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Tom Rogers, Ph.D. |
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| About the Author ... |
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Tom Rogers is Avionics Editor for AVweb.
Tom operates
Avionics West, Inc. at Santa
Maria, California, one of the finest radio shops on the West Coast. Tom is an
instrument pilot, an FAA Designated Engineering Representative (DER) for
avionics, and has a Ph.D. in nuclear physics. (We're not sure why he got the
doctorate, but we call him "Dr. Tom," and he seems to like that.)
You can send Tom your avionics questions at
avionics@avweb.com.
Tom's company, Avionics West,
is one of the worlds largest discounters of handheld and
panel-mount avionics for general aviation. They offer all leading brands of
aviation electronics at deep-discount prices. Every item sold by AWI is
covered by a 30-day no-questions-asked return privilege.
Avionics West features handheld GPS receivers from Garmin and Lowrance;
headsets from David Clark,
LightSPEED and Telex; and panel-mount GPS navigators from II Morrow. AWI
stocks these units and generally sells them at prices substantially below the
manufacturers' "Minimum Advertised Price" (MAP). Consequently, AWI is not
permitted to advertise these prices on its web site, but you can obtain them
by sending an email to the AWI auto-responder at
avionics-specials@avweb.com, or
by telephoning AWI at 1-805-928-3601 (M-F, 8-5 Pacific Time). |
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Distance 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.
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