View Full Version : Fiber Optic Gyros

11th Feb 2019, 05:34
First I have heard of it but, it looks like that could be the wave of the future for some airplanes.

According to an article I am reading about a new Embraer business jet, has Northrop Grumman LITEF LCR-100 fiber optic gyros that are private labeled as Rockwell-Collins AHS-40000 units(not sure what the privaye labeling means).

These units provide virtually all the inertial position, attitude Heading/Reference, and velocity outputs of top end laser IRS boxes., but at a much lower cost and weight. The 6 lb. units have a lower than 12 nm/hour drift rate, require no forced air cooling, and have a 15,000 hour MTBF.

The article does mention that there is an optional Honeywell LaserRef box that improves navigation performance in remote and oceanic regions by eliminating the need for GPS RAIM checks for dispatch and by prolonging the period during which the aircraft can maintain navigation performance without GPS or VOR/DME updates.

So perhaps the fiber optic gyro units could become more common on short/medium haul aircraft. The CRJ 1000 has approval for this.

From Wikipedia....

"A fibre optic gyroscope (FOG) senses changes in orientation using the Sagnac effect (https://en.wikipedia.org/wiki/Sagnac_effect), thus performing the function of a mechanical gyroscope (https://en.wikipedia.org/wiki/Gyroscope). However its principle of operation is instead based on the interference (https://en.wikipedia.org/wiki/Interference_(wave_propagation)) of light which has passed through a coil of optical fibre (https://en.wikipedia.org/wiki/Optical_fiber), which can be as long as 5 km. Two beams from a laser are injected into the same fibre but in opposite directions. Due to the Sagnac effect (https://en.wikipedia.org/wiki/Sagnac_effect), the beam travelling against the rotation experiences a slightly shorter path delay than the other beam. The resulting differential phase shift (https://en.wikipedia.org/wiki/Phase_(waves)) is measured through interferometry, thus translating one component of the angular velocity (https://en.wikipedia.org/wiki/Angular_velocity) into a shift of the interference pattern which is measured photometrically. (https://en.wikipedia.org/wiki/Photodiode)

From Rockwell-Collins...."The Collins Aerospace AHS-4000 Attitude and Heading Reference System (AHRS) features a distinctive solid-state design in a small, lightweight package. The embedded low-power fiber-optic gyro and triple-axis MEMS accelerometers ensure high reliability and low power consumption. Continuous monitoring of all functions and internal diagnostics gives you a high-integrity system. You can operate the AHS-4000 in several modes to provide your aircraft with the most accurate navigation data based on available inputs. After self-aligning, the system operates like an Inertial Reference System, providing free inertial navigation position and velocity outputs along with the basic AHRS functions. For higher precision calculations, the AHS-4000 can use GPS inputs."

The fiber optic may be very long but it is not straigh. It has thousands of turns(coiled) with the more turns, the better it is.

Any additional info would be welcome.

11th Feb 2019, 08:08
Standard on the Airbus A320 since 1987...

Station Zero
11th Feb 2019, 08:34
I believe the A320 ADIRU's (at least Honeywell one's) all are RLGs, not Fibre Optic Gyros.

Even more recent aircraft such as the 787 still utilised RLGs in its Earth Reference System components (IRU's and AHRU's).

Believe some if not all Integrated Standby Instruments (ISIS or ISFD) mostly use fibre optic gyros due to their relative size compared to RLGs and also if I remember right there are fibre optic gryos under the floor mid-cabin on the A340-600 for improving ride comfort.

11th Feb 2019, 22:12
Thanks Station zero...now that is an interesting reply. Anyone else.

FE Hoppy
12th Feb 2019, 16:32
Same principle as the Laseref from what used to be Honeywell and not much lighter. Must be cheaper. ISIS uses piezo gyros as far as I remember.

12th Feb 2019, 20:37
The 6 lb. units have a lower than 12 nm/hour drift rate
I think most people’s phones could do better than that!

12th Feb 2019, 20:46
Actually I read about them back in the 70's in an engineering journal, I thought that they were in use in INS many years ago!

Gauges and Dials
12th Feb 2019, 21:15
I think most people’s phones could do better than that!

Not without referencing signals from GPS satellites, they couldn't. The unit in question here does not depend upon any external signals

12th Feb 2019, 22:04
Not without referencing signals from GPS satellites, they couldn't. The unit in question here does not depend upon any external signals
Neither do the inertial units in modern phones/tablets. Yes, they use GPS, wi-fi, whatever to update/refine absolute position but the inertial bits don’t drift at up to 12kts in a random direction. I saw some research where they got a (fairly old) iPhone, took it for a stroll around for a while including buildings, stairs, cars, etc. and it had drifted by fractions of a metre (on purely inertial) by the time they got back to the starting position.

