DAL7e7
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Is INS Still Used?

Wed Nov 04, 2009 1:00 am

Hey guys,
I just finished a ground school lesson and I was wondering more about the INS. My instructor said that they are big and old and mainly used in airliners but not sure if they're still used. Also, I was wondering if anyone here has much experience with them because they seem interesting and I am curious to know a little more.

Any help is appreciated!

War Eagle!
Trey

(Edited for clarification; class ended)

[Edited 2009-11-03 17:04:52]
DAL7e7 is wondering... Do pilots take crash courses?
 
tdscanuck
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RE: Is INS Still Used?

Wed Nov 04, 2009 2:08 am



Quoting DAL7e7 (Thread starter):
I just finished a ground school lesson and I was wondering more about the INS. My instructor said that they are big and old and mainly used in airliners but not sure if they're still used.

I assume INS=Inertial Navigation System. Yes, they're still used in essentially all airliners.

The old style of INS used physical gyros; big, heavy, expensive, and hard to maintain. Modern INS uses laser gyros; small, light, (relatively) cheap, and reliable. Modern aircraft will calculate position using multiple sources (INS, GPS, DME, VOR, etc.) and then use a filter to combine them to the "best" position estimate (usually GPS is most heavily weighted). INS works great over short term, so it's the best thing to cover temporary gaps in the others, like during a maneuver.

Tom.
 
L-188
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RE: Is INS Still Used?

Wed Nov 04, 2009 4:28 am

God I hope so.

I wouldn't want to be on a GPS only aircraft in the middle of the ocean when the government decides to shut down civilian use of GPS for "National Security" reasons.

Hasn't happened yet, but it is nice to know that a fully self contained navigation system is available.
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flyinTLow
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RE: Is INS Still Used?

Wed Nov 04, 2009 2:03 pm

It is essentially used on all airliners nowadays. None fly without them. INS is where modern aircraft get their ENTIRE attitude information from, as an INS essentially is 3 gyros and 3 accelerometers, which measure acceleration. They also calculate a position, and the FMS usually mixes this position with other information, like GPS, DME-DME updates, etc., to calculate their own "FMS Position", which is then used for navigation.

In the good old days, the gyros esentially were the gyros you know from cessnas, etc. which actually stabilized a platform in order to keep the accelerometers level. Nowadays, as said before, these gyros are laser gyros.
They basically are triangular-shaped with mirrors in the corners, with two lasers going around this triangle in two different directions. Should there be any movement of the "gyro" around its axis, one light beam takes longer for the now "longer" distance to cover, the other less time. A photocell measures this difference, and a computer behind that does the math to it. The "INS platform" remains attatched to the aircraft without any stabilisation.

Hope that gives you a little more insight.

Thilo
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DeltaGuy
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RE: Is INS Still Used?

Wed Nov 04, 2009 4:14 pm

FlyinTLow, thanks for that good explanation of modern laser gryos. Nowadays you usually only have an 'on/off' switch, and no further knowledge of it after that. Neat to see the inner workings of it.

DeltaGuy
"The cockpit, what is it?" "It's the little room in the front of the plane where the pilot sits, but that's not importan
 
B727LVR
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RE: Is INS Still Used?

Wed Nov 04, 2009 4:47 pm

Yeah most lasernav systems on power up will get its present position from the GPS system, but you can still manually input your own cordinates, usually done as an all else fails sort of thing. I like tha lasernav units, they are so much lighter than the old mechanical gyros. Nothing like being in a hurry and send the "help" in to get the new gyro of the shelf, and they come running out carrying it like a briefcase... DOH!  headache 
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Tristarsteve
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RE: Is INS Still Used?

Wed Nov 04, 2009 4:56 pm

Yes, The Inertial Navigation Unit is the only source on a modern jetliner ( say from B757 onwards) that drives the horizon in front of the pilots. With the latest ISIS (Integrated standby indication system) even that horizon is driven by a laser gyro.(inside the unit)
The position outputs from the INS are integrated with GPS outputs and used for navigation, but where VOR and DME are available, these are usually used in preference.
So if the INS was removed, the laser gyros would still be required.
 
flyinTLow
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RE: Is INS Still Used?

Wed Nov 04, 2009 8:27 pm

No problem, I'd be glad to share some more knowledge on this. We had a pretty smart instructor on this at flight school, unfortunatly I forgot a lot of it by now except for the A320. If there is some more stuff you want to know (including the math behind the calculation of the INS position :P ), just let me know...

