I have to say, that since I mentioned this in another post here, I have been thinking it over. This is not something I have read in a navigation tutorial, nor does it involved complex geometery to do.I made this up, and I wonder if anyone can give me comments on whether it would work in practice, and if so, under what cirumstances would it fail to work.It's not very good for navigating from Point A to B, but it will tell you where you are (or were) at a fixed point in time, fairly accurately, say, within a sqr mile.http://www.campbell-multimedia.co.uk/temp/dme.gifIf you know you distance from 2 DMEs on the sectional chart, then you can visualise or pencil in a circle around each of the 2 (or 3) DMEs to reflect you distance from them.You will find that the circles over lap, with at least 1 contact point, but more than likely 2 contact points. To determine which of the 2 points you are at you need to draw an arrow for you heading at the time of reading and then note your speed to or from the DMEs. The diagram shows this better than it can be explained.Any comments? As I said, I just made this up, although I am almost certain it must have been used in the past or even today for navigation.

I seem to recall reading in our training notes that the correct intersection point is simply resolved by looking at basic IRS position. If the IRS position is closer to A than B, then it will choose A.The FMC is smart enough to know when not to use a particular DME pair (if it can't resolve its position by using these two particular DME's and IRS position). It will simply resort to VORDME or some other navigation solution.Cheers.Ian.

Yes, this is one of the methods that the 767 uses to locate itself in space (or I guess confirm itself in space), the others being (off the top of my head), gps, iru, two adfs, an adf and a vor, two vors, or one vor and a dme. Obviously when you have more than this avaliable, as you do over most land, then the best solution is two vor/dmes. I would be surprised if the 737 works in a different way.

Hmm. I have spent a lot of sim hours in Knok Medias Concorde, well worth a look if you like realism. Lot, read LOTS more hands on systems than the NG and it's FMC. The INS (Inertial Navigation System) that the original Concorde had, (though I am convinced later models had GPS fitted) was set to a gate position, using 3 seperate gyro and control units, which were intermixed for accuracy.After flying across the atlantic though the accuracy index is at the max deviation between the gyros and usually your course is out by a nm or two on each. The mixed position might be more accurate than any one INS position.To update it, you can...Enter a position in space you will overfly and when you overfly it, tell each INS to reset to that POS.Relevent to this discussion, you wouldn't normally do the above, it's too inaccurate and only deserved when the gyros are way out. What you do is tune a NAV radio to a DME emmitting NAV aid, VOR/DME and enter it's LON/LAT position into the INS and tell it to update. It uses the position and distance change against it's gyros to calibrate itself. You can get even better results using 2 DMEs, preferable one either side of you course, not on your trk.Usually Concorde would do this arriving at the US coast, at Newfoundland and Nantucket areas, or whenever available when over land to make certain the gyros didn't slip much. Having, as said, played with Conc a lot, it's So, so so easy to mess up the gyros and end up completely lost with the gyros reading differences of over 30 degrees N and W. LOL Without DME updating you can always correct using VORs when you near your destination, even if you are a few miles out of course.

"the others being (off the top of my head), gps, iru, two adfs, an adf and a vor, two vors, or one vor and a dme. "On the NG....GPS, yes.IRU, yes.Two ADF's, no.ADF isn't used for position updating either by itself or in combination with something else.Two VOR's, no.One VOR and a DME, yes.Two DME's, yes.Localizer deviation, yes.Various combinations GPS, IRU, VOR, DME and Loc can be used, but there are combinations which can't be used (E.g. VOR + Loc).VOR and LOC must be manually tuned to a suitable station before they can be used. DME is autotuned.More facts required here, IMHO, and less "off the top of your head" answers.... to prevent misinformation and confusion ;-)Cheers.Ian.