Measuring The Miles

From star-gazing to satellite navigation systems


The aircraft interrogates the ground transponder with a series of pulse-pair interrogations and after a precise time delay of 50 microseconds, the ground station replies with an identical sequence of reply pulse-pairs. The DME receiver in the aircraft searches for specific pulse pairs using 12 microsecond spacing and with the correct time interval between them. This pattern is determined by each individual aircraft’s particular interrogation.

The aircraft equipment locks on to the DME ground station once it comprehends the particular pulse sequence and verifies alignment with 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 the lock.

A radar-mile
A radio pulse takes 12.36 microseconds to travel 1 nautical mile to and from. This is also referred to as a radar-mile. The time difference between interrogation and reply 1 nautical mile minus the 50 microsecond ground transponder delay is measured by the interrogator’s timing circuitry and translated into a measurement in nautical miles which is then displayed in the cockpit.

The distance formula, Distance = Rate x Time, is used by the DME receiver to calculate its distance from the DME ground station. The rate or speed in the calculation is the velocity of the radio pulse, which is the speed of light (186,000 miles per second). DME transponders transmit on a channel in the 962 to 1,150 MHz range and receive on a corresponding frequency between 962 to 1,213 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 with spacing of all channels at 1 MHz and a signal spectrum width of 100 kHz.

Morse Code identifier
DME facilities identify themselves either audibly or visually using a 1,350 Hz Morse Code three letter identity. Some newer displays can produce the identifier for visual reference while older systems require listening to the DME receiver and interpreting the Morse Code letters. In the event of reported malfunction, this check is a quick means of telling if at least the system has a station responding. If collocated with a VOR or ILS, it will have the same identity code. Additionally, the DME will identify itself between those of the parent facility. The DME identity is 1,350 Hz to differentiate itself from the 1,020 Hz tone of the VOR or the ILS localizer.

The original specification for the ground based equipment was to have adequate capacity to be able to communicate with up to 100 aircraft at a time. More modern equipment can handle twice that. Above the design 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 of the DME station when it is overloaded and cannot accept more than 100 aircraft is called “Station Saturation.”

Considering the similarity in principle and operating frequencies of DME and ATC, most aircraft including both kinds of equipment incorporate a DME inhibit feature called a “Suppression Bus.” Anytime the ATC transponder is replying to an interrogation the DME is placed on standby for the duration of the ATC transmission. A malfunction in this interconnect may impede DME operation.

Many of today’s interrogators have the ability to lock on up to three different ground stations. Associated DME control heads include a “DME Hold” switch. When actuated, the DME will continue to communicate with the last station entered while the navigation radio can be tuned to another facility. This feature too, can provide for flight squawks and should always be checked prior to beginning serious diagnostic techniques.

In more modern aircraft, DME information is also utilized by long-range navigation and flight management systems (FMS). Another troubleshooting technique is to enter the FMS “Sensors” page and check the status of the respective DME. This often provides clues if a problem is associated with an indicator versus an interrogator. There are strong probabilities that DME will remain a useful tool for navigation well into the next generation.

I still like the star gazing concept even when I don’t have to steer by the constellations.

Jim Sparks has been in aviation for 30 years and is a licensed A&P. He can be reached at sparks-jim@sbcglobal.net.

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