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How Does INS Work?  
User currently offlineBio15 From Colombia, joined Mar 2001, 1089 posts, RR: 7
Posted (6 years 9 months 2 weeks 5 days 8 hours ago) and read 9476 times:

Hi. I've been reading about the INS and get the basic idea of what it does. I understand the theory behind gyroscopes, but I don't see how they can be used to determine the position over the earth's surface.

Well, I have three questions:

1) How does it work?
3) What is the accelerometer in the INS?
3) Which position information can be gathered by the INS itself, prior to the pilot's initialization input?

The first question pretty much wraps the other two  Smile



Thanks in advance
regards,
Alfredo

31 replies: All unread, showing first 25:
 
User currently offlineJetMech From Australia, joined Mar 2006, 2635 posts, RR: 53
Reply 1, posted (6 years 9 months 2 weeks 5 days 7 hours ago) and read 9490 times:

It was mainly the older aircraft that used an INS system. The newer aircraft seem to use GPS based systems as the primary navigation tool, with an Inertial Reference System (IRS) as a back-up.

The INS was an electro-mechanical device consisting of accelerometers and gyroscopes. The accelerometers were mounted on a gimballed platform such that the axes of the devices sensitive to acceleration were mutually perpendicular. At the beginning of the flight, an INS is aligned. The pilots would input the current position of the aircraft into the INS. The INS would store this information and also align the gimballed platform to a set attitude with reference to the Earth. The gyroscopes were set up to measure the rotational displacement and velocity of the aircraft about the longitudinal, vertical and lateral axis of the aircraft.

Basically what happens is that the gyroscope signals are used to mechanically rotate the gimballed platform in a direction opposite to what the aircraft is doing. This maintains the accelerometers in the same attitude relative to the Earth that they were established in during the alignment process. Thus, if one accelerometer was aligned East-West, and one North-South, this alignment would be maintained no matter the attitude of the aircraft (not sure if there was a vertical axis accelerometer). Thus, the accelerometers would generate signals proportional to the accelerations in the E-W and N-S directions. This acceleration data can be directly integrated once to get velocities in the E-W and N-S directions, and integrated once more to get displacements, or navigation data in the E-W and N-S directions.

An IRS works on the same principle, but uses a different technique and equipment to get navigational data. The INS uses solid state gyroscopes and accelerometers that are firmly attached to the aircraft structure, and hence, an INS is know as a strap down system. The gyroscopes in this case are Ring Laser Gyroscopes (RLG's), which are again used to measure the rotational displacement and velocity of the aircraft about the longitudinal, vertical and lateral axis of the aircraft.

The accelerometers are different however. In the case of the INS, because they are strapped down, they measure the acceleration of the aircraft's Cof G along the longitudinal, vertical and lateral axis of the aircraft with respect to the attitude that the aircraft is in for that instant. The signals from the RLG's are then used to "mathematically gimbal" the outputs of the accelerometers to relate the aircraft accelerations they are measuring to accelerations with respect to the Earth. This "mathematically gimballed" acceleration data can then be integrated twice to get displacement or navigational data.

Regards, JetMech

[Edited 2007-07-05 05:48:28]


JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlineMandala499 From Indonesia, joined Aug 2001, 6590 posts, RR: 75
Reply 2, posted (6 years 9 months 2 weeks 5 days 7 hours ago) and read 9469 times:

Quote:
3) Which position information can be gathered by the INS itself, prior to the pilot's initialization input?

Last recorded position before shutdown...  Smile

How does it determine it's position over earth's surface?
Here's a basic layman's method of explaining... It has a datum which is it's original position... it then measures continuously it's displacement from the datum. If it knows the datum, it can calculate the present position in flight.

Laser ring gyros measure the acceleration through minute differences in the time it takes for a laser to go around the gyro going one way (let's say clockwise) and the one going the other way (let's say anti clockwise)... Mount this on 3 axis and presto... U get a 3D accelerometer...

There's the simplified version... there are complex calculations to get to the end results though...

