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Inertial Navigation Old Timers/Experts

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Inertial Navigation Old Timers/Experts

Old 23rd Sep 2008, 22:22
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Inertial Navigation Old Timers/Experts

Hi all,

Just a quick question and i apologize if this has been asked in the past.

If the INS was not updated at the gate first flight of day, can the INS reset itself to be accurate after departure, in flight, with an FMS (no GPS on board)? If so, can it 'right' itself within 10, 20 mins after departure? In other words, if the INS can 'right' itself while underway, how long would you anticipate accuracy? And/or can INS be considered accurate if not reset in a stationary position?

In further words, if the pilots werent able to update their INS at the gate (whether too lazy or behind schedule) and the INS was off by 1,000-2,000-3,000 ft at the gate, can they rely on the INS to update itself in flight and/or even perform an update in flight to be accurate?

Please explain for either answer.

Thanks in advance...

Last edited by A320Slave; 23rd Sep 2008 at 22:45. Reason: clarity
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Old 23rd Sep 2008, 23:07
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The old INS and their accuracy

I flew for PanAm - started flying early 1969, until their end 1991.
And I flew with their early INS systems - on the 707s...
And later flew the 747s, also equipped with the INS.
xxx
Early INS systems were the "Litton LTN-51" or "Delco Carrousel C-IV"
We had a maximum of 9 waypoint memory... then had to enter subsequent waypoints.
Their accuracy degraded with time "in navigation", if not updated.
As an example, 1 NM error per hour of flight could be expected as average.
JFK to LHR - some 7 hours in "NAV" - INS generally would drift 7 NM off...
xxx
We sometimes updated the INS over a known point, such as a VOR.
As an example, on the way from JFK to Europe, we would do it overhead Gander.
Then again maybe, overhead Shannon VOR, if our destination was still far.
Then later, INS were improved with "automatic update" operating with DMEs.
xxx
With the 747, we had 3 INS systems (only 2 with the 707 generally).
So we could "triple mix" the navigation between the 3 systems.
It averaged the "inherent error" of the 3 systems and resulted in more NAV accuracy.
xxx
I still fly the 747 - We have the Litton LTN-92s now.
Our 3 INS are extremely accurate now, in "triple mix" updated by 2 GPS receivers.

Happy contrails
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Old 23rd Sep 2008, 23:20
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Thanks for your reply BelArgUSA,

So if i have your above reply correct, INS can update in flight, even if the INS was inaccurate at the gate and never updated until in flight? And this is done by DME? Mainly due to the fact DME is extremely accurate, at least more accurate than INS... (from my experience with DME anyway...)

??

Thanks again for your reply...
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Old 23rd Sep 2008, 23:50
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INS "position update"

You can do a "manual update" - i.e. overflying a known point (a VOR)...
That was the first form of update that existed on early models.
xxx
A couple of years later, started to have capability of tuning a DME and be updated.
So in the mid-1970s until mid-1990s, no DME = NO update (over the ocean).
And since some 15 years ago, GPS is now used, instead of DME update. Works everywhere.
xxx
Hope that answers your questions...

Happy contrails

P.S.
Warning - DME updates - i.e. France.
Numerous DME not co-located with VOR (are not "VORTACs")
Be careful entering DME (TACAN) coordinates, not the associated VOR...
Danger of updating and entering some 5 or 10 NM error at some locations.
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Old 23rd Sep 2008, 23:56
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Much of the position updating and velocity correction ability of an INS is determined by the firmware/software in the system. That also determines whether a manual "position update" or re-alignment can be accomplished in flight. For example, the LTN-92s in current 747 Classics cannot be re-aligned in flight, and have no effective manual position updating capability.

The closer the Initial Position is to reality when the INS is aligned, the better it will navigate. While I don't know how far off the IP has to be to adversely affect one of these newer Ring [email protected] Gyro INS systems, some of the old spinning gyro types would start building stray velocities (nav error) if the IP was more than about 1/2 mile off.

AFAIK, the newer INS systems can be dynamically updated using GPS or VOR/ILS/DME, but the update applied by the radio nav instrument will "fade away" after the update signal is lost. Our LTN-92s will occasionally drift as much as 3 miles crossing the Atlantic or Pacific without updates (measured by the track error when updates are regained on the other side), even when there was "zero error" while updating at the start. Triple-Mix helps to smooth out these errors.
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Old 24th Sep 2008, 00:01
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Litton LTN72R's could be position updated in flight using DME/DME data.
Works good, lasts a long time.
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Old 24th Sep 2008, 00:21
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Just to add for 320 he may not understand that the on board navaids are auto tuning, ie seeking the nearest aids enroute for INS update
The priority used to be DME/DME, DME/VOR

I quit flying before GPS came along so guess thats one handy gadget now?
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Old 24th Sep 2008, 00:34
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kijangnim
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Greetings,

There is a big difference between updating a position , which is not a problem at all, and Re-Aligning an IRS in flight which is not possible .
 
Old 24th Sep 2008, 00:38
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Not so fast, the LaserRef V in the Challenger 605, among others, can be aligned in-flight. Haven't tried it and is supposed to take awhile.

