Basic Principles of Ground Avionics DME

1. Basic Principles of Ground Avionics DME

The purpose of Distance Measuring Equipment (DME) is to provide distance information between a flying aircraft and a DME ground station. The distance is determined by measuring the propagation delay of a radio frequency (RF) pulse that is emitted by the aircraft transmitter and returned at a different frequency by the ground station.

DME equipped aircraft transmit encoded interrogating RF pulse pairs on the beacon's receiving channel. The beacon replies with encoded pulse pairs on the airborne equipment’s receiving channel, which is 63 MHz apart from the beacon’s channel.

The aircraft’s receiver receives and decodes the transponder’s reply. Then it measures the lapse between the interrogation and reply and converts this measurement into electrical output signals. The beacon introduces a fixed delay, called the reply delay, between the reception of each encoded interrogating pulse pair and the transmission of the corresponding reply. The interval between the interrogation emission and the reply reception provides the aircraft with the real distance information from the ground station; this information displays on the cockpit indicator.

In addition to distance pulse pairs, the transponder periodically transmits special identification pulses which the aircraft decodes as morse code keys to identify the transponder beacon. Further, the transponder can respond to interrogations from 100 aircrafts, each at 27 pulse pairs per second (pps). This is equivalent to a response of 2700 pps. Whenever the number of pulse pairs fall below 2700 pps, the transponder reacts by generating random pulse pairs called squitter to fill in and maintain a constant load of 2700 pps

It is important to understand that DME provides the physical distance from the aircraft to the DME transponder. This distance is often referred to as 'slant range' and depends trigonometrically upon both the altitude above the transponder and the ground distance from it. Slant range error is most pronounced at high altitudes when close to the DME station.

2. Operational Theory

Aircrafts 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.

A low-power DME can also 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. The DME system is composed of a UHF transmitter/receiver (interrogator) in the aircraft and a UHF receiver/transmitter (transponder) on the ground.

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 reply pulse-pairs. The DME receiver in the aircraft searches for pulse-pairs (X-mode= 12 microsecond spacing) with the correct time interval between them, which is determined by each individual aircraft's particular interrogation pattern. The aircraft interrogator locks on to the DME ground station once it understands that the particular pulse sequence is the interrogation sequence it sent out originally. Once the receiver is locked on, it has a narrower window in which to look for the echoes and can retain lock.

A typical DME transponder can provide distance information to 100 aircraft at a time. Above this limit the transponder avoids overload by limiting the gain of the receiver. Replies to weaker more distant interrogations are ignored to lower the transponder load. The technical term for overload of a DME station caused by large numbers of aircraft is station saturation.

DME frequencies are paired to VHF omnidirectional range (VOR) frequencies and a DME interrogator is designed to automatically tune to the corresponding DME frequency when the associated VOR frequency is selected. An airplane’s DME interrogator uses frequencies from 1025 to 1150 MHz. DME transponders transmit on a channel in the 962 to 1213 MHz range and receive on a corresponding channel between 1025 to 1150 MHz. The band is divided into 126 channels for interrogation and 126 channels for reply. The interrogation and reply frequencies always differ by 63 MHz. The spacing of all channels is 1 MHz with a signal spectrum width of 100 kHz.

Technical references to X and Y channels relate only to the spacing of the individual pulses in the DME pulse pair, 12 microsecond spacing for X channels and 30 microsecond spacing for Y channels.

DME facilities identify themselves with a 1350 Hz Morse code three letter identity. If collocated with a VOR or ILS, it will have the same identity code as the parent facility. Additionally, the DME will identify itself between those of the parent facility. The DME identity is 1350 Hz to differentiate itself from the 1020 Hz tone of the VOR or the ILS localizer.

Radio-navigation aids must keep a certain degree of accuracy, given by international standards, FAA, EASA, ICAO, etc. To assure this is the case, flight inspection organizations check periodically critical parameters with properly equipped aircraft to calibrate and certify DME precision. ICAO recommends accuracy of 1.25% of the distance measured.

3. Sequence of Operation

1. The aircraft interrogates the ground station by transmitting a series of pulse-pairs (interrogations) on the receiver frequency of the ground station. The pulse-pairs have a constant time interval (T1 = 12µsec or 36µsec) between pulses. The interrogator randomly varies the time intervals between pulse-pairs to enable it recognize only the replies corresponding to its interrogations.
2. The ground station receives the series of pulses-pairs and, after a precise time delay (50 µsec), the ground station replies with an identical sequence of reply pulse-pairs. The reply from the ground station to the aircraft interrogator is made in frequency of 63 MHZ above or below the interrogator frequency.
3. The DME receiver in the aircraft receives the reply and measures the elapsed time from when it sent the interrogation until when it received the reply. It subtracts the 50 microsecond delay that the ground station introduced to come up with the round-trip time.
From this, the airborne receiver can calculate its exact distance from the ground station, given the fact that Distance = Velocity x Time. The DME equipment then displays the computed distance.

4. Considering that the ground station is replying interrogation from several aircraft at the same time, the DME interrogator need to sort its own pulse-pair out from the ground station replies. To achieve that the DME airborne receiver examines the ground station replies looking for a sequence with the same randomly jittered signature. When it finds that, it knows they're replies to its own interrogations.
In addition the DME uses rate of change of the distance to calculate the ground speed and time to station.  Once the ground speed is known, the time to station can be calculated. DME displays distance in nautical miles, groundspeed in knots, and time-to-station in minutes. Beware, however, that DME groundspeed and time-to-station are only accurate when you are flying directly to (inbound) or from the ground station (outbound).

6. Monitoring of the DME signal

There must be away of monitoring DME transponder signal to ensure that its parameters are correct and free of errors. Therefore, every transponder is fitted with an inbuilt monitoring system. The monitoring system checks the following parameters of the signal: Pulse spacing, system delay, Efficiency (% number of replied interrogations), PRF (pulse repetition frequency), RF power level and Identification. If any of this parameter deviates outside the threshold, then the monitoring system initiates a control signal to shut down the transponder. The monitoring system comprises of two independent monitors which must agree to effect a shut down. The nominal values of these parameters should be as follows:

1. Pulse spacing = 12 microseconds
2. System delay = 50 microseconds
3. Efficiency = 70%
4. PRF = 800 – 5400
5. RF power levels = 100W (LP), 1000W (HP)
6. Identification = 1350 Hz