Thilo

P.S.: Unless you are a mathematics geek, you really don't want to know  Smile
- When dreams take flight, follow them -
 
bri2k1
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RE: Is INS Still Used?

Wed Nov 04, 2009 8:35 pm

From what I recall reading, most airlines use the INS record of the beginning of pushback from the gate as the official departure time of the flight. Does anyone know how true or prevalent this is?
Position and hold
 
tdscanuck
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RE: Is INS Still Used?

Thu Nov 05, 2009 3:14 am



Quoting Bri2k1 (Reply 8):
From what I recall reading, most airlines use the INS record of the beginning of pushback from the gate as the official departure time of the flight. Does anyone know how true or prevalent this is?

Several airliners use the INS to get groundspeed, because the air data system doesn't work when you're going slow (or backwards). So it's true in the sense that the official departure time of the flight is the Out time, when the wheels start rolling.

Tom.
 
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747classic
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RE: Is INS Still Used?

Thu Nov 05, 2009 8:32 am

As a 747 Flight engineer, starting at the mid 70's we had the very extended navigational technical course and had to know everything about the then new inertial navigation systems.

The old (big and heavy ) Inertial Navigation Systems (as installed on the 747,DC10, L1011 and Concorde) worked with gimbals to keep the gyro's in an upright position and used accelerometers to compute the actual position by dead reckoning. (most famous : Delco Carousel IV INS ). The stabilized platform moved free from the aircraftstructure. These systems were extremely accurate and often used in triple installations by civil airliners. From all three platforms the intermix position was used for even more increased accuracy. Position updates were needed, when the ocean crossing started. The only draw back was the relative low MTBF (many tiny moving parts) and the very high maintenance costs for an overhaul. In most remaining aircraft from this generation (70's) the old INS systems are now replaced by laser gyro INS or IRS (inertial reference systems) systems, with GPS update, controlled by FMS, to meet the increased navigational requirements.

The present laser-gyro based systems are strap-down systems (strapped to the aircraft structure) and don't have moving parts. These systems were possible by the increase in computing power in the last 2 decades. However these systems are not so accurate as the old (very expensive) INS systems. So the calculated INS position had to be updated more frequently.
At first auto DME updating was used (eg 747-400 etc). Later multiple GPS receivers are used to update the computed actual position of the INS systems.
On some (updated) aircraft there is even primary GPS navigation, while the INS nav position is updated every 5-10 minutes for standby purposes (total GPS failure).
On all aircraft the INS is still used for attitude info.

[Edited 2009-11-05 01:20:43]

[Edited 2009-11-05 01:31:32]
Operating a twin over the ocean, you're always one engine failure from a total emergency.
 
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Faro
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RE: Is INS Still Used?

Thu Nov 05, 2009 5:37 pm



Quoting 747classic (Reply 10):
However these systems are not so accurate as the old (very expensive) INS systems.

Quite counter-intuitive: why should a 70's-era electromechanical gadget be more accurate than contemporary laser ring installations? I don't understand...

Faro
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Tristarsteve
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RE: Is INS Still Used?

Thu Nov 05, 2009 6:30 pm



Quoting Faro (Reply 11):
Quoting 747classic (Reply 10):
However these systems are not so accurate as the old (very expensive) INS systems.

Quite counter-intuitive: why should a 70's-era electromechanical gadget be more accurate than contemporary laser ring installations? I don't understand...

Faro

Thats what I was thinking.
I wonder if 747classic has any proof.
Looking at the IRS drift at the end of the flight, which I don't do very often, i would say it is much better now than before. But, the reliability is hundreds of times better. On the Tristar, we used to change IRS units every week. They were so unreliable that we fitted a tray with a DG and a VG in the Nbr 3 posn, as there were so many IRS away on repair. In my 25 years at ARN, I have changed 3, two on B734 and 1 on A320.
 
B747FE
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RE: Is INS Still Used?

Thu Nov 05, 2009 6:54 pm



Quoting Tdscanuck (Reply 9):
Several airliners use the INS to get groundspeed

Indeed. We use the GS readout from the INS's during taxi on the B747.

Quoting 747classic (Reply 10):
(most famous : Delco Carousel IV INS ). The stabilized platform moved free from the aircraftstructure.

