DME, Inside and Out

 

A lot of misunderstanding surrounds the concepts and use of Distance Measuring Equipment (DME). Also, the correct use of GPS equipment in lieu of DME can be confusing. This month, I’ll attempt to explain how this equipment works and hopefully dispel any myths you may have heard or be passing along to others.

Basic DME Concepts

One of the first myths to debunk is that DME is a VHF navigation facility. Its not! In fact, despite the frequency you tune in to receive DME information, it is by design a UHF navigation facility.

The operating range of DME is somewhere around the range of 960 MHz and 1215 MHz, while VORs and VHF communication frequencies are far lower in the MHz band. So how is it that we can tune in what appears to be a VHF frequency and receive DME information?

Although many DME facilities are co-located at VOR stations, they are just that… co-located. It’s a separate piece of gear that is transmitted to using the paired UHF-VHF frequency. Aircraft with a UHF TACAN unit will receive the DME information automatically and do not need a separate DME unit like the rest of us.

Unfamiliar with the concept of frequency pairing? Do a Google search and you can see how the frequency pairing is set up.

Anyway, when you tune in the appropriate VHF frequency, the DME unit in your aircraft sends out paired pulses while waiting for the ground station to reply. The ground station then transmits a new set of paired pulses at a frequency offset of 63 MHz, at the same spacing as what the aircraft sent, back to the aircraft.

The time required for the round trip of a paired pulse is then measured in the aircraft’s DME unit and translated into distance. This calculation is relatively easy to do since the radio signal moves at the speed of light and that value is well known. With these two values, a time to the station and groundspeed can also be easily calculated by the aircraft unit and displayed to the pilot.

One sure way to confirm that DME is not like the other VHF navigation radios is to check your antennae arrangement. Even if you have two VHF nav radios, they will both likely use the same antenna since they only receive radio signals.

Since the DME both transmits and receives information, it must use it’s own antenna; it cannot share an antenna with the other nav radios. Likewise, each VHF communication radio must use its own VHF antenna since they both transmit and receive.

But how exactly does the DME ground station keep track of all those aircraft transmitting paired pulses, requesting a reply for distance information? Simply put, the spacing between paired pulses coming from the DME unit in the aircraft is not symmetric, and as such creates a unique pattern that the ground station can keep track of.

The ground station responds with the same pattern of pulses. The aircraft’s DME unit knows what pattern of pulses to look for and searches all the incoming patterns for a match. In this way, only the aircraft that originally sent a particular pattern can use the information that is pertinent to its current location.

A final note about basic operation is that DME sends out Morse Code identification about every 30 seconds. This differs from other navigation facilities so you have to be a little more patient in waiting to hear the DME code.

Errors and Limitations

DME is not a perfect navigation signal, so pilots need to be aware of the errors and limitations when using this equipment.

The first error is not really an error at all, but easily perceived as one. DME does not give a measure of ground distance to a station, but rather the line-of-sight distance that we commonly refer to as slant-range. At long distances from the ground station, the difference between ground distance and slant-range distance is small and generally unimportant. Where pilots need to be aware of the difference is close-in to the station.

As the aircraft arrives over the top of the DME station, the DME will only “click” down to the approximate altitude (in NM) of the aircraft. For example, if the aircraft flies over the station at approximately one NM high, the DME reading in the aircraft will click down towards roughly one NM and then start back up after station passage. So, the pilot should not assume the DME distance will click down to zero prior to station passage.

Many DME units feature the additional information of groundspeed and time-to-station. Groundspeed readout is only accurate when flying directly to or away from the station. This is a necessary limitation since the only information the DME unit has to work with is time (converted to distance).

If the aircraft is flying a constant distance from the receiving station (such as during a DME arc procedure), although the distance information is still accurate, there is no trend of time change between one update of the distance information to the next. During a properly executed arc, no change in time-to-station is seen as synonymous with an aircraft standing still since the DME station has no idea what bearing you are on, or that you are in fact crossing bearings; hence the groundspeed drops to near zero.

In fact, one sign of a perfectly flown DME arc is virtually zero groundspeed readout.

One other groundspeed readout problem occurs as DME station passage nears. Since the actual distance from the station starts to slowdown as the ground station is neared (due to altitude) so will the groundspeed. After station passage the groundspeed will climb back up to normal. This problem is due to the use of slant-range distance as opposed to actual ground distance.