Mandala499



When losing situational awareness, pray Cumulus Granitus isn't nearby !
User currently offlineJetMech From Australia, joined Mar 2006, 2635 posts, RR: 53
Reply 3, posted (6 years 9 months 2 weeks 5 days 4 hours ago) and read 9430 times:

Quoting Mandala499 (Reply 2):
Laser ring gyros measure the acceleration through minute differences in the time it takes for a laser to go around the gyro going one way (let's say clockwise) and the one going the other way (let's say anti clockwise)... Mount this on 3 axis and presto... U get a 3D accelerometer...

Yes, but RLG's are only good for measuring rotational displacements / velocities. I am not sure that a RLG can measure linear acceleration. The double integration of linear acceleration information is what gives displacement information relevant for navigation. The RLG's provide inputs to accomplish the complex calculations you mentioned, namely; converting linear accelerations with respect to the aircraft to linear accelerations with respect to the Earth.

Regards, JetMech



JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 4, posted (6 years 9 months 2 weeks 5 days 3 hours ago) and read 9423 times:

If you know where you are and then know how much you accelerated for how long and in which direction, you will know your speed. If you know where you were, know what speed you have been travelling at and in which direction you can calculate your position.

That is how an INS/IRS works. Gyros, be they RLGs or traditional mechanical gyros, for direction and accelerometers for acceleration.

Quoting JetMech (Reply 1):
It was mainly the older aircraft that used an INS system. The newer aircraft seem to use GPS based systems as the primary navigation tool, with an Inertial Reference System (IRS) as a back-up.

It's still IRS though, with GPS used to update the position. You do not want to do any serious flying on GPS alone. To rely on a single-source external signal for position reference isn't a good idea. I know there are aircraft out there doing it, with IFR certified GPS systems, but I do hope that stays out of the commercial transports.

Quoting JetMech (Reply 3):
Yes, but RLG's are only good for measuring rotational displacements / velocities. I am not sure that a RLG can measure linear acceleration.

You are correct. They cannot.



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineTristarSteve From Sweden, joined Nov 2005, 3930 posts, RR: 34
Reply 5, posted (6 years 9 months 2 weeks 5 days 1 hour ago) and read 9410 times:

Quoting JetMech (Reply 1):
The pilots would input the current position of the aircraft into the INS.



Quoting Mandala499 (Reply 2):
Last recorded position before shutdown...

How does it determine it's position over earth's surface?

When you align an IRS you put in the present position, on an airliner this is done through the FMC. The IRS will then align for about 10mins before it starts to work. During this time it does some sums. First it checks that the position you entered is roughly the same as the position it was when it was shut down. Second it measures the earths rotation as it aligns. From this it can check that the Latitude you entered is correct. It cannot confirm longtitude. If you enter an incorrect Latitude, the IRS will not align. If you enter an incorrect Longtitude it will ask you to confirm it. At the third entry it will give up and agree with you.
To enter position from an FMC is just a matter of entering the Airport ICAO designator, EGLL or KEWR, or, on an Airbus, the route i.e LHRJFK1, and then pressing the align key. This is much easier than on the old Carousels of the 70s when you had to type in the whole position manually, and into each INU separately. But it can cause problems when the pilots don't check the position offered by the FMC. Three times in the past year I have been called out to an A320 when the INS will not accept the present position. In all cases I have found the pilots trying to enter the wrong position. They had flown LHR-ARN, were trying to enter ARN, but the FMC was offering them LHR! It gets too easy, so they don't check. But the safeguard is that the IRS would not align.


User currently offlineSailorOrion From Germany, joined Feb 2001, 2058 posts, RR: 6
Reply 6, posted (6 years 9 months 2 weeks 4 days 23 hours ago) and read 9393 times:

Quoting Bio15 (Thread starter):
1) How does it work?

Seeing that the acceleration vector is the second derivative of the position vector, a double integration of the acceleration vector will give you the position, provided you supply initial velocity and initial position.

Quoting Bio15 (Thread starter):
3) Which position information can be gathered by the INS itself, prior to the pilot's initialization input?

None, because of the mathematics involved. You need initial velocity and initial position.