I've heard that is exactly why they named it "Carrousel"

Quoting DAL7e7 (Thread starter):
but not sure if they're still used.

Yes, I still get to fly in an aeroplane with triple Delco CIVA installation.
They are old, extremely bulky and heavy but the perform really well. In a nutshell, all the navigation computer needs is barometric altitude and TAS from the CADC (Although is perfectly capable to operate without them) and present position input for alignment.
Based on what the accelerometers and gyros are sensing, the computer calculates and solves the equations associated with great circle navigation, supplies control information to the inertial platform to keep it earth referenced and monitor itself. In return it provides true HDG; TAS; GS; Wind speed; Wind on nose; Ground track angle; Drift angle; Cross track distance; Present position; Distance; Desired track and supplies guidance to the autopilot/FD system to keep the airplane in a great circle path.

A truly magnificent equipment, completely independant from ground stations/satelites.
It can even operates with battery power in case of emergency.

Regards,

B747FE.
"Flying is more than a sport and more than a job; flying is pure passion and desire, which fill a lifetime"
 
B747FE
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RE: Is INS Still Used?

Thu Nov 05, 2009 6:59 pm



Quoting TristarSteve (Reply 12):
Thats what I was thinking.
I wonder if 747classic has any proof.

In my experience, LTN 92 are both more accurate and reliable than Carrousels...way better.

Regards,

B747FE.
"Flying is more than a sport and more than a job; flying is pure passion and desire, which fill a lifetime"
 
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747classic
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RE: Is INS Still Used?

Thu Nov 05, 2009 8:40 pm



Quoting TristarSteve (Reply 12):
Quoting Faro (Reply 11):
Quoting 747classic (Reply 10):
However these systems are not so accurate as the old (very expensive) INS systems.

Quite counter-intuitive: why should a 70's-era electromechanical gadget be more accurate than contemporary laser ring installations? I don't understand...

Faro

Thats what I was thinking.
I wonder if 747classic has any proof.

Why do people always think that old is bad and new is good.

After more than 25 years experience with the old (Delco Carrousel IV) INS on the 747, I still come to the conclusion that the old system was more accurate or at least equal, when it worked ok. The MTBF was the real problem together with the alignment time during start up of the system.
I have flown the 747 classic with the Delco, Litton 72 and 92 and the tolerance on all three was 3+3T for removal. (T = time in nav mode)
Normally, with 3 normally functioning systems we had an radial error of only a few NM when crossing the Atlantic with the Carrousel IV.


You can get more accurate ring-gyro lasers if you want, but the cost of such a system will be high and it's not necessary to be that accurate, due to the continuous updating capability.
In fact in new aircraft it's for navigation a back-up system.
And,as always, the airlines don't want to pay more than necessary.
Operating a twin over the ocean, you're always one engine failure from a total emergency.
 
JohnM
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RE: Is INS Still Used?

Thu Nov 05, 2009 10:47 pm

Older "legacy" C-5s still have the triple delco INS. I have worked the system for a pretty long time and it is a good system. Easy to troubleshoot, ops check, and repair on the aircraft. Pretty reliable compared to some of the POS systems that plane has. It is being replaced with a laser ring gyro system which I must admit, does work very well and doesn't break much. I always thought the Delco system was so good because it came from the civil aviation world and Uncle Sam didn't modify or screw with it. Somebody else worked the bugs out before the AF got a hold of it!

The only issue in my opinion with old delco stuff is: the frequent (60 day) replacement cycle required for the battery units, and it is difficult and expensive (as mentioned earlier) to fix the box (nav unit) when it goes tits up.
 
B747FE
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RE: Is INS Still Used?

Thu Nov 05, 2009 11:53 pm



Quoting 747classic (Reply 15):
Why do people always think that old is bad and new is good.

Not always.
We still use the good old Performance Management System in our planes. Works really good and I am pretty sure we'll never get an FMS on the 747 fleet.

However, the fact remains. While Carousel IV units get the job done, strap down inertial platforms are just better.


Regards,

B747FE.
"Flying is more than a sport and more than a job; flying is pure passion and desire, which fill a lifetime"
 
BigSaabowski
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RE: Is INS Still Used?

Fri Nov 06, 2009 1:03 am



Quoting Bri2k1 (Reply 8):
From what I recall reading, most airlines use the INS record of the beginning of pushback from the gate as the official departure time of the flight. Does anyone know how true or prevalent this is?

