View Full Version : Dme Dme Update

3rd Feb 2005, 23:04
On some ILS charts we can read a note "DME reads 0 at threshold." What I understand from this, the DME position is electronically displaced. So, the actual position of the airplane with reference to the geographical position of DME antenna and measure of the distance from antenna for position updating, will be different. If I am correct, this will cause the DME-DME position updating to be erroneous and eventually map/position shift will occur. To prevent from this, when we see this note on the approcah, we should select that DME station to be inhibited from position updating.

What you guys think all above this?

Old Smokey
4th Feb 2005, 07:00

When the chart reads "DME reads 0 at threshold", that is because the DME is located at the threshold. No DME that I know of has the means to "electronically" reposition itself, the DME reading will always be with respect to the the correct geographic position of the DME site. Consider an ILS/DME, the Localiser is at the 'Far' end of the runway, the Glide Path transmitter at the touchdown point, and, ideally, the DME is abeam or very close to the touchdown point, hence Zero reading at the threshold.

If you examine the FMC navaid data-base, you will see 2 locations for the one ILS/DME ident, one for the localiser, and one for the DME (The Glide slope transmitter doesn't rate a mention, as no position updating occurs from the Glide Path signal).

There is no need to inhibit the DME portion of the ILS/DME for DME-DME updating. You may continue to use the DME with confidence.


Old Smokey

Alex Whittingham
4th Feb 2005, 07:54
My understanding is different. The principle of DME operation is that the aircraft sends pulses to the DME, the DME replies, and the time taken is used to determine slant range. The time delays are small, one thousandth of a second of timing error would produce a range error of 80 odd NM.

The process recognises that retransmission from the DME ground station would not be instantaneous, and even a small delay while the wiggly amps were working down the wires would cause a predictable over read of range. To allow for this the DME ground station holds onto the signal before retransmitting for a fixed period of 50 microseconds, this delay being taken account of in the range calculation back at the aircraft so that a true range is indicated. To make a DME under read by half a runway length, a bit less than a NM, the DME only has to reduce the fixed time delay by 10 microseconds or so. When the aircraft unwittingly compensates for the standard delay you get an under read.

The ideal used to be to position the DME between two thresholds so that it was zeroed at both ends. When this is not possible it is zeroed to the main instrument runway and the approach plate on the opposite runway reads something like 'DME reads 0.2NM at the threshold'. I understand some modern equipment has the capability to change the timing correction on the DME as the ILS is changed from one runway to another.

I also understand that DMEs associated with an ILS can only be used in the localiser coverage area and up to 25,000ft. If the FMS is programmed only to use the DME in the coverage area, that is to say on the approach side, that would explain a 'virtual position' at the threshold.

I can't reference this, its accumulated from snippets of data. I would be interested if anyone can confirm or deny?

5th Feb 2005, 00:51
Hi again.

Thank you for replies, meanwhile, sorry for my English if difficult to understand.

Is your understanding different than mine or Old Smokey?
What I briefly understand, you say: The geographical position of the DME station is not necessarily to be at or near threshold. In the case where DME station is far away from threshold, if it is desired that the airplane to under read the DME as "zero" when it is approaching to threshold, 50 microsecond fixed time delay is reduced by 10 microsecond.

Doesn't the latter mean that the exact position of the DME station and the position where the FMC think the station is, may be different?

Old Smokey,
I have come across with term "electronically displaced" during my Innsbruck operation. Maybe it is no so appropriate but it meant exactly what Alex has explained. So what would be your comments about my above question? Wouldn't it induce a wrong position updating? As respecting your vast experience, a final question: For what I can basically use the DME inhibit function in the FMC.

Thanks in advance.

Old Smokey
5th Feb 2005, 05:31

I'm certainly glad that I included the phrase "No DME that I know of has the means to "electronically" reposition itself" in my post, Alex Whittingham's remarks that "I understand some modern equipment has the capability to change the timing correction on the DME as the ILS is changed from one runway to another" come as a complete surprise to me. I'm not sure if Alex is referring to the ground equipment or the aircraft equipment.

My best response, in view of the doubts that Alex has cast, is that this DME correction would PROBABLY arise only in ILS mode, and not affect the "pure" DME mode if used in isolation to ILS mode, that is, for a DME-DME update. Reference to the FMC Navaid data-base certainly indicates differing locations for the Localiser and the DME, you can bet your last Euro that I'll be looking at the Nav Data-base for Innsbruk next time I fly.

With respect to your last question, I would not inhibit a DME unless shown as unreliable in a NOTAM. VOR inhibit is another story.

JABBARA, my experience is not all that vast, but sufficient to know when to stand correction. Don't apologise for your English, it's better than mine - I was edjercated in Awstralya, diddun cha know?.


Old Smokey

5th Feb 2005, 09:56
My understanding of DMEs associated with an ILS is the same as Alex. These DMEs will read zero at the glide slope origin although the transmitter, which can serve both runway ends, may not be physically at the glide slope origin.
I also understand that this type of DME is encoded in FMS databases in a manner that could enable a DME/DME updating system to choose to automatically ignore them.

5th Feb 2005, 10:45
This information can be found to much detail in ICAO Annex 10.

DME transmissions are sent in pairs of two pulses, in order to make it possible to use DMEs with the same frequency closer together, as well as to minimize errors from noise or other aircraft DMEs. The time difference between pulses vary with different DME stations, see in Annex 10.

Before the DME ground station (the transponder) replies, it pauses either 50, 56 or 62 microseconds, depending on the type of DME (the aircraft receiver will "know" this, you can see the letter next to the channel indicating either X, Y, W or Z). This delay can be adjusted to calibrate the DME, as well as adjust the distance readout for ILS or MLS.

