Distance Measuring Equipment
DME is a pulse ranging system first developed in the 1950s. DME is operating in the UHF frequency band between 960-1215 MHz; this part of the spectrum is also commonly known as the Lower L-band. The DME was originally ‘paired’ with a VOR or ILS Localiser to provide a range source to a bearing signal. Originally, each DME channel had to be manually selected but today this automatically done by the aircraft’s avionics.
DME uses exactly the same principle as secondary radar, that is to say an interrogation is made and a response is transmitted. In secondary radar the interrogation is made from the ground and the aircraft responses. In DME, the interrogation is initiated from the aircraft and the ground station responds.
Once the pilot, or the avionics, has selected the correct channel (each of the 252 channels available is associated to a frequency between 960 and 1215 MHz) the aircraft transmits a stream of pulse pairs and is then looking for a response from the ground-based transponder. The transponder receives the interrogation pulse train and re-transmits them after a 50 microsecond (μs) delay period on an associated frequency which is either +/- 63 MHz from the interrogation frequency.
The aircraft interrogator identifies its own stream of returned pulses and then measures the time taken from transmission of interrogation to time of receipt. The aircraft’s interrogator then corrects for the fixed time delay on the ground, 50 μs, to calculate the time taken for the signal out and back. Half the time taken then translates to a ‘slant’ range from the station.
Each DME ground station is capable of transmitting 2700 pulses per second, this is referred to as the Pulse Repetition Frequency (PRF). If no interrogations are received then the DME transponder randomly transmits 2700 single pulses per second; these single pulses are known as ‘squitter’. If a valid interrogation is received by the ground station then the squitter pulse is replaced by a pulse pair. Once all squitter pulses are replaced by pulse pairs then further interrogations can not be replied to and the DME station is considered ‘saturated’. Furthermore, additional interrogations force the ground station to reduce its sensitivity to maintain the PRF of 2700 and this results in only aircraft closer to the station receiving and tracking the signal.
A single DME ground station can nominally respond to a maximum of 100 aircraft at any one time before reaching saturation. When the airborne DME unit has identified to its own responses from the squitter pulses, this is achieved by randomly changing (jittering) the airborne interrogation pulse repetition rate to ensure a unique series of pulse pairs such that it that it rejects all other responses, it is considered to be “locked-on” or “tracking” the ground station. The aircraft will not lock-on to any other DME reply because the pulse pairs are not syncronised with its own interrogations.
For identification, at least every 40 seconds, the DME ground station will temporarily replace all pulses with pulse pairs transmitted at the rate of 1350 PPS. These pulses are detected as a 1350 Hz Morse-code identifier, and can be used by the aircrew to identify the station. As no pulse pairs are transmitted during this time, the aircraft goes into memory mode to prevent the loss of ‘lock on’.
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