SailorOrion


User currently offlineMandala499 From Indonesia, joined Aug 2001, 6590 posts, RR: 75
Reply 7, posted (6 years 9 months 2 weeks 4 days 23 hours ago) and read 9390 times:

Quoting JetMech (Reply 3):
Yes, but RLG's are only good for measuring rotational displacements / velocities. I am not sure that a RLG can measure linear acceleration.

Indeed!!!! I was just giving a simplified explanation!

Quoting FredT (Reply 4):
It's still IRS though, with GPS used to update the position. You do not want to do any serious flying on GPS alone. To rely on a single-source external signal for position reference isn't a good idea. I know there are aircraft out there doing it, with IFR certified GPS systems, but I do hope that stays out of the commercial transports.

Correct... an IRS with GPS position updates.

What bothers me on this is the systems on the 777, I am told it takes GPS pos updates before the navaid pos updates with the IRS pos updates being last... (albeit it uses 2 GPS pos to determine its own pos)

Quote:
The FMC determines present position from these navigation systems:
ADIRS, GPS, and navigation radios. When receiving reliable GPS data, the primary mode of navigation is from a GPS updated FMC position. If GPS data is not available, cannot be validated, or is inhibited, the FMC position is updated using navigation radios. When navigation radios are not available or not reliable, the FMC position comes from the ADIRU.

I wonder why they didn't pick Navaid before GPS...



When losing situational awareness, pray Cumulus Granitus isn't nearby !
User currently offlineBuyantUkhaa From Mongolia, joined May 2004, 2828 posts, RR: 3
Reply 8, posted (6 years 9 months 2 weeks 4 days 22 hours ago) and read 9370 times:

Quoting SailorOrion (Reply 6):
None, because of the mathematics involved. You need initial velocity and initial position.

Exactly. Since initial is zero (relative to the earth) all you need is a position. And I had never thought of that, but of course it can check your latitude. In 10 minutes you have rotated 360deg/24h/6 = 2.5 degrees so it would be able to pick that up (and the linear acceration towards the earth's axis would then give the distance to the Earth's axis, and thus latitude). Interesting.



I scratch my head, therefore I am.
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2532 posts, RR: 24
Reply 9, posted (6 years 9 months 2 weeks 4 days 21 hours ago) and read 9350 times:

Quoting TristarSteve (Reply 5):
First it checks that the position you entered is roughly the same as the position it was when it was shut down. Second it measures the earths rotation as it aligns. From this it can check that the Latitude you entered is correct. It cannot confirm longtitude. If you enter an incorrect Latitude, the IRS will not align. If you enter an incorrect Longtitude it will ask you to confirm it. At the third entry it will give up and agree with you.

One other key thing the IRS/INS alignment period provides is true heading. "Alignment on the move" is now possible using GPS data. With this method IRS alignment does not require a stationary platform, allowing re-alignment of an IRS in flight, or rapid alignment on the ground (useful to the military).

Quoting Mandala499 (Reply 7):
What bothers me on this is the systems on the 777, I am told it takes GPS pos updates before the navaid pos updates with the IRS pos updates being last... (albeit it uses 2 GPS pos to determine its own pos)

GPS position is likely to be consistently very accurate, whereas navaid position correction accuracy is dependent on the relative position of the aircraft to the navaid. Over the ocean there won't be any navaids anyway. GPS is only being used to eliminate steady state errors arising from gyro drift, and a grossly erroneous GPS position will be rejected.

Quoting JetMech (Reply 1):
An IRS works on the same principle, but uses a different technique and equipment to get navigational data. The INS uses solid state gyroscopes and accelerometers that are firmly attached to the aircraft structure, and hence, an INS is know as a strap down system.

I think your last two references to INS were meant to be IRS. However you can have a strapdown INS too (e.g. the Litton 92). The key difference is that an INS is a navigation system, whereas an IRS provides position information to the FMC. It does not navigate as such. Both INS and IRS can provide attitude and heading references.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlinePhilSquares From , joined Dec 1969, posts, RR:
Reply 10, posted (6 years 9 months 2 weeks 4 days 18 hours ago) and read 9308 times:

Quoting Mandala499 (Reply 7):
I wonder why they didn't pick Navaid before GPS...

