INS for Dummies

[airgator]

All of my instrument books either don't mention INS or have a paragraph on it that doesn't explain much. How exactly does INS work? For example, when flying LA to Hong Kong, is INS your primary means of navigating. When I had a reserve interview and was getting showed around the C-5 sim, I was told about the "triple INS", etc. What does that mean?

 

[Gulfstream 200]

No, everyone uses GPS these days.

might have meant "IRS" instead if "INS". most newer equipment may have a Laserref "IRS" (basically bounces lasers off mirrors to reduce error in "gyroscope") instead of old heavy bulky IRS's that were around before you or I were born.

But Im just a pilot, I dont really know how most of the stuff works.....it just does.

 

[Andy Neill]

INS uses the technology of inertial sensors to determine where the aircraft is in the world. With a known starting point, known acceleration inputs from a multiple axis processor, and a clock, the set will use the formula

d = (at^2)/2

to determine where it is now.

 

[bigsky]

I dont know if they have had any updates but I would guess the c-5 probably had the old ins. I am pretty sure that is what is in the 141's or at least was. As descibed above, it use laser gyros and accelerometers and has a data base. It can be updated via land navaids but when triple mixed it is pretty accurate- I would guess you lose maybe 4 or 5 miles crossing the pond but it does a pretty good job. We had several 10s that still had that and even the ones that have gps they were going to put one INS onboard as the gps has a tendency to be very unreliable in several areas of the world( mainly middle east and that could have something to dowith the military) but I have had the gps lose all signals for 5 minutes or more at a time

 

[Slug]

Triple INS

INS is a term that the military uses instead of IRS in the civilian world. Basically they are the same thing that was described above.

The triple INS system is used by the C-5 (at least when I left). It is 3 INSs installed on the aircraft. The 3 INSs "talk" to each other to refine the assumed position. If an INS begins to drift due to internal errors the other two will "drag" it back into alignment. You can kind of think of the triple mixed position as the average of the three lat and long positions that each INS head derives for itself. While flying the C-5 I seldom saw more than a couple of miles error over an 8 hour flight.

On the other hand the C-141 had only 2 INS heads that did not talk to each other. Therefore if one of the INSs started drifting the aircrew had to figure out which INS was bad (hard to do if you've got no other navaids) and then disregard the bad INS.

Hope this helps


Slug

 

[Draginass]

An INS is a self contained navigation system in a box. It measures acceleration and vector and has a computer to direct the pilot along a pre-loaded flight path. The INS is a relatively old technology that is very expensive to maintain. The reason for triple INS' was that if you only have one and it started to drift you don't know it. If you have 2 INS' and one starts to drift, you won't know WHICH one. With three, you take the two that are closest together as the correct ones. Triple INS' have algorithms to update and "triple mix" the position to come up with the most likely accurate position.

An IRU is an Inertial Reference UNIT. They are typically much more reliable and simple than INS', but also use three for the same reason as the triple INS setup. It also measures acceleration and vector, but only keeps track of the present position, velocity, vector, and attitude of the aircraft. It does not itself have a self-contained navigation computer. In aircraft that use IRU's, there is another computer called the FMS, or Flight Management System. This is the navigation system similar to that integral to the INS box. A FMS also does many more things than an INS, such as fuel computation/predictions, temperature displays, etc. The IRUs simply provide the constantly updated position, speed, and vector of the aircraft to the FMS', which then do all the number crunching to navigate the aircraft.

Since both INS' and IRU's do "drift" slightly, various forms of updates can be provided to bring the system accuracy back. INS' typically use updates from DMEs. IRU's also use DME updates but many use GPS now also.

Hope that's clear.

 

[metrodriver]

INS=inertial navigation system. You program the unit with your coordinates at the gate and during flight there are usually 3 gyro's who's precession is messured to calculate your course. Why 3? The unit can compare the inputs from the gyro's and detect the one that's not so accurate. You cannot do that with 2. On very long flights the accuracy goes down, and that's why GPS is now the preferred method of figuring out where you're going

 

[airgator]

Very interesting. Thanks for the input guys.