INS and GPS.
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INS and GPS.
Hi everyone,
I found a technical interview question burried in one of the threads, can't remember which, from the Interview and Jobs section of the forums. The question is the following: What is an INS and how does it differ from a GPS? I have read through my ATPL material and understand the first part of the question but I can't expand any further on the second part. In general terms an INS is a self-contained navigation system which requires no external source input whereas a GPS does (i.e. that of satellites). Can anyone else add or expand on the possible differences?
Thanks.
I found a technical interview question burried in one of the threads, can't remember which, from the Interview and Jobs section of the forums. The question is the following: What is an INS and how does it differ from a GPS? I have read through my ATPL material and understand the first part of the question but I can't expand any further on the second part. In general terms an INS is a self-contained navigation system which requires no external source input whereas a GPS does (i.e. that of satellites). Can anyone else add or expand on the possible differences?
Thanks.
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I can think of one difference:
INS computes position parting from sensed acceleration in the north-south and east-west axis. It integrates it to obtain GS and track, and integrate those to obtain distance flown and position.
GPS computes position directly, and then can calculate with time to obtain GS and track.
GPS is good for static position, but in motion, INS (or IRS, too) are better in the short term. That is why FMS systems with GPS use a position which is the GPS position "enhanced" with the ability of the IRS to calculate change in position. or something like that...
INS computes position parting from sensed acceleration in the north-south and east-west axis. It integrates it to obtain GS and track, and integrate those to obtain distance flown and position.
GPS computes position directly, and then can calculate with time to obtain GS and track.
GPS is good for static position, but in motion, INS (or IRS, too) are better in the short term. That is why FMS systems with GPS use a position which is the GPS position "enhanced" with the ability of the IRS to calculate change in position. or something like that...
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In general terms an INS is a self-contained navigation system which requires no external source input whereas a GPS does (i.e. that of satellites).
I would add...GPS is continually updated for accuracy, via data received from those satellites, whereas, raw INS data is not updated, and therefore errors creep in over time, and in so doing degrade the overall accuracy of the INS unit.
The way around this small problem with INS, was to have the raw INS data fed into a navigation computer, and in turn, that unit uses an external source (for example DME/DME triangulation) to update the computed aircraft position.
The first widebody type to use this sort of latter described arrangement (a flight management system, IE: FMS, which also included full time engine thrust management in the mix) was...the Lockheed TriStar.
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The first widebody type to use this sort of latter described arrangement (a flight management system, IE: FMS, which also included full time engine thrust management in the mix) was...the Lockheed TriStar.
This post in no way intends any disrespect towards the mighty ten eleven 411.
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My own personal summary (which consolidates a lot of what people have said already)............
INS/IRS begin with a known position (the parking bay), and via very sensitive accelerometers compute movement from that initial position to calculate Ground Speed, Position etc. As the accelerometers are very sensitive, they provide EXCELLENT instantaneous Ground Speed etc. data. The problem is (as 411A points out) that they do tend to slowly deviate from absolutely correct position, within the 3+3T allowance for a serviceable unit (That is, 3 miles + 3 miles per flight hour). On a 10 hour ocean crossing, a unit at it's allowed limits could be up to 33 miles in position error. This is perfectly safe as that would put the aircraft within normal Radio Nav coverage at the destination, but totally unacceptable for the modern RNAV approach. The earlier fix was to input accurate Radio Nav information (such as DME/DME fix) to pull the INS/IRS position back "into line".
Thus INS/IRS is an excellent short term navigation reference, but had it's limitations for long term.
GPS provides instantaneous position with EXTREME accuracy. It operates by taking a series of fixes a few seconds apart, and computing the Ground Speed and tracking between these series of fixes, a bit like "Join the dots" in kid's games to get the full picture. "Instantaneous" data is merely an average of what happened over the last few seconds.
Thus GPS is an excellent long term navigation reference, but had it's limitations for short term.
Now, provide the INS/IRS with it's long term position problems, with the GPS with it's excellent short term position resolution, and you have close to the perfect system..... a system which is excellent for both short and long term navigation information. Both systems are "quite good" standing alone, but about as "good as it gets" in partnership.
vapilot2004, I'm sure that you knew, but for those who don't, Triple mixing is a bit of a misnomer. The 3 positions are not averaged (as the name would suggest), but the MEDIAN Latitude and MEDIAN Longitude of the 3 systems are used to eliminate possible "bad" data from one of the units (A dual system could not do this).
