RFDME vs TACAN: Understanding Air Navigation Systems
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This article compares DME and TACAN, two systems used in air navigation. We’ll cover the basics of each, highlighting their differences. Both systems facilitate aeronautical communication between aircraft and ground stations. Let’s dive into the details, including channel frequencies, components, and specifications.
DME - Distance Measuring Equipment
DME stands for Distance Measuring Equipment. As the name suggests, it’s used to determine the range (distance) between an aircraft and a ground station. It operates in the frequency range of 960 to 1215 MHz.
A DME system consists of two main components:
- Aircraft-based interrogator: This part transmits a pair of pulses and then waits for a response from the ground beacon to calculate the distance. The aircraft uses an omnidirectional antenna for transmitting these pulses.
- Ground beacon: This acts as a responder to the interrogator signal.
Because the system relies on pulsed signals, it’s often referred to as a pulsed ranging system. The DME equipment (or TACAN equipment, in some cases) is located in the aircraft cockpit, where bearing and range information are displayed.
TACAN - Tactical Air Navigation
TACAN stands for Tactical Air Navigation. This system was developed to provide both range and azimuth (bearing) measurements of the aircraft relative to the ground station. The azimuth information is obtained through the use of an antenna that rotates at approximately 900 RPM (Revolutions Per Minute).
Like DME, TACAN also involves an interrogator and a ground beacon, functioning similarly to DME in terms of range measurement.
Both DME and TACAN can operate in four modes: X-mode, Y-mode, W-mode, and Z-mode. The following table outlines the frequencies used by the interrogator and ground beacon in each of these modes. The pulse pairs used in both systems are Gaussian in shape, with a duration of about 3.5µS. The separation between the 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
DME/TACAN interrogators transmit pulse pairs on one of 126 frequencies within the 1025 to 1150 MHz band. These frequencies are spaced 1 MHz apart. The reflected pulses are then received at the aircraft. Ideally, the round-trip time of these pulses should be shorter than the interval between pulse pairs. This allows the interrogator to receive a response before it transmits the next pair of pulses.
In search mode, the interrogator operates at a rate of 40 to 150 pulse pairs per second. This rate decreases to 10 to 30 pulse pairs per second in track mode.
Beacon
A TACAN or DME ground station beacon serves aircraft within a radius of approximately 100 to 300 nautical miles. When the beacon receives a pulse pair, it transmits a pulse pair back to the aircraft that sent the original signal. The delay between reception and transmission is approximately 50 µS for mode-X and 56µS for mode-Y.
The beacon’s transmission frequency is approximately 63MHz +/- of the interrogation frequency. DME beacon peak power typically ranges from 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: 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
