Topic Author
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INS Alignment

Sat Nov 02, 2002 1:49 am

As those of us that work with INS systems know, during the INS alignment period, the aircraft cannot be moved. In most systems, the INS will warn you with a CDU message such as "excessive motion" if the alignment process has been stopped due to motion of the platform. Obviously, the system cannot make an accurate measurement of the earth's rotation if it is being moved about. So consider this: How do aircraft aboard naval vessels align their INS systems while the ship is underway? Regards,
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RE: INS Alignment

Sat Nov 02, 2002 2:04 am

OK, this is bit of a guess, combined with something I might have read once. (Sounds good so far,eh?) I think the INS is warmed up, and when they are ready to go, the position information is downloaded from the ships nav system.
This could be totally wrong, because I've never worked with INS.
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RE: INS Alignment

Sat Nov 02, 2002 2:06 am

That is a very good question, do you know for a fact that they do align their systems whilst in motion? Because I can't see how that would work at all.

Are they not all on GPS these days too?
I used to love the smell of Jet-A in the morning...
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RE: INS Alignment

Sat Nov 02, 2002 2:51 am

Perhaps the oldfashioned way? Figure out your current latitude and which direction is north, then feed that into the system. GPS updates will take care of the former, a flux rate gyro, variation database and GPS fix gives you the latter.

Ships are on INS these days? Hmm... the drift must be quite a factor. Oh well, not if they keep updating from GPS - and why shouldn't they? After all, ships don't require the degree of independence an aircraft does - they can always stop and figure things out!

I thought I was doing good trying to avoid those airport hotels... and look at me now.

RE: INS Alignment

Sat Nov 02, 2002 6:09 am

Got the courtesy of visiting a containership some years ago - yes they had INS - made by Litton - I think they had 4 or 5 of them, all were updated by several GPS - pretty much same idea in our planes... The units resembled aircraft equipment...
I assume they align their INS before moving in harbor, then they stay "ON" and get updated constantly for their typical 10 days or 2 weeks-long trips...
On our 747, we have 3 LTN-92s, which are used in "triple mix" in cruise, and there are 2 independent GPS updating the INS 1 and 2, and the 3rd INS gets updated since it is mixed with 1 and 2... and we still can do DME update in case GPS would fail... but in that case we are no longer good for RNP-5...
These ships have everything (airplane pilot's dream) - should see their radio equipment...
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RE: INS Alignment

Sat Nov 02, 2002 7:14 am


I was smart enough to buy a DVD from ITVV, which gave an insight into the (nowgone) Virgin Atlantic 747 classics. Looks quite spectacular to me!  Smile
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RE: INS Alignment

Sat Nov 02, 2002 1:53 pm

>So consider this: How do aircraft aboard naval vessels align their INS
>systems while the ship is underway?

Normally done by performing a SINS alignment. The Ship's Inertial Navigation System (SINS) sends a signal (by RF or a long cable) to the aircraft's INS to sync the aircraft's platform with that of the SINS platform (hidden in the bowels of the ship very near to the ship's center of moment. This provides "Rough Alignment" --platform level, Lat/Long position and the true north pointer. 20 years ago this took an average 3-4 minutes. One can operate with a Rough Alignment as drift is supposedly <4nm/hr.

"Fine Alignment" begins as soon as Rough Alignment ends. Fine Alignment is when the aircraft's INS calculates the three moment arms that make up the physical difference in SINS and Aircraft positions on the ship. Again, 20 years ago Fine Alignment took an average 2-4 minutes depending upon aircraft position on the deck and sea state (more movement = quicker results). Fine Alignment provides a published drift of <2nm/hr --my experience was that a properly Fine Aligned system would perform at well under 1nm/hr drift.

If SINS is not available a Manual Alignment is performed. One manually inputs last known position, ship's magnetic course, deck spot (position on the flight deck), and spotting angle (aircraft heading relative to ship's heading). The aircraft INS will then attempt to align itself similar to a ground based alignment but biasing out ship movements. With no significant ship heading change(s) a Rough Alignment would take 10-15 minutes. Fine alignment was not possible using manual alignment procedures.

The above was how things worked 20 years ago using mechanical INS'. LRG systems and GPS have sped up the process considerably since then.

*NO CARRIER* -- A Naval Aviator's worst nightmare!
Topic Author
Posts: 176
Joined: Wed Nov 29, 2000 3:05 am

RE: INS Alignment

Tue Nov 05, 2002 10:13 pm

Everyone: AAR90 has it. The next time you see a picture of aircraft on an aircraft carrier flight deck, if you look closely you will see several cables coming up over the deck edge, and connecting to the aircraft. One of these is the cable that connects the aircraft's INS to the vessel's INS system. The ship tells the aircraft it's current position, and the aircraft INS system takes it from there. Thanks to AAR90 for a very good explanation. Regards,
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Joined: Fri Jan 21, 2000 11:51 am

RE: INS Alignment

Wed Nov 06, 2002 9:56 am

>One of these is the cable that connects the aircraft's INS to the vessel's INS system.

I am told yellow remains the color for SINS cables. All other electrical cables are black.

>Thanks to AAR90 for a very good explanation.

Glad an entire deployment of MIDRATS with the TechReps didn't go to waste.  Laugh out loud
*NO CARRIER* -- A Naval Aviator's worst nightmare!

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