Yes, seilfly, Vertical Gyros are a thing of the past with IRS aircraft. Attitude comes straight from the IRS. Some INS aircraft did use this, but INS came along as an "add-on" to many existing aircraft with VGs, and thus the INS' capability for attitude provision was not used on these aircraft (but could have been).
Now, for a bit of rest, I have a long ocean to cross in a few hours time (the big one) with GPS/IRS of course....... and I know that the FMC track line will be "bang on" with the Localiser at the destination, no more map shift for me!
I hope that this ramble has not muddied the waters.
Regards,
Old Smokey
INS/IRS begin with a known position (the parking bay), and via very sensitive accelerometers compute movement from that initial position to calculate Ground Speed, Position etc. As the accelerometers are very sensitive, they provide EXCELLENT instantaneous Ground Speed etc. data. The problem is (as 411A points out) that they do tend to slowly deviate from absolutely correct position, within the 3+3T allowance for a serviceable unit (That is, 3 miles + 3 miles per flight hour). On a 10 hour ocean crossing, a unit at it's allowed limits could be up to 33 miles in position error. This is perfectly safe as that would put the aircraft within normal Radio Nav coverage at the destination, but totally unacceptable for the modern RNAV approach. The earlier fix was to input accurate Radio Nav information (such as DME/DME fix) to pull the INS/IRS position back "into line".
Thus INS/IRS is an excellent short term navigation reference, but had it's limitations for long term.
GPS provides instantaneous position with EXTREME accuracy. It operates by taking a series of fixes a few seconds apart, and computing the Ground Speed and tracking between these series of fixes, a bit like "Join the dots" in kid's games to get the full picture. "Instantaneous" data is merely an average of what happened over the last few seconds.
Thus GPS is an excellent long term navigation reference, but had it's limitations for short term.
Now, provide the INS/IRS with it's long term position problems, with the GPS with it's excellent short term position resolution, and you have close to the perfect system..... a system which is excellent for both short and long term navigation information. Both systems are "quite good" standing alone, but about as "good as it gets" in partnership.
vapilot2004, I'm sure that you knew, but for those who don't, Triple mixing is a bit of a misnomer. The 3 positions are not averaged (as the name would suggest), but the MEDIAN Latitude and MEDIAN Longitude of the 3 systems are used to eliminate possible "bad" data from one of the units (A dual system could not do this).
Yes, seilfly, Vertical Gyros are a thing of the past with IRS aircraft. Attitude comes straight from the IRS. Some INS aircraft did use this, but INS came along as an "add-on" to many existing aircraft with VGs, and thus the INS' capability for attitude provision was not used on these aircraft (but could have been).
Now, for a bit of rest, I have a long ocean to cross in a few hours time (the big one) with GPS/IRS of course....... and I know that the FMC track line will be "bang on" with the Localiser at the destination, no more map shift for me!
I hope that this ramble has not muddied the waters.
Regards,
Old Smokey
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... and INS can provide you with an attitude source. No clue if modern birds use that any more though.
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mixing INS and GPS
The concept that INS is a good navigator in the short term but slowly drifts (typically 1nm/hr) while GPS is always accurate (how accurate depends on type of augmentation) is true.
But GPS is easily jammable(low signal strength) and no doubt nasty people will do so in the future when we are relying to a greater extent on GPS.
One possible solution may be to revert to INS,when jamming is detected.However ,drift can be reduced by "calibrating" the sources of INS drifts against the GPS results before the jamming starts
Keith
But GPS is easily jammable(low signal strength) and no doubt nasty people will do so in the future when we are relying to a greater extent on GPS.
One possible solution may be to revert to INS,when jamming is detected.However ,drift can be reduced by "calibrating" the sources of INS drifts against the GPS results before the jamming startsKeith
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Not sure how it works with new aircraft, but it is possible to couple at least one INS to one GPS to make sure the GPS "tells" the INS if it (when) it is starting to drift off. -As in... lets say the P-3
Our INS actually drifts back and forth, correcting towards the correct position/course all the time. What timeframe and the amount of drift before it reverts the drift; I do not know. And we do use the INS's as the primary source of attitude, heading (syntetic) and position; the P using INS1 and CP INS2.