GF
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Old 24th Sep 2008, 01:23
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Greetings
Galaxy Flyer, the subject is OLD INS
 
Old 24th Sep 2008, 02:49
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The INS in the OLD A-6 could be re-aligned in flight, UNTIL a newer/better software package removed the capability. However, we didn't miss it too much, because it made the initial alignment MUCH more reliable...
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Old 24th Sep 2008, 03:05
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kijangnim:

Yes that's what I was going to say

The IRS/INS/ADIRU whatever you use does not update after alignment.(on the ground you can get a fast align ( manual or automatically on the 777 )to remove the GS errors ) It's the NAV SYSTEM that updates. In my case the on the 777 the ADIRU drifts and the FMC uses the best info to update itself and that is always the GPS. Unless on App then it's LOC-GPS or LOC-DME
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Old 24th Sep 2008, 05:23
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Old INS position updates

With the older INS (such as Delco Carousel C-IVA) - 1980s
Using triple mix and DME update...
On typical NAT track between YQX and SNN, was nearly 4 hrs without DME update.
About the same time between NHA (Noronha) and SAL (Cape Verde) DMEs
From US West Coast to HNL was somewhat closer to 5 hrs.
xxx
It took the GPS update to make these old INS compliant with RNP-5

Happy contrails
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Old 24th Sep 2008, 05:26
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This is a technology that should never be allowed to fade away. One day...or maybe night, there will be a solar flare that destroys all the satellites within a line of sight. We need that back up science, especially now that it can be contained in a small device.

I used to feel more confident about inputting an update slap-bang over a VOR, cos the vibration and basic acceleration during take-off made me wonder if the system would be affected.
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Old 24th Sep 2008, 05:42
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I didn't think the ADIRU in a 777 was going to "fade away" as you put it. All the GPS systems could shut down and we still could find our way around. except without RNAV arrivals and departures, it would be like it was around 7 to 10 years ago.
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Old 24th Sep 2008, 05:49
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When all else will fail...

An old E6-B stays aligned for quite long...
If in metal, can even put it for cleaning in a dishwasher.

Happy contrails
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Old 24th Sep 2008, 14:34
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As kijangnim has pointed out, the 'experts' are tending to confuse INS/IRS update with position update. Two separate issues. Probably confusing A320Slave too!

Most 'old' systems were indeed capable of position update. The alignment for a platform requires steady conditions over several minutes which can rarely be achieved in flight. Normally the best 're-alignment' in flight gives a fairly crude attitude reference. Often it requires a repeat at intervals.

I do not see any reference to 'OLD' systems in the post (just 'no GPS'), so for A320Slave - it is by no means unusual for an 'oldish' (no GPS) IRS/INS to be aligned at start of day and left untouched all day for several sectors. No loss of navigational (or indeed attitude accuracy) will normally occur in a 'rich' environment where the position update is easily obtained and kept, and most systems can update position at take-off on a defined runway as well. Even with the 'errors' IGH mentions, the resulting accuracy of navigation is still streets ahead of the old star/sunshot/Doppler drift/LORAN/piece of seaweed systems. The only time when it is really important to align accurately is before a longish flight in a region where updates are not available - eg ocean crossings. Of course, now-a-days the REQUIREMENTS for navigation accuracy have tightened, so frequent re-alignments are more advisable.

The really 'old' INS on the early Harriers was position updated by 'pickling' over a map point, but once the platform had lost its alignment and had rushed off into 'enemy territory' at 200kts, there was little point, so we just turned the map off and got lost manually.

Last edited by BOAC; 24th Sep 2008 at 14:45.
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Old 24th Sep 2008, 15:29
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If the INS was not updated at the gate first flight of day
Must go back to the original question. For the first flight of the day you must turn the INS on. It would never be left on on an unattended aircraft. If the ground power failed then the INS would so go pear shaped with no cooling, and would be running on the aircraft (or its own) battery.
When you turn on an INS you must enter the correct coordinates or the ALIGN light will start flashing at you.

But I bet most short haul pilots never fly on the INS. There is even a Caution message on the A320..INS NAV.. to tell you that you have lost DME updates.

Also on an A320, there is no INS. There are three IRS sensors that supply position to the FMS, along with the 2 GPS and the 2 DME and 2 VOR. The FMS decides which sensors to use and starts with DME/DME.
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Old 24th Sep 2008, 16:07
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Hey............cough cough...............read post 12 again.


The 777 starts with GPS, never seen it use DME DME in flight EVER.

Only on approach it may use LOC DME or LOC GPS
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Old 24th Sep 2008, 17:20
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Greetings,

FMS selects the update source from the most precise to the least, i.e., and in order please, ILS, GPS, DME DME, DME VOR (VOR VOR is a NO NO situation) then IRS.
On the Smiths FMS world they use Kalman Filtering (So does Litton on the Flag Ship AIME) this enables an error modeling from which the position will be corrected
IRS Alignement on the ground it is highly recommended (even for a fast align) NOT TO MOVE THE AIRPLANE

An error in position input during initial alignment, a part from obvious bias, will give the wrong "G" initial value and induce wrong vertical acceleration (delta between the sensed real one and the erroneous one) throughout the flight dixit Korean Air B747
 

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