None that I'm aware of.
ACARS is used for that. There are several inputs into ACARS, the two most common being the parking brake position and boarding door status. Those two are usually used to determine the out and in times (i.e.: Out = Boarding doors closed with the parking brake released).
 
rcair1
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RE: Is INS Still Used?

Fri Nov 06, 2009 3:19 pm

HP MEMS Could Shake up Motion Sensing

You may find this interesting. A new type of inertial sensor. These are really quite amazing devices...

http://www.pcworld.com/article/18149...could_shake_up_motion_sensing.html
rcair1
 
bri2k1
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RE: Is INS Still Used?

Fri Nov 06, 2009 3:45 pm



Quoting BigSaabowski (Reply 18):

Interesting, thanks. I suppose that does make more sense since there's already someone (probably a dispatcher) monitoring ACARS. It would probably require another crewmember action, or some additional communication between the FMS and ground ops, to get the data from the INS.
Position and hold
 
B727LVR
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RE: Is INS Still Used?

Sat Nov 07, 2009 1:54 pm



Quoting Bri2k1 (Reply 20):
Interesting, thanks. I suppose that does make more sense since there's already someone (probably a dispatcher) monitoring ACARS. It would probably require another crewmember action, or some additional communication between the FMS and ground ops, to get the data from the INS.

Maybe, but thats what the log book is for. The crews write down their OUT/OFF times. And once airborn (from home station) they call back to the ground and give their times. When they are away, depending on the Capt. they will either call using the sat phone, or wait until the next stop to call back. Atleast mine do. Our aircraft haven't been fitted with ACARS. We haven't quite got caught up to the reast of the world.
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Fabo
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RE: Is INS Still Used?

Sun Nov 08, 2009 11:21 pm

Guys, dont mix it up for the bloke. Now I*NS and I*RS have a different letter in them for a reason. NO IRS can possibly be used for NAVIGATION - it is just a referential unit for some other unit (FMC).

Also, typically, IRS relates to laser gyro stuff, and INS to clockwork magic. Though I am not sure it is like that exclusively.
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tdscanuck
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RE: Is INS Still Used?

Mon Nov 09, 2009 12:53 am



Quoting Fabo (Reply 22):
Also, typically, IRS relates to laser gyro stuff, and INS to clockwork magic. Though I am not sure it is like that exclusively.

It's not. Current generation airliners have IRS and INS, but no mechanical gyros.

Tom.
 
KELPkid
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RE: Is INS Still Used?

Mon Nov 09, 2009 1:45 am



Quoting B727LVR (Reply 5):
Yeah most lasernav systems on power up will get its present position from the GPS system, but you can still manually input your own cordinates, usually done as an all else fails sort of thing. I like tha lasernav units, they are so much lighter than the old mechanical gyros. Nothing like being in a hurry and send the "help" in to get the new gyro of the shelf, and they come running out carrying it like a briefcase... DOH!

Interesting. I wonder if this is why, in SIN, there is a sign with the LAT/LON coordinates for each gate, at that gate...  Wink
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mandala499
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RE: Is INS Still Used?

Mon Nov 09, 2009 5:10 pm



Quoting Faro (Reply 11):
Quite counter-intuitive: why should a 70's-era electromechanical gadget be more accurate than contemporary laser ring installations? I don't understand...

There is some truth to it... because the role of the inertial systems has changed.

The clue is here:

Quoting Fabo (Reply 22):
NO IRS can possibly be used for NAVIGATION - it is just a referential unit for some other unit (FMC).

INS = Inertial NAVIGATION System
IRS = Inertial REFERENCE Unit

An Inertial system consists of 3 axis gyro (laser or whatever), and 3 axis accelerometers.

For an Inertial system to know where it is... you tell it where it is to start with, and then it'll measure the displacement you've experienced from it using the accelerometers and timekeeping and the orientation...

The INS, uses the above information to feed into it's own pocket calculator, and tells you where it is in LATLON, and where you need to go to go to the next waypoint in the pocket calculator... it can tell you, or it can tell the flight guidance system... whichever. The measurement and calculations are done within the INS...

The IRS, uses the above information, and feeds it into separate systems... The orientation (attitude) to the aircraft's instrument (and/or through other guidance systems)... and the displacement data to the Flight Management system... So, all the IRS does is tell whoever needs the data... I think am here... this is my calculated groundspeed, this is the attitude, roll and heading... etc...