Another possible limitation, rarely experienced by most pilots, has to do with the absolute limit to the number of aircraft that the DME ground station can respond to simultaneously.

Some sources indicate that DME stations can only respond to a set number of aircraft at one time, and that if that number is exceeded, only the strongest signals will be responded to. Likewise, if you have a weak DME transmitter, you may not get a reply from the ground station if enough stronger aircraft transmitters are closer to the station.

I’ve heard of this problem from a few experienced pilots over the years but I can’t speak to the veracity of the explanation. However, one older book I have talks about this problem and indicates that about 100 aircraft can simultaneously use the same DME station.

Regardless, if the signal is lost due to station overload, there will be no indication other than the loss of all DME information. The DME ident code will still be received, since the station is still operating. In any event, this particular problem appears to be quite remote.

Since DME is line-of-sight, it is susceptible to loss of signal during maneuvering flight. As well, since it operates in the same band of frequencies as transponders the potential for interference between the two devices exists.

To solve both problems, the antenna is usually mounted on the bottom of the aircraft and at a reasonable distance from the transponder antenna, and suppression equipment is installed in the aircraft to keep the DME and transponder from transmitting and receiving at the same time.

If the DME and transponder antennae are too close together or the suppression equipment is malfunctioning, either the DME and/or the transponder may not work properly.

Finally, the DME antenna is rarely out of the line-of-sight for more than a few seconds during maneuvering flight, but when loss of signal does occur, the DME will float the last reading for a short period of time prior to giving up and replacing the value with dashes or showing a NAV flag.

The aircraft unit will rely on its last received data for about 20 seconds and then floats the last DME value for about 10 more seconds. Bottom line is, if your DME info doesn’t change in 20 seconds, it might just flag within the next few seconds due to loss of signal. I think this method of losing the signal is pretty rare given the time that the aircraft’s antenna needs to be blocked for the loss to occur.

GPS distance vs DME distance

Before concluding for this month, a little needs to be said about GPS distance and DME distance. For starters, they are NOT the same thing!

Although the general consensus is that GPS distance can be used in lieu of DME, that is only true when the DME fix has a five letter fix name (both en route and for overlay instrument approaches) or when using a navigation facility that is preprogrammed into the unit.

It is not acceptable in some cases to simply select a VOR on the GPS and use it’s distance like it was DME distance. In fact, the GPS does not give you slant-range, which is what is taken into account for the charted distances.

The error between the two distance sources increases with altitude. So when flying en route, the correct way to use GPS distance is to program the route with the fix names and VOR names from the onboard GPS database.

For instrument approaches, only those VOR/DME approaches with named DME fixes (such as stepdowns) can be flown using the GPS. In fact, those approaches will be loadable as overlays and you’ll use the distance info from fix to fix as indicated by the GPS.

Flying a VOR/DME approach using the GPS with the DME distance to guide the step-downs and missed approach point is not a legal way to use GPS. In fact, you are likely to be confused using the distance off the VOR when using the GPS for course guidance since the distance reading from the GPS is to the next active waypoint as opposed to a fixed navigation aid (although in some cases the next waypoint is the VOR).

Also remember that when flying an overlay instrument approach with the GPS, the distances between fixes as shown in the waypoint list may not exactly match what is printed on the chart. This is due to the difference between the slant-range distances for the charted conventional navaid approach and actual GPS ground distance when the approach is flown as a GPS overlay.

Fly the GPS waypoints as programmed; they provide the correct obstacle clearance, stepdown and missed approach points locations.

For now, DME is still required for much instrument flying around the world, at least if you don’t have an approved GPS unit. As a result, it’s important to understand how it works and how to properly use the features available from DME. Hopefully, I’ve given you a little better understanding of this useful navaid.

This month’s Pilot Primer is written by Donald Anders Talleur, an Assistant Chief Flight Instructor at the University of Illinois, Institute of Aviation. He holds a joint appointment with the Professional Pilot Division and Human Factors Division. He has been flying since 1984 and in addition to flight instructing since 1990, has worked on numerous research contracts for the FAA, Air Force, Navy, NASA, and Army. He has authored or co-authored over 180 aviation related papers and articles and has an M.S. degree in Engineering Psychology, specializing in Aviation Human Factors.