Do not confuse the transmission delay with the delay between the pulses in the pulse pair.

Since it takes 12.36 microseconds for radio waves to travel one nautical mile and back (a "radar mile"), the 50 to 56 microseconds could cater for adjustments of several nautical miles, if desirable. If the aircraft is closer than this distance, I suppose the DME would read zero, but I have never tried it nor found anything about it. Usually the corrections are only a few tenths of a nautical mile, to make the DME read zero at the threshold. If the adjustment is more than a certain value it should be published for the DME.

How much does a few tenths of a mile matter to updating INS/IRS? Even if on a RNP1 airway, it shouldn't be a problem.

From Annex 10, page 150 or so:

7.1.6 Siting of DME associated with ILS or MLS The DME should, where possible, provide to the pilot an indicated zero range at touchdown in order to satisfy current operational requirements. The optimum site for a DME transponder is dependent upon a number of technical and operational factors. DME/N may be installed with ILS or MLS where operational requirements permit. DME/P, which provides higher accuracy and coverage throughout the entire runway region, is required to support the more flexible and advanced operations that are available with MLS. In the case of DME/N, the provision of zero range indication may be achieved by siting the transponder as close as possible to the point at which zero range indication is required. Alternatively, the transponder time delay can be adjusted to permit aircraft interrogators to indicate zero range at a specified distance from the DME antenna. When the indicated DME zero range has a reference other than the DME antenna, consideration should be given to publishing this information.

Old Smokey
5th Feb 2005, 16:21
And there you go, if you wait long enough someone will come through with the correct answer, well done LGB.

LGB's last highlighted sentence says it all - "When the indicated DME zero range has a reference other than the DME antenna, consideration should be given to publishing this information. That is, unless NOTAMed or specified otherwise in publications, there should be no problem with using a NORMAL ILS/DME for DME-DME updates.

We live and learn, I sure did.

Old Smokey

7th Feb 2005, 08:13
So JABBARA asked a very good question.

If the time delay is modified to electronically displace the zero reading, actually this will generate a circle of zero reading, around the antenna.
And that's the point.

Of course, when the airplane is aligned, or almost aligned with the ILS, that's not a problem, but if approacing from a side, like in a long base turn, let's say, the airplane will think he's closer to the runway than actually is...

Isn't it why sometimes the map display is shifted during the approach?
Isn't it why, on FMCs without GPS, it is forbidden (or should be forbidden) to intercept the Localizer on LNAV?

The airplane could just fly a parallel virtual LOC and never really capture the real thing...

Anyway, I will never bother to inhibit some DMEs before the approach:bored:
Just select Heading mode for localizer interception.

7th Feb 2005, 08:50
Whether approaching from the side or front course, the position will have the same magnitude of error, although with different directions.

Does the FMS update take into consideration that DMEs are slant range and not true ground distance? If so, does it measure true altitude, comparing it to the DME station elevation? The higher your level and the closer you are to the station, the greater the error. If you are 6000 feet above the DME station, the slant range error will be as much as a nautical mile when you pass over it. In this sense, a DME displaced by a few tenths of a mile should not make such a great difference?

And as you write, LEM, you are not using LNAV to fly the entire approach anyway, and even if you were, the INS also updates from the localizer, not just the DME. Since the FMC knows of available DMEs, it might even disregard displaced ILS DMEs in the first place, or know what to correct for if they are displaced?

I found this is in a Boeing 757/767 manual (http://www.boeing.com/commercial/caft/reference/documents/RNP757767.pdf page 11, updating without GPS):

The displayed ANP (ACTUAL) is computed knowing the geometry relative to the navaids for radio updating and
time since last radio update for inertial only mode. There is conservatism in the displayed ANP for radio updating in
terms of cross track position error. The sources of the conservatism are as follows:

1. ANP does not accounting for baro altitude corrections in the terminal area for the DME slant to ground
range conversions.

2. Along track latency in the navaid data due the average effect of the airplane track versus bearing to navaid.

7th Feb 2005, 10:26
OK, to sum this up, the NAV database contains information to whether the DME is displaced or not.

It is called DME bias, and the format is like this (example from http://x-plane.org/home/robinp/AptNavFAQ.htm#nav.dat ):

34.987022 Latitude of nav-aid in decimal degrees
-106.620384 Longitude of nav-aid in decimal degrees.
5304 Elevation of nav-aid in feet above MSL.
247 Frequency of nav-aid. (Always an integer so VOR and localiser frequencies multiplied by 100).
50 Range of the nav-aid in nautical miles (first introduced in file version 740)
0.000 Used for several purposes, depending upon the type of nav-aid:

Indicates the slaved variation of a VOR/VORTAC in degrees (ie. how the VOR is aligned compared to true north). This value often differs slightly from the local magnetic variation. Eastern variations are positive, western ones are negative. In the above data, Albuquerque VORTAC has a slaved variation of 13.000 degrees East.

Indicates the heading of a localiser in true (not magnetic) degrees. Also required for glideslopes (when it is combined with the glideslope angle see below for details) and marker beacons.

Indicates the DME bias in nautical miles often used for DMEs associated with an ILS that are not located near the runway threshold. This bias must be subtracted from the calculated distance to the DME to give the desired cockpit reading. This is typically used for a DME located in the centre of an airport that serves multiple l ILS approaches, so that the DME will read zero at the threshold of a runway. This is a common set up in Europe.

ILT Identification of nav-aid (broadcast in Morse code in X-Plane). Note that these are not unique.
Isleta NDB Name of nav-aid.