Where you need to go from there to the next waypoint, is calculated on a different box... the FMS... which takes in position updates from DME/DME, RAD/DME, RAD/RAD, LOC/DME, and... GPS... and recently, uses GPS as primary position and IRS with RadNav as backup for all phases except approaches.

So... what's the point? Why make it more complicated than a good system?

Remember, Inertial systems' accuracy (on displacement, not orientation) degrade over time. It is no longer good enough to be within 5NM of your calculated INS position at the end of your flight. So, you need position updates from an external source to calculate the position error from the inertial system to produce a continuous and accurate positioning with each update.

This enables you to have "cheaper" IRUs comparing to expensive INS units... why? if there's a position error, the FMC will track the IRU pos error and remember the error so it'll give you a more accurate position when you loose an external source update than if you never gave it position updates (eg: through radnav tuning)...

One 733 ended up landing 500km from where it should be one day because the crew didn't understand the above... and thought nothing of the "IRS NAV ONLY" in the FMC. Another recent one was one 734 trying to land 100NM from where it should be... and realized they were not where they were when they lined up on the approach and realized it was a different layout... why? They forgot to allow the system to have a position update from an external source... and allowed the FMC to continue relying on straying IRS data with no correction updates. The 733 flight above was scheduled for 02H15, the 734 flight above was scheduled for 01H45...

So you see the stark difference between that... and explains why the old INSs are better than the newer IRS... as long as you remember that they are not the same, and for very good reason!

To be pedantic... Inertial systems are still used... but, good INSes are going and going... crappy IRSes are here and more so, slapped on with an FMC and more recently, GPS.
When losing situational awareness, pray Cumulus Granitus isn't nearby !
 
MD-90
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RE: Is INS Still Used?

Mon Nov 09, 2009 7:49 pm



Quoting Mandala499 (Reply 25):
So you see the stark difference between that... and explains why the old INSs are better than the newer IRS... as long as you remember that they are not the same, and for very good reason!

To be pedantic... Inertial systems are still used... but, good INSes are going and going... crappy IRSes are here and more so, slapped on with an FMC and more recently, GPS.

So what would happen to international flying if for some reason the GPS system failed and modern 777/A330/A380/etc were forced to get by without GPS signals? Hypothetically speaking, of course, and I promise I am not asking another question that is going to come true!
 
tdscanuck
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RE: Is INS Still Used?

Tue Nov 10, 2009 2:37 am



Quoting MD-90 (Reply 26):
So what would happen to international flying if for some reason the GPS system failed and modern 777/A330/A380/etc were forced to get by without GPS signals?

The airplane would attempt to get some combination of DME & VOR and get a position from that, and it would keep track of it's position via IRS integration. The IRS error would grow with time, but if you're not in range of any DME/VOR then you probably don't care about precision navigation anyway. You could happily fly inertial position (aka good dead reckoning) until the plane was back in range of some radio source, at which point the INS/FMC would figure out where you really are and update the IRS.

Tom.
 
Fabo
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RE: Is INS Still Used?

Tue Nov 10, 2009 4:05 am

Also known as The Very Same Way As Before GPS.
The light at the end of tunnel turn out to be a lighted sing saying NO EXIT
 
mandala499
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RE: Is INS Still Used?

Tue Nov 10, 2009 5:01 am

Btw, "Crappy IRSes" does not mean they're hopeless...  Smile

737 Classics still fly on dual IRS with RadNav position updates... provided they're maintained correctly, they'd do fine even in areas of little or poor dme/dme environment.

We've had 777s who've had GPS prim screw ups, and had to fly internationally on triple IRS with radnav updates... no big deal (albeit it was disturbing to the crew)... And several 320s have flown with the aircraft binging "GPS PRIM LOST"... again, triple IRS with radnav updates.

As far as I know, many 744s and 763s still fly across the pond on IRSNAV only whilst waiting for radio nav pos update to correct the IRS error once they cross the pond.

Over-reliance on GPS as sole POS sourse does have it's dangers. A friend descended in a remote area using GPS whilst waiting for the local terminal VORDME to come online... as he came over the expected holding point, instead of being in a gulf with an island holding the VORDME below him... it was nothing but sea and nothing else was seen... He was "a bit early" due to tailwinds, and waited while holding until the scheduled VORDME "on time" came... when it came on, the DME was 40 - 50NM... so much for GPS...

