Short two-bullet answer (1)
Differences between VOR/DME and TACAN include: TACAN uses UHF (VOR uses VHF) to increase bearing determination accuracy; a single frequency for range and bearing (VOR/DME uses two frequencies); two small rotating drums with parasitic antenna elements (VOR uses a large circular array of antennas).
A VOR/DME station actually uses the DME portion of a TACAN, that's why it is common to see VORTAC which are complete dual VOR+TACAN stations rather than simple VOR+DME. A TACAN is small and can be installed on ships or mobile stations (a VOR is large); a TACAN is usually more powerful than a VOR and has a larger range of use.
While a VOR transmits bearing information continuously, a TACAN only sends RF samples keyed by reference and DME reply pulse pairs (see the explanation below).
Overall differences and compatibilities
The TACAN is a military radio navigation aid to determine aircraft relative range and bearing. The distance measurement part (DME) is also available to civilian aircraft. Bearing and distance signals require only a single transceiver on the ground and on the aircraft. They also use the same UHF frequency. The TACAN aerial shape is (has been for a long time) a vertical cylinder.
A TACAN mobile station in Alaska during an exercise:
The VOR/DME is the civilian counterpart in range/bearing determination. The VOR and DME equipments work on separate, but paired, frequencies: VOR in VHF, as name implies, and DME in UHF. This requires two receivers.
Lambourne doppler VOR/DME, with the VOR circular phased array and, at the center, the vertical DME antenna. (source: Wikipedia)
As you can see a VOR-DME is not going to be mobile easily, that's a first difference.
The DME part being compatible with the DME part of the TACAN. It's usual to see VORTAC stations that combine a TACAN and a VOR, the DME part being common to both:
Electromagnetic principles used
For range determination
The DME principle is to measure the time a radio signal takes for a roundtrip to the ground station. "The aircraft interrogates the ground transponder with a series of pulse-pairs (interrogations) and, after a precise time delay (typically 50 microseconds), the ground station replies with an identical sequence of pulse-pairs." (Wikipedia).
By measuring the total time, removing the 50 µs delay, and dividing by 2, one is able to determine the distance between the two equipments.
The DME signal is sent using a simple vertical antenna, visible at the center of the VOR array, and at the center of the TACAN drums.
Bearing determination, theoretical principle
The common principle of bearing/azimuth determination is to send a signal from a ground rotating directional antenna. The aircraft receives the signal when the antenna orientation is favorable (facing the aircraft), and there is a gradually increasing, then decreasing signal as the antenna comes close to or moves away from the aircraft direction.
While the signal rotates, the ground station also transmits an information that informs receivers which direction the antenna is currently facing, the reference being usually the magnetic North.
By knowing the direction the station antenna currently faces, and sensing the strength variation of the received signal, the aircraft is able to determine its own azimuth relative to the station (but not its heading), e.g. if the maximum strength coincides with the time the station says its antenna is currently broadcasting to 180°, then the aircraft knows it is located South of the station.
Both VOR and TACAN use this basic principle, albeit they realize it differently.
VOR bearing determination
The ground station antenna doesn't physically rotate (in modern Doppler VOR). Instead of having a single aerial, the signal is sent to a circular array of identical antennas. If all antennas received the same signal from the transmitter, then the signal would be received with the same strength by an aircraft, regardless of its bearing relative to the station. Instead the signal sent to each antenna varies in phase according to the current bearing:
The "current direction" is changed so that all directions are swept at 1,800 RPM (30 Hz). The phase at a given antenna varies accordingly, and the system behaves exactly as if there was a single physically rotating beam antenna broadcasting a constant signal.
A reference signal is also sent, but this time with the same power in all directions (omnidirectional). This signal transmits the information about the "current direction" of the pseudo-rotating array. The directional information is sent in FM (actually this frequency shift is obtained by a Doppler effect, so the name Doppler VOR).
The bearing is determined by comparing the phase of the variable signal and the reference.
A TACAN is based on a stationary antenna plus a rotating parasitic system. As already mentioned the base antenna is vertical and common the distance and bearing measurement instruments.
Parasitic elements in the aerial field refer to passive antenna elements added to the actual active radiator. A reflector decreases the gain on its side, a director increases the gain on its side (more). The well-known Yagi directional antenna (here in an horizontal polarization) has the two types of parasitic elements:
These elements are used in the TACAN, but they are rotating around the active element:
The central element, which is the one also used for the DME portion, transmits a constant amplitude signal.
A rotating drum with a reflector electrically adjusts the radiation pattern, adding a signal dip (low gain) that rotates at 900 RPM, which is equivalent to a 15 Hz amplitude modulation. The radiation pattern in the horizontal plan takes the shape of a cardioid:
(Source: Advances in Electronics and Electron Physics, Volume 68, modified)
Another drum with a set of 9 directors, mechanically linked to the first one, creates a 135 Hz (9x15) additional amplitude ripple over the 15 Hz modulation:
(Source: Advances in Electronics and Electron Physics, Volume 68, modified)
Now we need to start again the reasoning taking into account that the TACAN signal isn't transmitted permanently, but only keyed (switched on/off) by bursts of information. Burst are of two kinds:
- Reference bursts
- DME responses.
Reference bursts are generated according to the orientation of the modulation pattern:
- When the 15 Hz peak faces North a main reference burst is sent. The burst consists of 24 pulses with an asymmetric duty cycle.
- When any of the 135 Hz peaks faces East, an auxiliary reference burst is sent. The burst consists of 24 pulses with a symmetric duty cycle.
(Source: Advances in Electronics and Electron Physics, Volume 68. Modified)
The duration of these bursts is only a portion of the 15 Hz cycle, meaning that if there is few aircraft DME interrogations, most of the time the TACAN signal is not keyed, therefore not transmitted. This lack of transmission would create a difficulty for the aircraft receiver:
- To adjust its receiver gain (AGC) to counter fading.
- To identify the 15 Hz and 135 Hz modulations.
To maintain the capability of reception, the TACAN signal is instead keyed at a constant rate of 2,700 pairs of pulses per second, adding squitter pulses if necessary to fill the blanks. The more the DME interrogations are received by the TACAN, the more DME reply bursts are sent, the less squitter pulses are necessary (more in MIL-STD-291).
The 135 Hz signal has been removed for simplification (Source)
The 135 Hz modulation is used for the bearing determination. By comparing the time between an auxiliary burst and the subsequent reception of one of the 9 signal peaks, it is possible to determine the aircraft bearing relative to the ground station. The main burst (15 Hz) is used to disambiguate which of the 9 lobes was used, and therefore which of the 40° (360/9) sector is actually relevant for the bearing.
In theory the use of the top end of the UHF band and the 135 Hz ripple increase the bearing accuracy by one order of magnitude compared to the VOR. In practical this is less, but still better than the VOR.
In more modern TACAN, the mechanical rotations have been replaced by electronically scanned arrays:
(1): Yes, there are three bullets...