Our INS actually drifts back and forth, correcting towards the correct position/course all the time. What timeframe and the amount of drift before it reverts the drift; I do not know. And we do use the INS's as the primary source of attitude, heading (syntetic) and position; the P using INS1 and CP INS2.
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Excellent summary Old Smokes.
My post did appear to allude to the triple mixing misnomer. Thanks for clearing that bit up for the uninitiated yoof among us.
My post did appear to allude to the triple mixing misnomer. Thanks for clearing that bit up for the uninitiated yoof among us.
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INS is certified as SOLE MEAN of NAVIGATION
GPS is certified as PRIMARY MEAN of NAVIGATION.
GPS is certified as PRIMARY MEAN of NAVIGATION.
In 1997, I was operating L1011 aircraft with a dual Honeywell HT9100 GPS installation, and....it was certified as a sole means of navigation, on oceanic sectors, worldwide.
Yes, even on the crowded north Atlantic.
Gosh, what an absolute surprise.
As to the lack of map shift that Old Smokey refers to...I have, for the last forteen years had my private twin engine aircraft fitted with a single GPS with a moving map, and....its accuracy is unsurpassed, and now with WAAS, as accurate as a category one ILS, depending of course, on the designated instrument procedure....fully IFR approved.
As the Brits would say...the bees knees of navigation equipment.
A very few RAIM failures...perhaps half a dozen during that 14 year period, due mostly to DoD exercises, out west, in the USA (Holloman AFB and China Lake NAS.)
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Greetings 411A
Sure, but in some areas... it doesnot 
, to be certified as sole mean, the equipment has to demonstrate at 95% of the time:
Availablity
Continuity
Integrity
and GPS always had an integrity issue, this is why the TSO129C (GPS) specifies that RAIM reciever autonomous INTEGRITY monitoring had to be built in.
SO GPS can only be PRIMARY mean of navigation until the integrity issue is solved.
Now it is possible that in the case of the equipment you mention that HWL had a Non Objection within a specified area, but it doesnot mean that it was SOLE MEAN OF NAVIGATION Certified.
Regards
Sure
, but in some areas... it doesnot , to be certified as sole mean, the equipment has to demonstrate at 95% of the time:Availablity
Continuity
Integrity
and GPS always had an integrity issue, this is why the TSO129C (GPS) specifies that RAIM reciever autonomous INTEGRITY monitoring had to be built in.
SO GPS can only be PRIMARY mean of navigation until the integrity issue is solved.
Now it is possible that in the case of the equipment you mention that HWL had a Non Objection within a specified area, but it doesnot mean that it was SOLE MEAN OF NAVIGATION Certified.
Regards
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SO GPS can only be PRIMARY mean of navigation until the integrity issue is solved.
In addition, with our ops we certainly had (and still have) more than 95% integrity, so this seems to satisfy the regulatory authorities in the enroute and terminal modes of operation.
WAAS does so with approaches, if fitted.
Couldn't say regarding EASA, however, in Euroland it apprars that the 'not invented here' syndrome is fully operable.
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Dunno really, GLS is operational in euroland, but not in the US. EGNOS is our own version of WAAS, however it is true that WAAS is not enough to do CAT I approaches here, you do need a ground based augmentation system for that at the moment, however i expect that to change. Since one GBAS is enough to provide around a lot different precision approaches within 50NM it is especially for areas with more than one airport a nice way to provide (in the future) CAT IIIc approaches for not a lot of money at all.
I reckoned that the coming of INS was the greatest single advance in aviation in my lifetime, almost like the invention of the chronometer for the old time seafarers, sure GPS gets it's information in a more modern and different way, but it doesn't really present you with anything that we couldn't get from INS, just does it a bit more precisely, I guess, and the colours and maps are pretty, too !
Couldn't be bothered re-programming the panel mounted GPS for some info. the other day, so pulled a handheld from my pocket - Sextants on 707's and now TWO GPS's - on a Microlight !! ( LSA to the USA readers ) how the World has changed.
A colleague once remarked that the wheel had turned full circle, some of the old ( younger than I am now, but they seemed old then ) Captains that we started with couldn't fly an instrument letdown and approach to save their lives, literally ( well, some of them ) but pop out of cloud and say " the runways' over there - Sir " ( of course ) and they would straighten up and fly an immaculate visual approach to a copybook landing, but our students of the time, brought up by Mr. Microsoft and his Flight Sim. could probably fly an instrument approach better than we ever would, but then the had to put a real aircraft back on to the real Earth - and had difficulty.
