Secondary radar principle
Transpornders are placed at ground stations, which are usually co-located with VOR beacons. Each station has a unique (within the UHF line-of-sight receiving area) receiving frequency in the UHF band between 962 and 1213 MHz. The pilot of an aircraft wishing to find its range from the station must select that frequency on his airborne equipment, which will send a series of pairs of pulses at random time intervals on that frequency to the station. The station transponder replies to every pulse it receives (after a fixed delay of 50 us) with a another pulse at a frequency 63 MHz removed from the received signal. Each pair of pulses is separated by 12s. The emission pattern of both signals is designated PON.
The aircraft transmitter ‘remembers’ the sequence of differing time differences between its transmitted pulse pairs and awaits a corresponding pattern of transponded pulse pairs on the different frequency. Once the pattern has been received, the computer in the airborne equipment measures the time from transmission to reception, and calculates the range. In fact, the frequency on which the equipment operates is not usually published as such. It is either identified as a channel number, such as 46X, or more commonly the VHF frequency which is, or would be, used by a co-located VOR beacon.
The transponder is constantly replying to whatever signals it detects. If there are no genuine aircraft generated signals, it replies to random noise. The transponder filters the received signals and replies to the strongest it receives, whether generated by an aircraft or by noise. In theory, the closest interrogating aircraft will have their signals replied to. The transponder is able to respond to approximately 2700 pulse pairs every second.
Because the transponding must be omnidirectional, the DME ground aerial is a single pole antenna, cut to the ideal length for 1090 MHz. When co located with VOR, it is placed on top of the VOR antenna. DME ground equipments are designated as X or Y channels. X channels are the most common. The transponded signal is 63 MHz higher than the received signal. Y stations reply at 63 MHz below the received signal, and the pulse pairs are differently spaced. X stations are paired with VOR beacon frequencies with whole number decimals (e.g. 114.30), whereas Y beacons are paired with frequencies using halved decimals (e.g. 114.35).
The ground station also transmits an identification signal on the same frequency, approximately every 30 seconds. During the identification per- iod, the transponded pulses are replaced by regularly spaced pulses, keyed with the beacon identification letters.
A military navigation aid called ‘TACAN’ (tactical air navigation) uses a DME station to provide its range facility.
In fact the airborne equipment automatically and progressively scans’ the timebase of its own radar receiver from a zero range time delay to its normal maximum range of 200 nm. It is looking for its own random prf pattern which it is transmitting at 150 pulses per second (pps). Once it finds that pattern, it ‘locks on’ to that time delay, and displays it as a range.
The airborne equipment subtracts the 50 us delay from the time between transmission and reception and displays that time as an equivalent range. The DME indicator shows the aircraft range from the station, so the pilot knows his aircraft is somewhere on the circumference of a circle with the beacon at the centre and the indicated range as its radius. DME therefore gives a circular position line, which can be combined with one or more other position lines to give a navigational fix. The accuracy of a position line, as required by ICAO at least 95% of the time, is +- 0.5 nm, or +- 3% of the aircraft’s range if greater. In fact, DME is the most accurate of the ‘classic navigation aids, which makes it the preferred input to area navigation systems. Assuming there is no beacon saturation, the maximum range is limited by the UHF formula. Most airborne equipment indicates a maximum of 200 nm, but some continue out to 300 nm.
The ranges calculated are line-of-sight ranges, or ‘slant ranges. Pythagoras theorem can be used to calculate the actual ground range if given the aircraft height and indicated range.
Co-located VOR and DME
A receiver in the transponding equipment is tuned to receive pulse modulated signals at a certain frequency. After a short time delay, the transponder sends its own signal on a slightly different frequency to be recev at the original transmitter. The signal from the transponder can be coded as a series of pulses, giving information to the interrogator. Range between the target and the interrogator can be calculated by subtracting the programmed time delay from the total time between transmission of the first pulse and reception of the first reply, and multiplying by the speed of radio waves.
Most DME stations are co-located, or paired’ with VOR beacons. These can be identified because they both have the same published VHF frequency and identification signal. In this case, using the VOR and DME together will produce a pair of position lines (the radius and circumference of the circle centred at the station) which meet at right angles and give simultaneous range and bearing navigation information from the combined VOR/DME station. Other DME stations may be located close to VOR beacons, but not co- located. These can be recognised by having the same published frequency, but a slightly different identification, in that the DME identification will end in Z instead of the last letter of the nearby VOR beacon’s identification. Because the two are not co-located, care must be taken when plotting navigation position lines.
ILS paired DME
A DME which is ‘paired’ with an ILS is designed to give accurate ranges from touchdown along the ILS centreline. The DME station of course cannot be situated at the touchdown point itself. The desired result is achieved by altering the transponder fixed delay so that the airborne equipment indicates an incorrect range from the actual station, but the correct distance along the centreline from touchdown. This means that a DME paired with an ILS will give incorrect ranges in any other direction. The DME published VHF frequency is usually the ILS frequency.
Range arc tracking
Certain instrument approach procedures include a portion of ‘arcing. This involves following a DME arc at the nominated range until arriving on a VOR radial. In order to reach such an arc prior to following it, the pilot would normally fly along a designated radial towards a co-located VOR DME. As range reduces towards that designated, he must anticipate reaching it, and start a turn in the required direction which will bring him on to the designated range after 90°. His turn must therefore be started at the designated range plus whatever is the radius of turn. A 90 turn would take him at a tangent to the radial, and therefore away from the DME station, so his range would increase. The turn should there- fore be through only 80°, so that he actually flies along a chord inside the required arc by a small distance. He should then maintain that heading (actually track) until the range displayed increases again to a little above that nominated (usually just under one mile), then turn towards the station to follow a further chord. On the RMI, while following the chord when the range is just above that nominated for the arc, the VOR pointer will appear a few degrees to one side of (behind) the 90° mark on the instrument. The pilot can use that RMI needle to help him with the procedure if he wishes. Having noted that angle between the VOR needle and the 90° mark, he should then turn towards the station until the VOR radial is the same amount on the other side of (above) that 90° mark. In this way, as in the technique above, the aircraft will follow a series of chords almost corresponding to the arc of constant range.