than the interval between the pulse pairs. Due to this, interrogator receives response before the transmission of next pair of pulses.
In search mode, interrogator conducts at 40 to 150 pulse pairs in a second. In track mode, it conducts at 10 to 30 pulse pairs in a second.
Beacon
TACAN or DME ground station beacon serves all the aircraft stations within radius of about 100 to 300 nautical miles. The beacon transmits a pulse pair back to the aircrafts from where the pulse pair have been received within its range. The delay between reception and transmission of pulse pairs is about 50 µS for mode-X and about 56µS for mode-Y.
Beacon frequency used for transmission is about 63MHz +/- of interrogation frequency. DME beacon peak power is about 1-20 KW.
Typical Specifications of a TACAN system
• Frequency range: 962 to 1213 MHz
• Low band(channels 1 to 63): 1025 to 1087 MHz(transmit frequency), 962 to 1024 MHz(receive frequency)
• High band(channels 64 to 126): 1088 to 1150 MHz transmit frequency, 1151 to 1213 MHz receive frequency
• IF frequency: 63 MHz +/- 70KHz
• Receiver sensitivity: -90dB
• Transmit pulse power 1KW (min), 2.5 KW (max)
• Pulse pair transmission rate of about 120 to 150 pps for search and 22 to 30 pps for tracking
• Channel selection time : maximum equal to 12 sec.
• Accuracy: Azimuth +/- 0.5 degree, range +/- 0.1 nautical miles including 0.2 percentage of distance
• Beacon tone: Morse code tone, 1350 Hz
• Operating range: 195 nautical miles with height of 40000 feet
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