I do wonder how some of them will cope one dark and dirty night when all the fuses have blown on their Yuppie, T.V. type instruments ?
( of course, you're going to tell me it will never happen. Yeah, right ! )
We were also told that the failure rate of the INS was about one in a Zillion hours - I had two fail on a 747 within 10 minutes of each other, dumped fuel and was on the ground smartish, to be criticised by the local outstation engineer for not returning to Base. Fat chance, how long was the third one going to last, and I didn't fancy a possible ILS in 200 metres with only the standby A.H. and compass. I'm on the ground, fix it Sunshine, sorry about your Saturday night party !
Couldn't be bothered re-programming the panel mounted GPS for some info. the other day, so pulled a handheld from my pocket - Sextants on 707's and now TWO GPS's - on a Microlight !! ( LSA to the USA readers ) how the World has changed.
A colleague once remarked that the wheel had turned full circle, some of the old ( younger than I am now, but they seemed old then ) Captains that we started with couldn't fly an instrument letdown and approach to save their lives, literally ( well, some of them ) but pop out of cloud and say " the runways' over there - Sir " ( of course ) and they would straighten up and fly an immaculate visual approach to a copybook landing, but our students of the time, brought up by Mr. Microsoft and his Flight Sim. could probably fly an instrument approach better than we ever would, but then the had to put a real aircraft back on to the real Earth - and had difficulty.
I do wonder how some of them will cope one dark and dirty night when all the fuses have blown on their Yuppie, T.V. type instruments ?
( of course, you're going to tell me it will never happen. Yeah, right ! )
We were also told that the failure rate of the INS was about one in a Zillion hours - I had two fail on a 747 within 10 minutes of each other, dumped fuel and was on the ground smartish, to be criticised by the local outstation engineer for not returning to Base. Fat chance, how long was the third one going to last, and I didn't fancy a possible ILS in 200 metres with only the standby A.H. and compass. I'm on the ground, fix it Sunshine, sorry about your Saturday night party !
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What worries me about GBAS, is the fact that some teenager hacker seduced or cheated by terrorists or common criminals could somehow send wrong info to the belly GPS antennas. With SBAS, they can't do such thing as the antennas are on top of the fuselage.
As for the GPS signal jamming, I thought that it was difficult, as it is a military designed and the pseudorandom noise is suposed to be difficult to be jammed. I also heard that the soviets (or russians) had jammers for GPS guided missiles. So it is possible, but I don't know if it is easy.
I would feel better with an ILS beam, to be honest.
As for the GPS signal jamming, I thought that it was difficult, as it is a military designed and the pseudorandom noise is suposed to be difficult to be jammed. I also heard that the soviets (or russians) had jammers for GPS guided missiles. So it is possible, but I don't know if it is easy.
I would feel better with an ILS beam, to be honest.
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Actually, GBAS is pretty similar to ILS but more simple and as far as i know safer. It sends correction information to the normal NAV-Receiver on a distinct channel as well as the geometry information (that part is similar to the ILS) and checksums (signal integrity information), in that is better protected than ILS. And since it can support easily very high precision signals it has a future for those fog riddled airports.
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Microburst,Denti,
GPS remains easy to jam, and neither form of augmentation can overcome that. Several trials have proved the susceptability to jamming. There is nogood in having a corrective signal if the basic one is non-existant.One possible solution might be to have a receiver antenna that cuts-off just above the horizontal, because terrorist jamming is likely to be from the ground.Trouble is that you have aircraft rolling and pitching in TMA
Keith
GPS remains easy to jam, and neither form of augmentation can overcome that. Several trials have proved the susceptability to jamming. There is nogood in having a corrective signal if the basic one is non-existant.One possible solution might be to have a receiver antenna that cuts-off just above the horizontal, because terrorist jamming is likely to be from the ground.Trouble is that you have aircraft rolling and pitching in TMA
Keith
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True, GPS can be jammed. Which just means it won't be available and you cannot fly that approach. As there will be another approach available (at least over here a non precision approach is required as backup for every precision one) that is basicly a non issue. Even if it is an RNAV approach that still can be flown without GPS, DME/DME or DME/VOR is enough for that.
By the way, how hard is it to jam a LOC or GP? Really do not know, but would think they send with quite a bit of power.
By the way, how hard is it to jam a LOC or GP? Really do not know, but would think they send with quite a bit of power.