View Full Version : gold star to the one who answers this

11th Apr 2003, 22:31
How do the IRS's or the INS's calculate heading information?

The crazy one.

oxford blue
11th Apr 2003, 23:15
Answering the question exactly as you have asked it:

They have high-grade gyros which are initially aligned to North and then they either continue pointing North (INS) or remember where North is (IRS), and then they measure the heading angle from it. It's as simple as that.

I suspect that what you mean is "How do they find out where North is when there's no magnetic compass input to an INS/IRS?"

In this case, the answer in that they have accelerometers which are sensitive enough to measure the spin of the Earth. The way that the Earth spins, no matter where you are (except the North or South Pole), the direction of Earth spin is due East. Ninety degrees left of this is North.

compressor stall
12th Apr 2003, 03:18
A ring laser gyro is an equilateral triangle and along each side it shines light from one corner to the other.

As the gyro accelerates in different directions, the wavelength of light appears to be stretched or compressed. The amount of this is measured by the aforementioned sensors, and computed into a position.

The compresssion of the wavelengths of light is known as doppler effect. Sitting in a train, the bells at a crossing are high pitched as you approach - the wavelengths are getting compressed from your viewpoint and appear to sound higher in pitch. Conversely when you are past the crossing, the pitch deepens.

12th Apr 2003, 03:33
(Hoping the Gold Star is a BSA motorcycle) my twopenceworth is to observe that once true north is 'found', one of the 3-axis platform gyros will measure angular acceleration in the yawing ('compass') plane, thereby giving the system a heading change.

12th Apr 2003, 16:58
Oxford Blue has the correct answer. He gets the Gold star and bragging rights.

Blue are you a pilot or engie?

12th Apr 2003, 22:05
All boils down to doppler shift, easy, next question!:8

13th Apr 2003, 00:35
Quick answer, PFM and it's all done with mirrors.

Dan Winterland
13th Apr 2003, 02:08
Not as simple as it first seems.

INs are divided into two categories. The earlier North aligned platforms are just as they sound. A platform which has the gyros and accelerometers mounted on it, finds true north during alignment (don't ask me how!) which can take up to 15 minutes and maintains this throughout it's operation. These platforms are not cleared for polar flight as the rate of heading change as you flew over of close to the pole is too much for the instrument to handle.

Later INs, the carousel for example, do nor align to true North. the carousel actually rotates slowly in an attempt to reduce it's wander error. Again, I don't know how it finds or maintains true North, but I seem to remember one aircraft I flew which had a Carousel fitted did have an input from the compass system..

IRSs use laser ring gyros. earlier ones (the Honeywells fitted to Boeings) employ accelerometers but later ones (such as Litton 92s) compute acceleration from the information derived from the lasers. Both compute true North during alignment, but the reference is an electronic one derived from the software.

INs were developed for the US space program. The INs fitted to the Apollo capsules used gyros to align, then when the capsule was on a steady tradgectory, the gyros were rundown to save power and the platform was then kept aligned optically by a device which tracked stars. I gather the attitude and heading reference it used was a arbitary one perpendicular to Houston. at a certain time of day.

I'm willing to be corrected on any of the above.

oxford blue
13th Apr 2003, 05:16

A former RAF navigator, now an FTO ground instructor and (when I can afford it) occasional pilot.

Dan Winterland:

I don't want to sound like a smartass, but you did say that you were willing to be corrected. I'll give you Miss Whiplash's phone number later. In the meantime, a few minor points:

The way it finds True North during alignment is just as I said in my previous post. It really is as simple as that - in principle at least. There are one or two practical points in the engineering, obviously, but basically, the idea is so simple, it's unbelievable. I just don't know why most textbooks make it sound so complicated.

Some non-rotating classical INS are cleared for polar flight. It depends on how the software is programmed. If the meridian convergency is dealt with digitally, you can fly over the pole.

All IRSs have accelerometers, to the best of my knowledge. All you can get out of a laser gyro is measurement of angular roatation. With 3 of them, you derive angular rotation from the initial datum in 3 axes. However, this is not acceleration. You still need accelerometers.

As for your final paragraph, not a correction, but an additional tit-bit of information. Astro-trackers were also used on terrestrial navigation systems. The B-58 Hustler's gyro heading system was aligned to a star. The photo-cell was so sensitive that it could actually continue to track the star in daylight, when it was well below the visibility threshold of the naked eye. The original idea was that the system should take a continuous astro position from the heavenly body ( Dammit, there's Miss Whiplash again) but this didn't work very well. However, using the heavenly body as a heading source did prove to be good and enhanced the accuracy available from the gyros of the day.

14th Apr 2003, 00:20
re. the need for accelerometers:

I would have thought it possible to do a double integration of the gyro change over time to derive acceleration?

Dan Winterland
14th Apr 2003, 23:19
Thanks Oxford Blue. And that telephone number?

15th Apr 2003, 08:39
Have a look at IRS = LONG. but how (http://www.pprune.org/forums/showthread.php?s=&threadid=9285&) !

17th Apr 2003, 17:34
For the polar navigation problem check this article from Boeing's Aeromagazine.


17th Apr 2003, 20:28
You donít NEED accelerometers to figure out latitude or true north.

The gyros provide a star-fixed reference system. This will appear to be spinning at about 15 deg/s relative to your aircraft due to the earths rotation. And, here is the good bit: The axis of rotation relative to your aircraft will be parallel to the rotational axis of the earth. Alas, it (or its projection in the horizontal plane, if you wish) points to true north.

The angular difference between this axis of relative rotation and its projection in the horizontal plane is your latitude (at least on a perfectly spherical planet, otherwise corrections might be required).

Yeah, that contained a bit of reiteration.


18th Apr 2003, 01:23
wow you guys are too smart for me. very deep stuff indeed.

what about this one...is the speed of sound the same at 1000 ft as it is at 40000 ft that is provided that it is -40 at both.(too easy i know) but it will hold you over for a bit while i think of something else.