Update of INS

[Spodman]

A few questions, please feel free to answer any or all.

1. How close to a VOR/DME do you need to get for an automatic update?

2. I understand INS will update within the range of two DME within 200nm of each other subject to a restriction on the angle between them, is this right, or are there additional restrictions?

3. How about manual?

4. Does updating happen more or less continuously, if the conditions are right? (Other than immediately after departure after a ground alignment.)

5. Do you get an indication when the machine is updated?

6. Do you get some sort of indication of the time since your last update.

[BOAC]

Spodman, as far as I know, IRS cannot be updated once 'aligned' without a ground re-alignment. (I am some years behind modern technology, so please someone correct me if I'm wrong).

What IS updated is the navigation system position that the IRS feeds. Any IRS errors that are there after alignment remain for the whole flight (or worsen).

IE If you have a moving map display atached, that 'position' can be updated, if it is an FMC, that can be (constantly) kept updated, but once the update facility (visual, DME, GPS, whatever) is no longer available, the kit reverts to IRS position only navigation. This function CAN be 'trained' to learn whatever errors it has acquired, while updating is available.

[tired]

Spodman, as BOAC and BIK 116.8 say, I think you're confusing INS and FMS (or FMC).

It's many years since I've used INS and memory grows dim :( but I'm pretty sure that there was no automatic update facility. We could update it manually, but this involved inputting the co-ordinates of a known point and then hitting the "update" button as we passed overhead. In practice, this meant using the co-ords of a VOR and updating when we hit the cone of silence. Bearing in mind the size of the cone at FL350 this didn't produce a particularly accurate fix (within a mile or 2 at best), so unless the INS had gone walkabout in a big way (and ours very rarely did) it wasn't of too much use.

On the other hand, FMS is a computer which accepts inputs from many sources, including INS or IRS, VOR, DME and GPS. If in range of radio navaids it will update itself automatically and continuously using DME/DME or, if 2 DMEs not available, then VOR/DME, or if no DME at all then VOR/VOR. There are restrictions regarding the angles between the aircraft and the naviads etc, but off hand I don't know what they are - if you think about it, I'm sure you'll see the logic. If out of range of radio navaids, then GPS, if installed, is the primary source of updating. There is no facility (on the one I use anyway) to update manually, but with all the above there's no need to. There's also no indication of when the last update was, since it's almost continuous anyway, but there is an estimate of the accuracy of the current position. The only time we see the nav accuracy wander away from "extremely blimmin' accurate" :) is if we are out of range radio navaids. (ie over the ocean, or somewhere like Siberia/Northern Canada) and there is no GPS installed (or it's U/S).

Hope the above is of use (and that it's accurate!! :) )

[Spodman]

Thanks for the info, more complicated than I thought....

Could I perhaps just tune in on one point and ask what proportion of INS/IRS flights out there would rely only on manual updating?

[tired]

Sorry for the delayed reply, I've been out earning my living!

To answer the question, almost none - .ooooo1% :)

I can't remember ever being forced to do a manual update in anger, and only ever did a couple just "for the hell of it". (And with the destination more or less in sight, in case we stuffed it up!! ;) )


A further point that might help you get your head around it - as BIK 116.8 says, an INS consists of 2 parts, the inertial reference platform and the nav. computer. The reference platform cannot be updated in flight - the aircraft needs to be perfectly stationary for the platform to align. Once it is aligned, it's accelerometers and gyros detect rate of change of movement (ie accleration) and the direction of this change. This info is then fed to the computer part of the INS, which integrates it with time to produce the distance the aircraft has moved in each direction. When you initially aligned the INS you gave it it's position in lat/long, so the computer knew where it was before it started to move - using the distance and direction moved the computer can keep track of where it has moved to.

So, when you update the INS you are actually only updating the nav computer part of the system - you are merely telling it that instead of being where it thinks it is, it is actually soemwhere else :) It then carries on using the outputs from the inertial platform to update this new position.

Don't forget that an IRS is merely an inertial platform, it does not contain a nav. computer, which is why you can never update an IRS postion, you can only align it when stationary.

Hope that all makes sense - my body is at 0 west, but my brain is somewhere else!!

[Spodman]

Thanks again tired.

[woderick]

Some A/C INS systems, the Litton ones spring to mind, have an installed Nav database and automatically update the Nav Computer part of the system via DME/DME. I think they have to have input from two separate, valid, ground stations less than 200 miles range to do this and obviously need a nav database to locate beacons. Certainly I am currently maintaining a DC10 that the crews constantly complain is not updating, and have had this in the dim and distant with a 727, both Litton INS equipped.

[411A]

The radio update feature of the Litton LTN-72R has never been all that swift, even in the best of times. Manual update works better.