Distance Measuring Equipment system
Distance measuring equipment (DME) is a transponder-based radio navigation technology that measures slant range distance by timing the propagation delay of VHF or UHF radio signals.
DME is similar to secondary radar, except in reverse. The system was a post-war development of the IFF (identification friend or foe) systems of World War II. To maintain compatibility, DME is functionally identical to the distance measuring component of TACAN.
Figure 2.6 DME antenna beside the DME Transponder shelter
Considered DME radio beacon construction principle is a basis of LRRB construction such as FSD (of FACE STANDART firms), which is wide-spread, and also in long range tract of RSSN equipment.
Aircraft use DME to determine their distance from a land-based transponder by sending and receiving pulse pairs - two pulses of fixed duration and separation. The ground stations are typically co-located with VORs. A typical DME ground transponder system for en-route or terminal navigation will have a 1 kW peak pulse output on the assigned UHF channel.
DME Range measuring
A low-power DME can be co-located with an ILS glide slope antenna installation where it provides an accurate distance to touchdown function, similar to that otherwise provided by ILS marker beacons.
Distance measuring radio-navigation system (RNS) includes ground equipment (ranging beacon) and avionics (aircraft range finders).
In international practice, such a system is called DME (Distance Measuring Equipment - Equipment Ranging). This is the name used in the aeronautical information documents Russia, although beacons manufactured by domestic producers may be quite another official name (for example, RMD - ranging beacon).
The operating principle of the rangefinder system in a simplified manner is as follows. Aircraft range finders on board emits electromagnetic pulses (radio waves) in all directions. Ground beacon and takes them through the fixed time delay (50 microseconds) emits a response signal, which is taken on board.
Figure 2.7 The operating principle of the rangefinder system
The time t between pulse emission and reception of his rangefinder as the sum of the impulse response of the pulse time "there" (from the aircraft to the beacon), the same transit time response signal "back" and the delay time. Knowing the speed of propagation of radio waves, it is possible to determine the distance to the lighthouse
Since VHF radio waves propagate in a straight line, then L in this formula - a slant range (in a straight line from the aircraft to the beacon).
In this case, it turns out that on-board equipment as it is requesting information from the beacon, that is a requestor (interrogator), and the beacon meets him, is the defendant (transponder).
This general principle of the measurement range, but in reality, of course, more complicated and more interesting. Rangefinder emits no single and paired pulses (interval between pulses in a pair, for example, 12 ms), and the beacon is "responsible" only if it received a pulse. Otherwise, he would have to answer all the random impulses that some other equipment transferred at this frequency (eg, cellular communications work in close range of frequencies).
All aircraft operating this beacon emits pulses at a single frequency, but the spacing between pairs of pulses in all the sun is different, each has its own pulse repetition frequency PRF (Pulse Repetition Frequency). Defendant beacon sends pulses with the same PRF, with which the signals received from the aircraft. This is to ensure that each aircraft has received a response to his signal, and not for other aircraft.
Figure 2.8 Aircraft DME Structure
Aircraft DME consist of:aircraft interrogator,interrogator pulses former,miter,indicator,decoder,aircraft receiver and antenna
Figure 2.9 Ground DME Structure
Ground DME consist of:receiver,decoder,coder,transmitter,control block and antennas
Furthermore, the beacon responds not to the frequency at which it received signal, and differs from it at 63 MHz. This is to ensure that on-board rangefinder not taken by mistake for a response signal beacon own impulses reflected from some objects (mountains, the clouds, the fuselage). Otherwise could be that rangefinder wink request is rejected, they are affected by the mountain range finder accepted them and found that this response pulses from the beacon.
When the DME airborne equipment it initially operates in the search mode, and transmits a request pulses at a frequency of 150 pairs per second. When the response is received Segal (typically 4-5 seconds), the pulse repetition rate is reduced to 25 per second.
The capacity of the ground is limited to the defendant, it may not have time to answer the entire set of planes that request it. Typically, the beacon is able to cater for 100 aircraft at the same time. If they are within range of the beacon is larger, the longer served by the weakest signals to the outermost planes.
For operation DME selected frequency range of 960 to 1215 MHz. It decimeter wave (UHF) band, VHF, which implies that they are distributed within the range of direct visibility. Therefore, they include everything that was said earlier about the maximum range of the VHF agents.
A radio signal takes approximately 12.36 microseconds to travel 1 nautical mile (1,852 m) to the target and back-also referred to as a radar-mile. The time difference between interrogation and reply, minus the 50 microsecond ground transponder delay, is measured by the interrogator's timing circuitry and converted to a distance measurement (slant range), in nautical miles, then displayed on the cockpit DME display.
The distance formula, distance = rate * time, is used by the DME receiver to calculate its distance from the DME ground station. The rate in the calculation is the velocity of the radio pulse, which is the speed of light (roughly 300,000,000 m / s or 186,000 mi / s). The time in the calculation is (total time - 50μs) / 2.