The reason is GPS updating is much more accurate than any navaid would be.

Quoting JetMech (Reply 1):
The newer aircraft seem to use GPS based systems as the primary navigation tool, with an Inertial Reference System (IRS) as a back-up.

Not quite true. The IRS position (3) information is fed into the FMC (Boeings), from that point the GPS updates the FMC followed by the Navaid updating. The aircraft then uses the calculated position as the aircraft position and navigates from that point.


User currently offlineStarlionblue From Greenland, joined Feb 2004, 16908 posts, RR: 67
Reply 11, posted (6 years 9 months 2 weeks 4 days 18 hours ago) and read 9308 times:

Quoting BuyantUkhaa (Reply 8):
And I had never thought of that, but of course it can check your latitude. In 10 minutes you have rotated 360deg/24h/6 = 2.5 degrees so it would be able to pick that up (and the linear acceration towards the earth's axis would then give the distance to the Earth's axis, and thus latitude). Interesting.

Wouldn't there be an error depending on airfield elevation? Unless you enter that.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineBuyantUkhaa From Mongolia, joined May 2004, 2828 posts, RR: 3
Reply 12, posted (6 years 9 months 2 weeks 4 days 17 hours ago) and read 9286 times:

Quoting Starlionblue (Reply 11):
Wouldn't there be an error depending on airfield elevation? Unless you enter that



Quoting TristarSteve (Reply 5):
To enter position from an FMC is just a matter of entering the Airport ICAO designator, EGLL or KEWR, or, on an Airbus, the route i.e LHRJFK1, and then pressing the align key.

Well I suppose if you enter the ICAO code of the airport, it knows the elevation (unless it's Courchevel  Wink)



I scratch my head, therefore I am.
User currently offlineMandala499 From Indonesia, joined Aug 2001, 6590 posts, RR: 75
Reply 13, posted (6 years 9 months 2 weeks 4 days 17 hours ago) and read 9274 times:

Quoting PhilSquares (Reply 10):
The reason is GPS updating is much more accurate than any navaid would be.

Thats always what everyone assumes... until one day my friend was 2.5NM off track due to a GPS position problem on his 777... something that couldn't be fixed so he went on the Inertials for the next 4 hrs when all of a sudden the GPS positions merged on the IRS positions... which was... err... weird... something the engineers couldn't solve till today...

Quoting FredT (Reply 4):
You do not want to do any serious flying on GPS alone. To rely on a single-source external signal for position reference isn't a good idea. I know there are aircraft out there doing it, with IFR certified GPS systems, but I do hope that stays out of the commercial transports.

And on the same day as that 777 GPS weird event... an F28 went 40NM astray on GPS navigation... went into holding above the "assumed" position until the navaid went on... 40 miles... Yucks!

The good news is... it RARELY happens...  Smile



When losing situational awareness, pray Cumulus Granitus isn't nearby !
User currently offlineTristarSteve From Sweden, joined Nov 2005, 3930 posts, RR: 34
Reply 14, posted (6 years 9 months 2 weeks 4 days 16 hours ago) and read 9228 times:

Quoting FredT (Reply 4):
To rely on a single-source external signal for position reference isn't a good idea.

Yet you would happily do a Cat 3 approach onto a single ILS?
Before GPS was invented, all narrow body jets (B737-200) flew around following a single VOR/DME.

I think single source is OK, but not a GPS.


User currently onlineMD11Engineer From Germany, joined Oct 2003, 13793 posts, RR: 63
Reply 15, posted (6 years 9 months 2 weeks 4 days 13 hours ago) and read 9177 times:

Quoting JetMech (Reply 1):
hus, if one accelerometer was aligned East-West, and one North-South, this alignment would be maintained no matter the attitude of the aircraft (not sure if there was a vertical axis accelerometer).

