How DME Works

Distance measuring equipment (DME) requires both ground-based and in-aircraft equipment. You'll usually find DME equipment co-located with a VOR or ILS/LOC. NAVAIDs like VORs and ILS systems transmit their navigation signals over VHF. On the other hand, DME is transmitted over UHF. The FAA has matched standard VHF NAVAID frequencies to associated UHF DME frequencies.

For example, all VORs operating on 113.0 will use the same DME frequency, regardless of where the VOR is. The FAA spaces out NAVAIDs to ensure mixed frequencies don't inhibit your receiver.

DME radios measure distance by timing the interval between the "interrogation pulse" from the transmitter and the "reply pulse" from the receiver. It's sort of like how submarines use sonar signals bouncing off objects to map the ocean floor.

DME is displayed in nautical miles and measured in terms of something called "slant-range distance." Slant-range measurements will always exceed planned distances due to how high the aircraft is flying.

For example, if an aircraft is flying directly overhead a DME transmitter at an altitude of 6,000 feet, the DME will read approximately 1 NM.

So how much DME slant range error is there for most aircraft operations? The rule-of-them is that if you're at least 1NM away from the station for every 1,000' AGL, slant range error is negligible.

So if you're flying at 5,000' above the DME station (AGL), and you're at least 5NM away from the station, your DME readout will be accurate.

DME requires line-of-sight between the aircraft and the ground station, and terrain and distance beyond the horizon will prevent DME from working.

Ground-based DME transmitters are also rated to handle roughly 100 aircraft at a time. If the equipment is overloaded by too many aircraft, those farthest away may not be able to pick up DME signals at all.

GPS DME vs. Traditional DME

Thanks to GPS, pilots are using traditional DME less and less. If you're flying IFR with an approved GPS, you can use GPS distance to substitute for DME.

For instance, if you're flying on an ILS and the DME for the FAF is 5 miles from the LOC antenna, and you're 2 miles away from the fix, your GPS DME will give you a 2 mile distance to the FAF.

If you want GPS DME to match traditional DME on an ILS approach, you could type in "Direct IXXX" to get distances that match your approach chart.

We don't recommend you do it, however, because you might end up placing a direct-to line on your map, leading to confusion about which course you're following to the runway.

Don't Forget The HOLD Function!

Have you ever flown an ILS that references distance information from a nearby VOR, instead of the LOC? For aircraft equipped with a DME radio, you should understand what the "HOLD" button does. When you're flying an approach that references DME off a nearby NAVAID, you have to first tune your NAV radio to the DME source, click "HOLD," and then tune the approach frequency for navigation data. Remember how we mentioned that DME UHF frequencies are always associated with individual VHF frequencies? If you "HOLD" the DME frequency from the NAVAID you have tuned, you'll lock the DME frequency from switching based on you tuning a new NAVAID.

Let's take a look at the Thief River Falls (KTVF) ILS to Runway 31. Fixes along the approach are identified using DME from the TVF VOR. As you arc to the localizer course, you'll reference DME off TVF. When you turn inbound, you'll switch to the I-HYZ LOC frequency of 110.5. But first, you must click "HOLD." This will lock the TVF DME readings in your DME radio since that's the distance information used for the approach. If you don't arc at all, the same principle applies. You must first tune TVF, click "HOLD," then switch to the I-HYZ LOC frequency.

Once you're done with the approach, don't forget to un-hold the DME frequency, so that future approaches don't reference incorrect distance information.

Which Airplanes Have DME Radios?

You probably won't find traditional DME radios on glass panel training aircraft. Most general aviation aircraft built today are equipped with multiple GPS receivers instead.

However, you will still find DME radios all over the industry. Most transport-category aircraft have DME, and quite a few older IFR-certified GA aircraft have DME too. In fact, FAR 91.205(d)(2) requires any aircraft certified to fly IFR about FL240 to be "equipped with approved DME or a suitable RNAV system."

Does your aircraft have DME? Does it only have GPS? Tell us in the comments below.