DME Basics

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Distance measuring equipment is among the most valuable black boxes on the panel, especially for the IFR pilot. But there's a lot more to DME than turning it on and watching the numbers count down. AVweb's avionics guru explains how DME works, its inherent errors and limitations, and what to do if it stops working.

AvionicsDistance measuring equipment—often called DME—is one of the most valuable pieces of avionics in the aircraft. The main purpose of the DME is to display your distance from a VORTAC, VOR-DME, or localizer. (Some NDB stations have colocated DME, but not many.) DME reduces pilot workload by continuously showing your distance from the station, accurate to within a half-mile or three percent (and usually better). In addition, most DMEs display time-to-station and groundspeed.

Combined with VOR, DME permits you to determine your exact position from a single ground station; VOR tells you what radial you're on and DME tells how far out on that radial you are.

DME is required by the FARs for flight at or above FL240 if VOR navigation is used. Some instrument approach procedures require DME—these always have "DME" in their title (e.g., VOR DME RWY 5 or LOC DME BC A). Other approaches use DME to define optional step-down fixes to permit lower minimums.

How It Works

You tune your DME by selecting a VOR or localizer frequency, either on the DME itself or on a VOR/LOC receiver to which the DME is channeled. Although you dial in the VHF frequency of a VOR or localizer in the 108-118 Mhz band, you're actually tuning the DME to transmit on a "paired" UHF frequency in the 978-1213 Mhz band (also called the "L-band") and to receive on a UHF frequency that is offset from the transmit frequency by 63 Mhz.

There are 200 DME channels which are paired with the 200 VOR/LOC channels. So for example if you tune to the VOR frequency 112.40 Mhz, your DME is automatically channeled to 1010.00 Mhz. But pilots normally never see the actual DME channel; only the paired VOR frequency.

Once you've channeled your DME, it starts transmitting a stream of interrogations to the ground station. Each interrogation is made up of a pair of RF pulses. When the ground station receives the interrogation, it waits for 50 microseconds and then sends a pair of reply pulses back to the aircraft. Incidentally, the purpose of the 50 microsecond delay is to eliminate the possibility of uncoordinated operation when the aircraft is very close to the ground station

Your airborne DME equipment receives the reply and measures the elapsed time from when it sent the interrogation until 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, it can figure out its exact distance from the ground station using simple arithmetic, given the fact that it takes 12.359 microseconds for a signal to go out and return one nautical mile. It then displays the computed distance on your DME readout.

If this sounds a lot like how secondary radar works, it is. Only backwards. Your DME unit is essentially a little radar interrogator, and the DME ground station is just a fancy radar transponder.

There's one little complication, however. The ground station may be replying to hundreds of other airborne DMEs in addition to yours. How can your DME sort out replies to its interrogations from replies to other aircraft? Here's where some clever magic comes in. Your DME doesn't send an equally-spaced stream of interrogations, but randomly jitters the spacing of its interrogations to create a unique "signature". It then 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 interrogations.

DME Ident

Most pilots were taught to listen to and verify the Morse code identification of VOR stations, but many pilots don't realize that DME also has identification. When a VOR and DME are co-located (as in a VORTAC or VOR-DME station), the DME transmits the same coded ident as the VOR, but sends it during the pause between successive VOR idents. The DME ident is also higher-pitched: 1350 Hz compared with 1020 Hz for a VOR ident.

Most audio panels have a switch that allows you to listen to the DME ident. I strongly recommend verifying the DME audio. It's quite possible to have your VOR receiver lock onto one station and your DME to lock onto a different station that shares the same frequency. This is most likely at high altitude over flat terrain, and it has happened to me more than once.

DME Errors

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 or from the ground station. If you are flying in any other direction, you will see groundspeed that is erroneously low and time-to-station that is erroneously high.

If you are flying away from the station, groundspeed will be accurate but "time-to-station" will actually show "time-from-station" and will increase as you get farther and farther from the station.

Another thing to remember is the DME measures the straight-line distance from the aircraft to the ground station. This is called "slant range" and is slightly more than the actual horizontal distance because of the difference in elevation between the aircraft and the station. The most extreme case of "slant range error" occurs when the aircraft passes directly over the station; instead of reading zero, the DME shows the altitude of the airplane above the station (in nautical miles).

For example, if the station is at sea level and you're flying at 9000 feet, the DME will show 1.5 NM as you pass directly over the station. When you're 2 NM away from the station horizontally, the DME will read 2.5 NM. At greater distances from the station, slant-range error is considered negligible.

Slant range error also affects groundspeed and time-to-station displays when you're close to the station. Displayed DME groundspeed drops below actual groundspeed as you approach the station and then climbs back to normal after you pass it. Displayed DME time-to-station may not count all the way down to zero as you fly over the station.

Effective Range

Like radar, DME is limited to line-of-sight. Regardless of how much power your unit has, if it's not within direct line-of-sight of the ground station, it's not going to work. That doesn't mean that you actually have to see the ground station with your eyes in order for the DME to work! At 5000 feet over flat terrain, you might expect to get about 60 miles of DME range. Over rough terrain, the range can be much shorter: 30 or 40 miles.

Another thing that can limit DME range is high air traffic density. The DME ground station can only respond to a certain number of interrogations in a given period of time. If too many aircraft are interrogating the ground station, it will automatically desensitize its receiver so that it can only hear and reply to the strongest interrogations. This can result in shorter-than-normal DME range, particularly for small aircraft with their low-powered DME units.

One nice thing about DME is that, unlike LORAN or ADF or even VOR, it is seldom affected by precipitation static buildup or thunderstorms.

Reliability

Reliability of DMEs have come a long way in the past twenty years. The newer all-digital DMEs like the King KN-64 or the Narco 890 are extremely reliable and seldom require service. Older analog DMEs like the King KN-65 or Narco DME-190 are far more troublesome and can be very expensive to repair. The older units also used lots of power and needed lots of cooling. So if your older DME is on the fritz, you might consider replacing it with a newer all-digital model.

Once you've flown with a DME, it's hard to imagine doing without it. Eventually, GPS may supersede the need for DME, but we're not at that point yet.