14
Distance Measuring Equipment (DME)
Slant Distance
Distance measuring equipment (DME) can provide pilots with extremely useful information: their distance from a DME ground station. DME uses radar principles to measure this distance, which is the slant distance in nautical miles, rather than the horizontal distance (or range). Distance measured using DME uses the expression “DME” rather than NM or nautical miles; for example, 12 NM measured by DME is referred to as “12 DME.” This is in contrast to GPS distance, which is referred to with units of distance; simply, “12 NM.” While not technically the same as DME distance, GPS distance can substitute for DME distance in most situations.
For most practical purposes, the DME distance can be considered as range, except when the airplane is within a few miles of the DME ground station. As a general rule of thumb, the DME distance may be considered as an accurate horizontal distance (with negligible slant range error) if the airplane is 1 NM or more from the DME ground facility for each 1,000 feet above the facility. For instance, if the airplane is 12,000 feet higher than the elevation of the DME ground station, DME distances greater than 12 NM will provide an accurate range. The greatest errors occur at high altitudes close to the DME ground station.
Passing directly over the ground station, the DME indicator in the cockpit will either show the altitude of the airplane above the ground in nautical miles (1 NM = 6,000 feet approximately), or the DME indication will go to dashes.
Figure 14-1 DME measures slant distance.
Figure 14-2 Passing over a DME ground station at 9,000 feet AGL.
DME Cockpit Displays
DME distance may be displayed in the cockpit as either a digital read-out, or by a pointer that moves around a calibrated scale. The pilot selects the DME by selecting the VOR or ILS frequency on the NAV/COM radio (since most DMEs are paired with a VOR frequency or a localizer frequency). Once the DME is locked on, and a DME reading and ident obtained, the DME indications can be used for distance information regardless of whether the VOR (or localizer) is used for tracking or orientation purposes.
Most airborne DME equipment is capable of computing and displaying the rate of change of DME distance (the rate of closure of the airplane with the DME ground station). If it is assumed that slant distance equals horizontal distance, and that the airplane is tracking either directly toward or directly away from the DME ground station, then the rate of closure read-out will represent groundspeed, a useful piece of information.
Figure 14-3 A digital DME panel.
Most DME indicators can also display time to the station (TTS) in minutes at the current rate of closure, by comparing the groundspeed with the DME distance. The groundspeed is accurate when the airplane is tracking directly toward or away from the DME ground station, while the TTS is accurate only when tracking directly toward the station.
If the DME equipment in the airplane does not give a groundspeed read-out, then the pilot can simply note the DME distance at two particular times, and carry out a simple calculation of groundspeed = distance/time either mentally or on the navigation computer. Again, this is only accurate when the airplane is tracking directly to or from the DME ground beacon.
Example 14-1
A pilot notes DME distance and time as the aircraft tracks directly toward a DME station. Calculate groundspeed:
Circular Position Lines
The DME provides a circular position line (referred to as a DME arc). If the DME reads 35 NM, for instance, then the pilot knows that the airplane is somewhere on the 35 NM DME arc.
Information from another NAVAID may provide a positive fix of the position of the airplane, provided the two position lines give a good cut (angle of intercept) — ideally as close to perpendicular as possible.
Figure 14-4 A circular position line from a DME.
Figure 14-5 Using two NAVAIDs to fix a position.
How DME Works
DME uses the principle of secondary radar. Radar is covered thoroughly in Chapter 10, where both primary and secondary radar are discussed. Primary radar detects its own transmissions that are reflected from some object; secondary radar detects responding transmissions from a transponder activated by an interrogation signal.
Distance measuring equipment operates, using the secondary radar principle, by the airborne transmitter (the interrogator) sending out a stream of radio pulses in all directions on the receiving frequency of the DME ground station, which acts as a transponder in the following way. At the target DME ground station, interrogation pulses from the airborne transponder are received, and this generates reply pulses from the ground station exactly 50 microseconds later. The reply pulse is received back at the aircraft but only passes into the circuitry if it clears an electronic “time gate.” Because the interrogation pulses are transmitted at random time intervals known only to the transponder, the time gate only opens to receive replies to its own interrogations. Thus, the airborne transponder differentiates between ground station replies to its own interrogations and replies to other aircraft in the area. The airborne DME transponder measures the time between the interrogation and its reply pulse from the ground station, and subtracts the 50 microseconds resulting in the “travel time” of the pulses to and from the DME station from which distance in NM is easily derived. When several iterations of interrogation/reply result in similar distances, the DME display comes alive and displays the distance and other relative data. At this point, the DME is said to have “latched on” or “locked on.”
Figure 14-6 Operation of the DME.
Each DME ground station can cope with about 100 different airplanes at any one time before becoming saturated. When saturation is being approached, the DME ground receiver’s sensitivity is decreased, cutting out the interrogations from the aircraft that are farthest from the station. Thus, the ground station is protected from saturation. So, in busy terminal areas if you don’t see your DME lock on until you get in a lot closer than usual, that’s probably why.