There is one thing more: Since earth isn't a flat disk, but a spheroid, we don't need a stabilised platform, which keeps it's position in space, but it needs to keep it's vertical axis firmly anchored in relation to the center of earth, The other two axises get oriented towards E-W and N-S. This is actually what the INS / IRS is doing during the alignment period. It monitors the rotation of earth for ten minutes and from this, it not only knows in which direction the earth axis falls, but also on which of two possible latitutes (one on the northern hemisphere, one on the southern) it is positioned. The entered figure will a) exclude the wrong hemisphere and b) give the system the longitude information it can not figure out by itself.

The vertical accelormeter is being used to imitate the physics of a Schuler pendulum, a pendulum with it's mass at exactly the center of earth. This pendulum will ALWAYS be aligned vertically to the center of earth. Since it is impractical to build one, the properties of this pendulum get mathematically imitated in the IRS. In the older systems with mechanical gyros, a motor was used to keep the gyro platform's vertical axis aimed at the center of earth, newer ones do it by calculation in the computer. BTW, back during my avionics class years ago I have learned that an INS is a self-sufficient system consisting out of a gyro platform and accelerometers (sensors) and a computer, which also did flight plan processing. The IRS is a set of (laser) gyros, plus the imediate support electronics, giving position and attitude data to an external flight management system.

Jan

Jan


User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 16, posted (6 years 9 months 2 weeks 4 days 10 hours ago) and read 9145 times:

Quoting Mandala499 (Reply 7):
wonder why they didn't pick Navaid before GPS...

I suspect an answer above was based on a meaning (of a word) that you did not intend.

"pick Navaid before GPS" could mean give navaid (like VOR) preference over GPS. It could also mean why didn't the technology use Navaids to confirm lateral navigation in the days before GPS came along.

If the second meaning is what you intended, the short answer is, they did.

IRS used both inertial (RLGs and accelerometers) guidance and VORs for guidance to get a present position from which to project its nav predictions and guidance. From the VOR/DME stations it used only the distance, the DME portion to derive a position. It tried to find stations nearly 90 degrees out from one another and even more, it tried to have three of them going any time reception permitted. Draw a circle 98nm radius around Cedar Lake and 66nm around Yardley and these will intersect at two different points - one on each side of a straight line between them. So a third VOR will play off the tie.

I have quite a few Atlantic crossings using this system without GPS. It worked just fine and may have been nearly as accurate.

Even GPS-equipped aircraft may well still have this procedures going on in the background and the computer will likely continually compare that result with the GPS one.



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User currently offlineN231YE From , joined Dec 1969, posts, RR:
Reply 17, posted (6 years 9 months 2 weeks 4 days 10 hours ago) and read 9138 times:

If I remember correctly, the 747 was the first civilian aircraft fitted with an INS.

It was so technologically advanced at the time, that it helped to eliminate the 4th man (navigator) on the 747, which was essentially like the transition from 3 to 2 man cockpits, more than 10 years later.


User currently offlineMandala499 From Indonesia, joined Aug 2001, 6590 posts, RR: 75
Reply 18, posted (6 years 9 months 2 weeks 4 days 8 hours ago) and read 9117 times:

Slamclick
Sorry, to prevent further confusion...
The 737 classics, the 744, 757, 767s all pick Navaid before IRS for positions...
The 737NG and the 777 use GPS before Navaids before IRS.

The question was... why didn't the NG/777 use Navaid before GPS before IRS instead...  Smile

Mandala499



When losing situational awareness, pray Cumulus Granitus isn't nearby !
User currently offlineJetMech From Australia, joined Mar 2006, 2635 posts, RR: 53
Reply 19, posted (6 years 9 months 2 weeks 4 days 7 hours ago) and read 9111 times:

Quoting Jetlagged (Reply 9):
However you can have a strapdown INS too (e.g. the Litton 92). The key difference is that an INS is a navigation system, whereas an IRS provides position information to the FMC.

Sure, an INS and IRS are both used to measure acceleration, which is then used to get displacements. What I was getting at is newer commercial aircraft tend to have an IRS, which due to the advancement of technology, tends to be a solid state strap down system with RLG's. I have no doubt that one can have a strap down INS as well. Is the Litton 92 a solid state or electro-mechanical system? Are you aware of any electromechanical strap down IRS or INS systems?