With the kind of specs quoted this device wouldn’t be useful even for RNP-10!

12th Feb 2019, 23:10

I go with FullWings on the accuracy. Sorry, Gauges, but I flew with INS systems 40 years ago that had plenty of mechanical components that needed bearing lube and constand calibrations and such. Think real gyros and tiny motors to keep the basic inertial platform "level" so the accelerometers could provide good movement rates for the nav computer. Our drift rate without GPS or other "real world" update data averaged less than 2 n.m. per hour. Figure less than 4 feet per second, and we could see this drift on our second bomb passes.

So 12 n.m per hour seems very high

I then flew a later jet (ten years later) that had less mechanical components and more sophisticated computer sfwe. It was even better than the orginal I learned on. No GPS updating or even a doppler to help keep the platform level. Drift rate was about 1 n.m. per hour with no GPS help.

The new stuff has an order of magnitude less moving parts and an order of magnitude more sophisticated sfwe and navigation algorithms than I grew up with.

So my bottom line is I am surprised a company is bragging about 12 n.m. per hour nav accuracy..

Gums sends...

12th Feb 2019, 23:39
We used to do quick align on every turn-around, most probably a hang up from the old 737-200. What do I know.
But some 5 years ago we quit doing it.
As I was flying out of the Canary Island on trips from 4 to 6 hrs I started looking at the drift of the two basic IRS positions as we turned the aircraft around and got more coffee.
Most times they were out with one to two miles after 6 hrs, seldom in opposite directions. So we are talking a drift on the 737-800 of less then 1/2 a mile normally. The odd time it was more then the 3 miles and a FMC warning would trigger on the ground.
If I am not mistaken the IRS position auto updates on TOGA selection on T/o on the 738.

12 nm/hrs drift sounds like a real bender on a Saturday after payday!?
Happy Navigating

13th Feb 2019, 08:24
Just curious - what are the advantages/disadvantages of a fiber optic gyro as compared to a ring laser gyro? The RLG have an impressively good history of both reliability and accuracy - so what's the carrot with the fiber optic? Hard to imagine 5km of fiber optics being much cheaper than the relatively simple RLG.

13th Feb 2019, 08:50
what are the advantages/disadvantages of a fiber optic gyro as compared to a ring laser gyro?

From https://en.wikipedia.org/wiki/Sagnac_effect

"Fibre optic gyroscopes versus ring laser gyroscopes[edit (https://en.wikipedia.org/w/index.php?title=Sagnac_effect&action=edit&section=10)]
Fibre optic gyros (FOGs) and ring laser gyros (RLGs) both operate by monitoring the difference in propagation time between beams of light traveling in clockwise and counterclockwise directions about a closed optical path. They differ considerably in various cost, reliability, size, weight, power, and other performance characteristics that need to be considered when evaluating these distinct technologies for a particular application.

RLGs require accurate machining, use of precision mirrors, and assembly under clean room conditions. Their mechanical dithering assemblies add somewhat to their weight but not appreciably.[citation needed (https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)] RLGs are capable of logging in excess of 100,000 hours of operation in near-room temperature conditions.[citation needed (https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)] Their lasers have relatively high power requirements.[30] (https://en.wikipedia.org/wiki/Sagnac_effect#cite_note-JuangRadharamanan-30)

Interferometric FOGs are purely solid-state, require no mechanical dithering components, do not require precision machining, are not subject to lock-in, have a flexible geometry, and can be made very small. They use many standard components from the telecom industry. In addition, the major optical components of FOGs have proven performance in the telecom industry, with lifespans measured in decades.[31] (https://en.wikipedia.org/wiki/Sagnac_effect#cite_note-Napolitano-31) However, the assembly of multiple optical components into a precision gyro instrument is costly. Analog FOGs offer the lowest possible cost but are limited in performance; digital FOGs offer the wide dynamic ranges and accurate scale factor corrections required for stringent applications.[32] (https://en.wikipedia.org/wiki/Sagnac_effect#cite_note-32) Use of longer and larger coils increases sensitivity at the cost of greater sensitivity to temperature variations and vibrations."

13th Feb 2019, 09:30
About this 12nm/hr drift rate, I think it must be a misprint or research error.
The B747 classic with its old Carousel IV units had a limit of 3+3T nm error for the sector. T was sector time in hours. Almost invariably they were way under that and this was before DME updating or any other stuff like that - raw INS.