Because the frequencies are carefully chosen so that stations with like frequencies are situated well apart geographically, there is very little likelihood of interference from the wrong DME ground station. The DME must still be identified, however, before you may use it for navigation. If your DME does not lock-on, try another nearby ground station in case the first one is saturated.
DME signals are line-of-sight transmissions (like VHF voice communications, radar and VOR), with an approximate usable range (in nautical miles) equal to the square root of (1.5 × altitude in feet).
Approximate DME range (NM) = 
1.5 × altitude in feet
Table 14-1 Altitude versus distance.
For example, if taking off from an airport 30 NM from the DME station (or VOICE-COM station or radar station) in flat terrain, don’t expect the system to work well until you have climbed to an altitude of about 1,000 feet AGL.
DME Frequencies
DME operates in the UHF (ultra-high frequency) band from 962–1,213 MHz which, with 1 MHz spacing, gives 252 possible frequencies. Each DME channel consists of two frequencies (an interrogation frequency from the airplane and a paired response frequency from the ground station).
There is no need for a pilot to know these details, since these channel numbers and frequencies are not used to select the DME — the correct DME channel is selected automatically when the frequency of an associated VOR or ILS station is selected on the NAV/COM set. If your DME is not channeled by the NAV/COM set, just select the VOR or ILS frequency on the DME set itself.
VOR/DME Pairing
Each VOR frequency has a specific DME channel paired with it (VORTAC). For instance, VOR frequency 112.10 MHz has DME Channel 58 paired with it, so that the VOR’s associated DME will automatically be interrogated when the pilot selects the VOR frequency 112.10 on the NAV/COM. The purpose of this pairing is to reduce the pilot’s workload in the cockpit, with only one selection instead of two required, and to reduce the risk of a pilot selecting the right VOR but the wrong DME station.
Figure 14-7 Fixing position with VOR and DME.
Co-located VORs and DMEs are frequency paired, and each will have the same Morse code ident, the VOR identifier modulated at 1,020 Hz and broadcast about every 10 seconds, and the DME identifier modulated at 1,350 Hz and broadcast about every 30 seconds. This is about one DME ident for every three VOR idents, with the DME ident being heard with a higher-pitched tone.
A single coded identifier received only once every 30 seconds, and not mixed in with another identifier broadcast every 10 seconds, means that the DME component of the VORTAC station is operative, but the VOR component is not.
VOR ground stations are often combined with TACAN installations (Tactical Air Navigation system), which provide azimuth and distance information to military aircraft on UHF frequencies. The combined VOR/TACAN facility is known as a VORTAC. Civil aircraft obtain azimuth (course) information from the VOR, and distance information from the DME component of the TACAN.
A paired VOR and DME (or VORTAC) can provide a very good position fix, consisting of:
- the radial from the VOR; and
- the distance from the DME.
ILS/DME Pairing
Some instrument landing systems (ILSs) have their localizer frequency paired with a DME located close to the runway. This provides the pilot with distances that can positively identify a fix along the approach path.
Some ILSs, however, are flown using the ILS for course and slope guidance, with a DME associated with some other nearby aid, such as a VOR, providing distance information. The need for this will be noted on the instrument approach chart.
Figure 14-8 Using a paired localizer/DME.
DME Arcs
Many instrument approach procedures use a DME arc along which the airplane should track to intercept the final approach course. DME arcs are sometimes used in Departure Procedures (DPs) as well.
The DME arc is a circular path centered on the DME ground facility. The airplane is flown to stay, at least approximately, at a fixed DME distance. Another aid, such as an RMI, is normally used to assist in tracking.
Figure 14-9 Example of a DME arc maneuver.
Review 14
Distance Measuring Equipment (DME)
1. What is DME the abbreviation for?
2. What does DME measure?
3. How is the DME selected?
4. How often is the coded identifier of the DME transmitted? What frequency is it modulated to?
5. How often is the coded identifier of the VOR transmitted? What frequency is it modulated to?
6. DME readings are accurate if the airplane is what altitude above the elevation of the DME ground station?
7. An airplane, cruising at 12,000 ft MSL, approaches a VORTAC situated on an airport with 2,000 feet MSL elevation. What is the minimum distance the plane should be from the VORTAC for the DME distance to be considered an accurate range?
8. For an airplane cruising at 15,000 feet MSL approaching an airport, elevation 210 feet, the DME read-out from a VORTAC sited on the airport may be considered an accurate range down to what DME distance?
9. How many times should you hear the VOR identifier each time you hear the DME identifier?
10. What will the DME distance indicate if an airplane flies 12,000 feet directly over a DME ground beacon?
11. What will the DME indicate if flying at 12,000 feet MSL and passing over a DME ground beacon at elevation 3,000 feet?
12. If an airplane tracking directly toward a DME is at 37 DME at time 0115, and at 27 DME at time 0120, what is its groundspeed?
13. If an airplane tracking directly away from a DME is at 22 DME at time 1223, and at 32 DME at time 1230, what is its groundspeed?
14. While flying abeam a DME ground station, the DME distance readings change in the following manner as you proceed: 25, 21, 17, 15, 14, 15, 17, 21. What was your distance from the DME ground station when exactly abeam the station?
15. What sort of position line can a DME provide?