Quoting MD11Engineer (Reply 15):
Since earth isn't a flat disk, but a spheroid, we don't need a stabilised platform, which keeps it's position in space, but it needs to keep it's vertical axis firmly anchored in relation to the center of earth, The other two axises get oriented towards E-W and N-S.

Yep, what I meant by a gimballed platform was one that was able to freely rotate about all three axes, not just the axis perpendicular to the plane containing the N-S and E-W accelerometers. Even if Earth was flat, you would still need a fully gimballed platform, as changes in aircraft pitch and roll would still have erroneous effects on the accelerometers unless it was a strap down system.

Quoting PhilSquares (Reply 10):
Not quite true. The IRS position (3) information is fed into the FMC (Boeings), from that point the GPS updates the FMC followed by the Navaid updating. The aircraft then uses the calculated position as the aircraft position and navigates from that point.

Sounds good! That was a point I was not too sure about.

Regards, JetMech

[Edited 2007-07-06 05:36:50]


JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlineFlynavy From , joined Dec 1969, posts, RR:
Reply 20, posted (6 years 9 months 2 weeks 4 days 7 hours ago) and read 9103 times:

The "Legacy" Hornets (A-D's) use INS as the primary navigation tool. On carrier decks, we can get an INS alignment from a number of sources, including directly from the INU (in carrier mode), over a SINS (shipboard INS) cable (which taps in the ship's own navigation reference), or via data link (with the receiver set in "align" mode). Our INUs also have a few other modes, including Ground (when operating from shore-based fields), IFA, and emergency.

Just my two cents.


User currently offlineTristarSteve From Sweden, joined Nov 2005, 3930 posts, RR: 34
Reply 21, posted (6 years 9 months 2 weeks 4 days 3 hours ago) and read 9086 times:

Quoting Mandala499 (Reply 18):
The 737 classics, the 744, 757, 767s all pick Navaid before IRS for positions...
The 737NG and the 777 use GPS before Navaids before IRS.

The question was... why didn't the NG/777 use Navaid before GPS before IRS instead...

When the older aircraft were designed, there was no GPS!
IOn older A320 the primary nav aid was DME/DME with the IRS used as back up. There used to be an ECAM message that announced NAV ON IRS to warn you that it had lost DME. They had no GPS fitted.
On newer A320 with built in GPS the nav supplier gets cloudy.
The standard nav tool is Triple IRS and two GPS, which is sorted by the FMC into a single position.
If GPS is not accurate enough then the next is Triple IRS with DME/DME.
After that it gets a bit vague. The FMC does calculations about the reliabilty of the various inputs and picks the best. I was just reading the FMC write up in the AMM, and I can't precis it better than that.

I assume that the reason that GPS is now given preference over DME is because it is more likely to be available all the time. DME is only available within range, and not over the ocean where the B777 was designed to operate.

One small point. We all talk about Ring Laser Gyros (RLG)> For those who are not sure, a RLG is not a Gyro. The old mechanical gyros in the Carousel and Litton INS were big spinning tops, but when the RLG was invented it was called a Gyro to explain what it does. There are no moving parts, and it does not exploit the Gyro effect to work.


User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2532 posts, RR: 24
Reply 22, posted (6 years 9 months 2 weeks 3 days 23 hours ago) and read 9062 times:

Quoting JetMech (Reply 19):
What I was getting at is newer commercial aircraft tend to have an IRS, which due to the advancement of technology, tends to be a solid state strap down system with RLG's.

Those newer aircraft also tend to have FMCs for navigation. Hence the use of the term IRS, in place of INS. An INS is an IRS with navigational capability, such as steering commands to the autopilot.

Quoting JetMech (Reply 19):
Is the Litton 92 a solid state or electro-mechanical system?

It uses RLG technology and digital electronics just like an IRS. Check this link:

http://www.nsd.es.northropgrumman.com/Automated/products/LTN-92.html

Quoting JetMech (Reply 19):
Are you aware of any electromechanical strap down IRS or INS systems?