13th Feb 2019, 09:57
The system originally quoted in this thread is not meant as an IRS:

LCR-100 Gyrocompass AHRS:

So while it basically is a low accuracy IRS meant to be used with GPS.

Even the other unit they have is meant for planes that usually don't have an IRS because of cost and weight:

Its cost effective and low weight design makes the LCR-110 the ideal alternative for classical IRS/INS. This is a major step towards NextGen and SESAR operations since it allows the introduction of the benefits of inertial navigation into aircraft classes usually not equipped with those systems due to cost and weight.

And for planes that typically don't have IRS, 12nmi / hour drift rate doesn't seem that bad.
(As far as i understand these units are meant to support RNP procedures where a certain RNP is required in case of GPS failure, so long term drift isn't a priority)

13th Feb 2019, 14:38

Thanks for a better/shorter explanation about the FOG's, Golden.

And from Wiede,
And for planes that typically don't have IRS, 12nmi / hour drift rate doesn't seem that bad.

I think Amelia Earhart would take issue with that.

But seriously, Wiede, it sure looks like a very good AHRS system with tiny angular errors and such. The lack of moving parts other than photons and maybe "tuning fork" rate/gee sensors increases reliability as well as size/weight/etc. The "dedicated" inertial nav systems today are an order of magnitude more accurate than what I flew 30 years ago, even without GPS aiding. And they are actually cheaper when considering inflation and such, kinda like the flat-screen HDTV sets at Best Buy.

Gums sends...

14th Feb 2019, 05:21
Small diversion into the world of academic research... CALTECH have had a group looking at improving MEMS devices for inertial sensing. They developed an idea where the position of the reference mass in an accelerometer was measured and cooled in a laser trap. The basic idea was to remove as much thermal noise from the device as possible. From memory they'd got it good enough to give very good drift rates, something like a few meters in a half hour. [An application for it discussed in the paper I read was that your mobile phone would still be able to work out which aisle you were stood next to in a supermarket half an hour after you'd gone into the shop]. Their plan then was to go onto the same idea for MEMS gyros.

Here's an article about the work of that group:

And I've heard nothing substantial since. Either it didn't pan out, or it couldn't be productionised, or someone governmental got wind of it and worked out the consequences of mobile phones having INS that good. I've not been keeping an especial eye out, but I'd expect to have seen stuff in the wider tech press by now had actual devices been prototyped.

I think laser fibre gyros are generally not quite as good as ring cavity laser gyros due to the dispersion of a laser pulse in the fibre. I may be wrong, but I think there is a compromise between the number of turns in the fibre coil (more is better for sensitivity) and the dispersion (which is proportional to the length of the fibre). Still, they're pretty good as these things go.

14th Feb 2019, 09:32
Got to be an error or mistake. Maybe it should be 1 nm/12hrs drift or 1.2nm/hr but 12 nm/hr is outside most if not all IRU unit limits!
Remember being taught the 3 + 3t years ago too.

Impress to inflate
14th Feb 2019, 12:05
Standard on the EC225 helicopter as well

14th Feb 2019, 12:08
Got to be an error or mistake. Maybe it should be 1 nm/12hrs drift or 1.2nm/hr but 12 nm/hr is outside most if not all IRU unit limits!
Remember being taught the 3 + 3t years ago too.

The 12 nm/hr (LCR100) is for an AHRS, not for an IRS.
The LCR110 is an IRS with the stats quoted below.
But it's not meant for long range inertial navigation, so the limits you are talking about don't apply.
It is just to ensure RNP capability by providing short term IRS for RNP approaches in case you lose GPS.

• Pitch / Roll 0.1deg
• True Heading 1.0 deg Typical 0.5deg
• Inertial Position 2 nm/15 min
• Hybrid Position GNSS accuracy
• Hybrid Coasting after loss of GNSS 0.1 nm/10 min, 0.3 nm/17 min, 0.5 nm/20 min, 1.0 nm/28 min
• Hybrid Velocity 0.5kts
• AAIM supports RNP 0.1 capability


Based on the navigation grade sensors and AAIM
the LCR-110 enables continuation of even critical
RNP approaches after loss of GNSS data, helping
the operator to reduce fuel consumption and save
time. The AAIM also ensures required integrity of
satellite based navigation at low level flight
operations in mountainous areas important for
helicopter SAR operations.

13th Apr 2023, 04:06
FOG has the advantages of no mechanical moving parts, no warm-up time, insensitive acceleration, wide dynamic range, digital output, and small size. In addition, FOG also overcomes the fatal shortcomings of ring laser gyro such as high cost and latching phenomenon.