DME/DME correction of INS & France?
Thread Starter
Join Date: Jun 2003
Location: EuroGA.org
Posts: 13,787
Likes: 0
Received 0 Likes
on
0 Posts
DME/DME correction of INS & France?
How does this work over say France, where most of the VORs don't have a DME?
I realise most ILS airports have a DME on the ILS but I would have thought those would be too short range to be useful for this - I can pick up TDMEs from only about 50nm max even when clearly high enough to be line of sight.
I realise most ILS airports have a DME on the ILS but I would have thought those would be too short range to be useful for this - I can pick up TDMEs from only about 50nm max even when clearly high enough to be line of sight.
Thread Starter
Join Date: Jun 2003
Location: EuroGA.org
Posts: 13,787
Likes: 0
Received 0 Likes
on
0 Posts
Interestingly, Concorde had no GPS and used single-DME corrections to the INS. This is what the crew told me after I went on one of the last flights.
I wonder how that worked. I can think of a few ways, all relying on solving simultaneous equations involving a previously computed GS and track.
I wonder how that worked. I can think of a few ways, all relying on solving simultaneous equations involving a previously computed GS and track.
a DME on the ILS
used single-DME corrections to the INS
Last edited by reynoldsno1; 20th May 2007 at 21:36. Reason: missed word
Join Date: Mar 2001
Location: I wouldn't know.
Posts: 4,498
Likes: 0
Received 0 Likes
on
0 Posts
Never had a problem with updating over France in our 300/500s. Actual navigation performance usually stays below 0.3NM unless we switch the NAVs to manual for the approach setup.
Join Date: Mar 1999
Location: UK
Posts: 367
Likes: 0
Received 0 Likes
on
0 Posts
In my innocence I've always beleived that DME paired with an ILS had an offset introduced to cater for the difference between the beacon and the runway threshold. Therefore 0 dme wasn't at the beacon, but rather a circle around it.
Thread Starter
Join Date: Jun 2003
Location: EuroGA.org
Posts: 13,787
Likes: 0
Received 0 Likes
on
0 Posts
I had some PMs from someone here, saying (basically) that the INS/IRS system is separate and its error is never adjusted all the time the a/c is airborne, and when over land the FMS derives its position from navaids but not from the IRS.
Is this true?
If it was, how could a jet transport meet the BRNAV requirements for example? Quite a bit of the time in Europe, one is out of the DOC of any navaid (maybe not at FL350 but definitely lower down). I asked this chap this Q but he didn't get back to me.
Surely the FMS *always* gets its position from the IRS, and the IRS error is corrected by navaids (DME/DME usually) when these are available.
So if e.g. you have crossed the Atlantic and have picked up a 2nm track error on the IRS, then pick up a few DMEs, that will fix up the error and if then you lose all navaid contact for a bit, you will still be flying along with (essentially) zero error. Your nav will *not* revert to the previous 2nm error.
Is this right?
(ignore GPS for now)
Is this true?
If it was, how could a jet transport meet the BRNAV requirements for example? Quite a bit of the time in Europe, one is out of the DOC of any navaid (maybe not at FL350 but definitely lower down). I asked this chap this Q but he didn't get back to me.
Surely the FMS *always* gets its position from the IRS, and the IRS error is corrected by navaids (DME/DME usually) when these are available.
So if e.g. you have crossed the Atlantic and have picked up a 2nm track error on the IRS, then pick up a few DMEs, that will fix up the error and if then you lose all navaid contact for a bit, you will still be flying along with (essentially) zero error. Your nav will *not* revert to the previous 2nm error.
Is this right?
(ignore GPS for now)
Per Ardua ad Astraeus
Join Date: Mar 2000
Location: UK
Posts: 18,579
Likes: 0
Received 0 Likes
on
0 Posts
Possibly a little confusion there. The FMS does indeed compute its postion from navaid updates whilst they are available. When no longer available, it reverts at a controlled rate to INS derived position which can, as you say, be 'significantly' in error. Hence the normal restriction on use of RNAV for approaches in such systems. What the PMs may have been telling you is that no update from navaids has ANY effect on the position that the INS thinks it is at. That is set at initial alignment and adjusted for manouevre thereafter by the acceleration derived inputs.
In 'mainland' Europe it is extremely unusual to be out of some sort of update coverage.
In 'mainland' Europe it is extremely unusual to be out of some sort of update coverage.
Join Date: Jun 2005
Location: London
Posts: 390
Likes: 0
Received 0 Likes
on
0 Posts
Just to finish the confusion clearing job from BOAC, if you also have integrated GPS (e.g. NGs), the FMC will:
1) Determine its position based on GPS (which means you don't have to enter a take-off shift for intersection departurers, like you do on the Classic)
2) If GPS not available, FMC will determine its position from both navaids and IRS (left), with a bias towards navaids (I think it's 80% bias towards NAVAIDS, but I'd have to check).
3) If no GPS or Navaids available, the FMC will come up with a "most probable" position based on what the IRS units are saying at that time. It determines the error margin based on the IRS position error it calculated when it last received a navaid update.
Again, this is for 737 NG. And again, no the IRS boxes never update their position.
P
1) Determine its position based on GPS (which means you don't have to enter a take-off shift for intersection departurers, like you do on the Classic)
2) If GPS not available, FMC will determine its position from both navaids and IRS (left), with a bias towards navaids (I think it's 80% bias towards NAVAIDS, but I'd have to check).
3) If no GPS or Navaids available, the FMC will come up with a "most probable" position based on what the IRS units are saying at that time. It determines the error margin based on the IRS position error it calculated when it last received a navaid update.
Again, this is for 737 NG. And again, no the IRS boxes never update their position.