Quoting Fabo (Reply 28):
Also known as The Very Same Way As Before GPS.

Hahaha! Nicely put!!!! Nicely put!!!!

With so many safety critical civilian systems being dependent on GPS, I don't think the US Govt is going to shut down GPS even for a few minutes without some serious thinking!

Even with GPS prims on your airplane, the POS update rate isn't as fast as inertial systems (talking about 1 - 10Hz vs 50 - 200Hz)...

Mandala499
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747classic
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RE: Is INS Still Used?

Tue Nov 10, 2009 7:53 am



Quoting Mandala499 (Reply 25):
This enables you to have "cheaper" IRUs comparing to expensive INS units... why? if there's a position error, the FMC will track the IRU pos error and remember the error so it'll give you a more accurate position when you loose an external source update than if you never gave it position updates (eg: through radnav tuning)...

That's the exactly the point I made before :

Quoting 747classic (Reply 15):
You can get more accurate ring-gyro lasers if you want, but the cost of such a system will be high and it's not necessary to be that accurate, due to the continuous updating capability

Mandala 499, I couldn't have explaned it better.

The incidents you mentioned are all caused by a lack of basic system knowledge, because it's fashion to talk about "blackboxes", without knowing excactly how they are interconnected. Basic navigation skills (always X-check your position from all available sources ) are teached in the class room and often forgotten or omitted. This is IMO the downside of (over)automatisation.

It's all OK, let's keep it on automatic, don't think and follow the magenta line.
Operating a twin over the ocean, you're always one engine failure from a total emergency.
 
bri2k1
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RE: Is INS Still Used?

Tue Nov 10, 2009 3:14 pm



Quoting Mandala499 (Reply 29):
DME was 40 - 50NM... so much for GPS...

I hate to get off topic, and I don't know the specifics here, but I would be interested to find out more. DME provides slant range, not absolute position like GPS. DME stations report distance to the interrogating aircraft no more accurately than 0.1NM, or 185m. A GPS receiver's position accuracy is primarily affected by the accuracy of the receiver's clock. A handheld unit uses math to roughly correct for the error, and using signals from just 4 satellites, typically determines 3-dimensional absolute position within about 6m. On-board installations in an aircraft, which may include data input from another source such as pressure altitude or an IRS or ground-based radio receiver, can be even more accurate. So it's quite surprising to me that you seem to be suggesting GPS might produce an error of 50NM (or over 9,000m!). The DME slant range is least accurate at high altitudes near the station, but this is typically only one or two NM at most (because most of the time, you don't fly a hold at FL400).
Position and hold
 
mandala499
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RE: Is INS Still Used?

Tue Nov 10, 2009 10:46 pm



Quoting Bri2k1 (Reply 31):

I hate to get off topic, and I don't know the specifics here, but I would be interested to find out more. DME provides slant range, not absolute position like GPS. DME stations report distance to the interrogating aircraft no more accurately than 0.1NM, or 185m.

The thing is, do you need to be more accurate than 0.1NM in cruise?
The GPS Pos received by the FMC is displayed on the ND in this case (777) by the ">O<" symbol... aka the "Tie Fighter"  Smile


When this photo was shot, the crew already purged/isolated the GPS signal from the FMC, and the aircraft was on IRS with RadNav update. How would one explain the 30NM GPS POS to the front and about 5nm to the left... and the other at 7NM to the rear left, if the accuracy is as simple as "6m on a handheld" ? It's not slant range error...

Btw Boeing still cannot explain the event despite the reports made and sent, and the request for explanation being thrown at them a few times... they put it down as "undeterminable anomaly" or something like that. The funny thing is, after going on IRS Prim for the rest of the flight, just prior to commencing approach, the 2 GPS pos symbols merged with the aircraft FMC Pos... and everything was back to normal.

Notice the 3 IRS pos (the *) are merged together with the 3 FMC POS (the white diagonal square) (which is IRS with NavRad update) (corrections on the symbol sources welcomed... am not 777 rated!  Smile ).

God knows where the FMC POS is if the GPS signal was still allowed to be taken by the FMC as per default... mind you, ATC did notify the aircraft was about 2 - 5NM off track (mostly, they don't know how far they went off on the max deviation) prior to the crew taking the isolation action after being informed by some very unhappy mainland China ATC guys...

---
the other case (the 40NM one), it was an F28 with a standalone aircraft GPS.