No I'm not. I don't think the problems with those were ever solved before the advent of the RLG. After that point, why bother? Easier to provide GPS position updating to improve the accuracy of a legacy electro-mechanical INS.

Quoting N231YE (Reply 17):
If I remember correctly, the 747 was the first civilian aircraft fitted with an INS.

Maybe as standard fit, but this link shows PanAm INS route proving in a 707.

Pan Am First Pass Flight Over North Pole (by MrFord Apr 14 2007 in Tech Ops)



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineStarlionblue From Greenland, joined Feb 2004, 16908 posts, RR: 67
Reply 23, posted (6 years 9 months 2 weeks 3 days 20 hours ago) and read 9028 times:

Quoting TristarSteve (Reply 21):
One small point. We all talk about Ring Laser Gyros (RLG)> For those who are not sure, a RLG is not a Gyro. The old mechanical gyros in the Carousel and Litton INS were big spinning tops, but when the RLG was invented it was called a Gyro to explain what it does. There are no moving parts, and it does not exploit the Gyro effect to w

True. A gyroscope uses the principle of conservation of angular momentum to function. A ring laser gyro measures the time light takes to travel between two points.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinePhilSquares From , joined Dec 1969, posts, RR:
Reply 24, posted (6 years 9 months 2 weeks 3 days 16 hours ago) and read 8983 times:

Quoting Mandala499 (Reply 18):
Sorry, to prevent further confusion...
The 737 classics, the 744, 757, 767s all pick Navaid before IRS for positions...



Quoting Mandala499 (Reply 13):
Thats always what everyone assumes... until one day my friend was 2.5NM off track due to a GPS position problem on his 777... something that couldn't be fixed so he went on the Inertials for the next 4 hrs when all of a sudden the GPS positions merged on the IRS positions... which was... err... weird... something the engineers couldn't solve till today...

Mandala,

I think you're somewhat confused about how the 744 works. IF the 744 has GPS, and all new ones do and it's a option to retrofit, the 744 will use the GPS. However, the IRS never uses anything for position updates. It's position is fixed when you align the IRS. The IRS position is continually fed to the FMC and THAT FMC position is then updated via GPS and then NAV AIDS. In the Navaids, it takes ILS DME first, then DME then VOR after that.

You have obvioulsy heard of map shift. 2.5 NM is nothing. When the 744 was first introduced you would get a mapshift going into HKG. The FMC was using navaids in China who's position wasn't accurate!

Just for your information, there was a recent Airbus/Boeing memo about GPS map shift. I will see if I can find it but it was due to a programming error on the GPS. The FMC shouldn't have accepted that update and both Airbus/Boeing are trying to figure out what happened. Your comments about the F-28 would indicate it was purely GPS since the FMC wouldn't accept an update that far out......there is a check with the IRS position. The reliability of the IRS isn't much better than the old INS, but it's 3 + 3T where T is the time the units have been in NAV. So a unit that's been in NAV can have an accuracy of 15NM! However the FMC accuracy is much better. In reality, I've never seen an IRS much more than 2 miles out and that's after a 15+ hour flight..


25 N231YE : I think you may be right. I stand corrected: the 747 was the first civilian aircraft to have INS standard.
26 Post contains images Bio15 : Hi everyone, thanks for the responses, I didn't expect so many! I clearly understand the mathematics on how to calculate your position based on accele
27 411A : With regard to GPS being the only source for long range navivation (IE: long range overwater operations), L1011 aircraft have in the past and now pres
28 Rwessel : Basically an RLG is a three mirror "ring" of tubes willed with a gas (often a mix of helium and neon) that can lase when stimulated by an electrical
29 Post contains images Mandala499 : *bangs head on table* Phil, What I meant as FMC position updates... *sorry... and yes, this isn't the first time I made this omission * It should be..
30 Viscount724 : Quite a few 707s and DC-8s operating international routes had INS installed in the late 1960s before the 747 went into service. I recall CP Air had I
31 TristarSteve : Well you learn something every day. All the L1011 I worked on had INS fitted. The GF and BA aircraft had dual or triple Litton, they were fitted with
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