P
Wunderbra
Join Date: Aug 2006
Location: Bedford, UK
Age: 44
Posts: 313
Likes: 0
Received 0 Likes
on
0 Posts
The DME works by a transmitter on the aircraft interrogating a ground station which then sends a reply. I would assume the reason your range seems to be limited is that the DME ground station is at capacity, so it is incapable of dealing with your request. They have a fairly limited capacity, and in busy airspace at an airfield this might well be reached fairly quickly.
Thread Starter
Join Date: Jun 2003
Location: EuroGA.org
Posts: 13,787
Likes: 0
Received 0 Likes
on
0 Posts
When no longer available, it reverts at a controlled rate to INS derived position which can, as you say, be 'significantly' in error.
Why does it work that way? Why not simply fix up the IRS when you have a verified DME/DME fix? Maybe there is a subtle safety reason why the IRS position is never modified.
Referring to the "controlled rate" above, let's say you have an IRS that is 5nm out, and you have been in navaid contact, then lose the contact. Your position will gradually drift to say 4nm off track. Then you get within range again, and your track error is again reduced. Does this actually happen, and if so doesn't it result in your track being a bit "wiggly"?
Why does it work that way? Why not simply fix up the IRS when you have a verified DME/DME fix? Maybe there is a subtle safety reason why the IRS position is never modified.
Referring to the "controlled rate" above, let's say you have an IRS that is 5nm out, and you have been in navaid contact, then lose the contact. Your position will gradually drift to say 4nm off track. Then you get within range again, and your track error is again reduced. Does this actually happen, and if so doesn't it result in your track being a bit "wiggly"?
Join Date: Feb 2002
Location: in a galaxy far far away
Posts: 82
Likes: 0
Received 0 Likes
on
0 Posts
IO540
It is impossible to update an IRS when moving. That is the whole point of IRS. You tell it its position before you start moving. It can then ony work out its position by knowing how far it is moving from that original point. If you are flying at 500kts at what point in time would you be able to say your exact position ? It would always be changing.
Just to clear up the confusion, on big boeing's 757/767/777/747 and almost certainly 737's, the FMC position is derived from radio aids and/or GPS. When these are lost (IRS NAV only) the FMC compares its last known position (with the aids) to the IRS position and then knowing the error, it maintains a parallel course with this error until it acquires more aids to fix on.
The FMC does not navigate using IRS it simply uses it as a reference.
It is impossible to update an IRS when moving. That is the whole point of IRS. You tell it its position before you start moving. It can then ony work out its position by knowing how far it is moving from that original point. If you are flying at 500kts at what point in time would you be able to say your exact position ? It would always be changing.
Just to clear up the confusion, on big boeing's 757/767/777/747 and almost certainly 737's, the FMC position is derived from radio aids and/or GPS. When these are lost (IRS NAV only) the FMC compares its last known position (with the aids) to the IRS position and then knowing the error, it maintains a parallel course with this error until it acquires more aids to fix on.
The FMC does not navigate using IRS it simply uses it as a reference.
Thread Starter
Join Date: Jun 2003
Location: EuroGA.org
Posts: 13,787
Likes: 0
Received 0 Likes
on
0 Posts
It is impossible to update an IRS when moving
That may be true in practice but is definitely incorrect technically. The system comprises of x/y/z gyros (laser ring nowadays) whose signals are integrated to give position in space. The lat/long is then calculated from that, noting the original (known) position it started from, and nothing prevents an offset to be added to that calculation.
The gyros themselves are obviously not messed with; they just happily carry on generating the pulses.
the FMC compares its last known position (with the aids) to the IRS position and then knowing the error, it maintains a parallel course with this error until it acquires more aids to fix on.
OK, that's exactly what I am saying above. You add an offset to the inertial-derived position, and carry on flying with that offset. That makes sense to me because it gives you seamless BRNAV capability no matter where you are.
I wish GA has this capability. Presently, if we lose GPS reception (very rare but possible) BRNAV goes down the pan immediately and we fall back to VOR/DME/vectors etc. A couple of years ago I was looking at a project to design an "IRS" for GA but the FOG gyros were still at least $5000 each and aren't likely to come down in the near future. They are also export controlled. A lot of other people have looked at this; the cost of the gyros is the issue. Everything else has been done many years ago.
That may be true in practice but is definitely incorrect technically. The system comprises of x/y/z gyros (laser ring nowadays) whose signals are integrated to give position in space. The lat/long is then calculated from that, noting the original (known) position it started from, and nothing prevents an offset to be added to that calculation.
The gyros themselves are obviously not messed with; they just happily carry on generating the pulses.
the FMC compares its last known position (with the aids) to the IRS position and then knowing the error, it maintains a parallel course with this error until it acquires more aids to fix on.
OK, that's exactly what I am saying above. You add an offset to the inertial-derived position, and carry on flying with that offset. That makes sense to me because it gives you seamless BRNAV capability no matter where you are.
I wish GA has this capability. Presently, if we lose GPS reception (very rare but possible) BRNAV goes down the pan immediately and we fall back to VOR/DME/vectors etc. A couple of years ago I was looking at a project to design an "IRS" for GA but the FOG gyros were still at least $5000 each and aren't likely to come down in the near future. They are also export controlled. A lot of other people have looked at this; the cost of the gyros is the issue. Everything else has been done many years ago.
It is impossible to update an IRS when moving
Per Ardua ad Astraeus
Join Date: Mar 2000
Location: UK
Posts: 18,579
Likes: 0
Received 0 Likes
on
0 Posts
The system comprises of x/y/z gyros (laser ring nowadays) whose signals are integrated to give position in space.
Align-in-motion is indeed techncally feasible now and produced, I believe, by Honeywell at least. Damned clever little black boxes! I cannot see it ever being applicable to the 'basic' DME/DME updated systems as on the Classic 737s.