Quoting Bri2k1 (Reply 31):
On-board installations in an aircraft, which may include data input from another source such as pressure altitude or an IRS or ground-based radio receiver, can be even more accurate. So it's quite surprising to me that you seem to be suggesting GPS might produce an error of 50NM (or over 9,000m!). The DME slant range is least accurate at high altitudes near the station, but this is typically only one or two NM at most (because most of the time, you don't fly a hold at FL400).

OK, you are assuming that GPS does not and cannot yield position errors... unfortunately, it's not perfect...
He was holding at 4000ft and supposed to be over the VORDME (which was switched off, he arrived at 0545 LT over the VOR, instead of 0605 LT as per schedule, and the VORDME came alive on schedule at 0600LT, he was 40NM off from where he should have been. It's not slant error... it's plain and simply a GPSPOS error.

GPS is nice, but, it's not perfect either... it can, and does screw up from time to time!

Interestingly, both the 777 and the F28 GPS "adventures" happened within 24hrs of each other... and both in Asia (although nowhere near any warzones *grin*)

Btw, this is an interesting previous topic on this subject... Does 777 Use VORs (by Gopal Nov 27 2006 in Tech Ops)
When losing situational awareness, pray Cumulus Granitus isn't nearby !
 
bri2k1
Posts: 952
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RE: Is INS Still Used?

Wed Nov 11, 2009 4:41 pm

Interesting, Mandala499, thanks. I am not yet rated on the 777 either ( Smile), but do you know why there would be more than one GPS location symbol? GPS position is calculated by a receiver by receiving the signal from multiple (at least 4) different satellites, computing the elapsed time since the signal was transmitted according to timestamps in the signal, and working out the distance based on orbit information also encoded in the signal. It would make sense to me that there could be a position with a certain amount of error, such as if the FMC determined one or more of the signals was outside of the standard deviation for signal strength, time offset, or other source of error. But to compute and display multiple, and wildly differing, GPS positions? I can't think of any situation in which this would be beneficial, and it should probably result in disregarding the GPS location until the error is within limits.

Quoting Mandala499 (Reply 32):
you are assuming that GPS does not and cannot yield position errors

Just to be clear, I'm not saying this at all. I'm just saying that the sum of all the errors is usually less than 6m. There used to be a "selective accuracy" "feature" in GPS that would purposely de-tune the accuracy to about 30m for certain periods, but the examples you've given are obviously much worse. I believe this feature was decommissioned some years ago. I'm as stumped as you are. I'd really like to know if the bulk of this error is a Boeing thing or a GPS system thing, or something else entirely. It doesn't sound like I'll ever have that luxury though.
Position and hold
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Is INS Still Used?

Thu Nov 12, 2009 6:27 am



Quoting Bri2k1 (Reply 33):
I am not yet rated on the 777 either ( Smile), but do you know why there would be more than one GPS location symbol?

Because you've got more than 1 GPS. I think the 777 has 3.

Quoting Bri2k1 (Reply 33):
But to compute and display multiple, and wildly differing, GPS positions? I can't think of any situation in which this would be beneficial, and it should probably result in disregarding the GPS location until the error is within limits.

You have 3 GPS receivers. One is dying. By looking at the display, you can probably tell which one.

Tom.
 
mandala499
Posts: 6458
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RE: Is INS Still Used?

Thu Nov 12, 2009 6:42 am

Bri2k1,
The 777 has 2 GPS receivers (GPS A, GPS B, or is it GPS L and GPS R... can't be bothered to open the book), and 3 IRS, with the multiple radnav receivers.

Given the velocities involved, the errors from the GPS is much greater than a few meters of CEP/ANP accuracy... hence, for say, the Airbus, each GPS receiver (they have 2 aswell, and the NG has 2 aswell) I think require 6 satellites to prevent significant shifts in the GPS signal timestamps. I can't remember exactly what it is but this one of the stuff I had to deal with when trying to get vious deviations before the GPS POS was purged from the FMC POS calculations... which the intercepting authorities would like to have quick answers for if it ended up in an intercept (which is one reason to carry a camera on board, not just for sightseeing from the front!  Smile )

Selective accuracy has been disabled... yes... but, freak offsets still occur... albeit rarely

I'll have a dig through my documents and archives to look for the "error in motion" stuff... see if I can find it again... If so, it's only one of the possibilities as above.

Mandala499
When losing situational awareness, pray Cumulus Granitus isn't nearby !

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