View Full Version : Why are we still using ILS?

25th Oct 2007, 07:44
There have been initiatives for quite some time to use GPS for precision approaches. The benefits are obvious - you can have an 'ILS' for any runway you like whether or not it has a genuine one and whether or not it is serviceable. I believe the CAA are dabbling with non-precision approaches but are you aware of bolder moves? What might be holding things up?

25th Oct 2007, 08:05
How can we thrust a system (which normally works perfect) that can be manipulated by the Americanos with one touch of the button??

stator vane
25th Oct 2007, 08:17

that's why, or why not for most anything in aviation.

25th Oct 2007, 08:23
USA have a big OFF S/W they can throw any time they like.

25th Oct 2007, 08:33
Because there will be temperature limitations with PRNAV Arrivals and GNSS/RNPRnav approaches.

Because it's taking ages for the national authorities to grant approvals to do them.

Because not every aircraft is yet equipped with GPS and a capable-enough FMGC/FMS.

Because GPS is not accurate enough. GNSS/RNPRNav approaches are NPA.
How do you fancy doing a CATIII in 75m RVR with GPS? I don't.

Will that do?

25th Oct 2007, 08:43
So far ILS is the only thing accurate enough for Cat II & Cat III approaches and until this changes ILS will remain the primary approach aid at most airfields.

Say again s l o w l y
25th Oct 2007, 09:30
As has already been said. GPS approaches aren't anywhere near mature enough to consider changing from an ILS.

ILS' are under control of the authority, can be easily calibrated and are more accurate.

Simple really!

green granite
25th Oct 2007, 09:41
Probably because the Americans weren't getting their way with MLS, so they decided to kill it by saying that GPS would be the preferred successor to ILS :}

25th Oct 2007, 14:26
Bear in mind that with ILS/MLS, you are constantly flying closer to the transmitter the lower you get hence the guidance signal gets stronger and more accurate, whereas with GPS, you are moving away from an already weak signal, and if you're in a mountainous area, one or more satellites might suddenly go below your horizon, thus degrading the accuracy of the signal.

25th Oct 2007, 14:48
The Instrument Landing System was conceived in 1938 by Sperry and Reed Pigman (jointly) and has proven to be very precise over the years, and is a known quantity...very important for aircraft navigation.
GPS on the other hand, does indeed have its limitations, and especially so with precision approaches.
The wide area augmentation system is being implemented in the USA now, and indeed one can equip the aircraft today with this kit.
WAAS provides VNAV for many GPS approaches, thus allowing lower minimums.
LAAS will provide landing minima to corresponding CATI ILS minima if trials now in progress are successful.

Now, many outside the USA generaly tend to throw verbal stones at GPS, and the simple reason is...they suffer from the 'not invented here' syndrome, and of course that is their problem...and they are welcome to it.

Stand alone GPS sets, however successful they have proven over the years, need to be treated with a good deal of respect, just as you might with a modern FMS, and the old saying 'garbage in-garbage out' is just as applicable...they require careful detailed operator programmimg to function properly and reliably.

In short, don't expect ILS to disappear anytime soon.

Capn Bloggs
25th Oct 2007, 14:49
To get even Cat 1 capability with GPS, you need ground augmentation of the GPS signal. That means a ground station near the airport. As for cat 2 and 3, you'll be waiting for a long time methinks.

25th Oct 2007, 15:14
I think WAAS currently provides Cat I whereas LAAS will give Cat III.
I of course accept ILS will be kept for those runways that can justify the cost but it's sad the authorities throw away the >99.9% benefits of GPS just because of the <0.1% chance of non performance. Approved units report loss of necessary accuracy if it occurs and have means of ensuring co-ordinates are correctly stored.
Incidentally I recall, many years ago on a crude version of Flight Simulator, an ILS display that was a series of moving squares that defined a virtual tube in the sky. The squares would keep coming past you, so you were trying to position yourself in the middle of each square as you 'flew' through it. If, for example, if you were too low the squares would pass 'above' you until you pulled your self back up into the glidepath whereupon you would be going down the middle of them again. It always struck me as rather better than crossed needles. The beauty of such a display is it could define curved approach paths as you could see the position of the glidepath ahead whereas needles only tell you were you are right now.

old,not bold
25th Oct 2007, 15:19
I was under the impression that ground augmentation stations (for GPS) have a long range, and so don't need to be "near" the approach being flown.

Pity that MLS got killed off, really. It had some practical difficulties, and conversion was going to be expensive on the ground and in the aircraft, to put it mildly.

But it has huge benefits over GPS, mostly mentioned above in one way or another; accuracy and not being subject to deliberate degradation are two, and the more efficient use of airspace being another. Every time I see the straight line of landing lights approaching LHR for several miles I wonder why we still operate so inefficiently.

Nathan Parker
25th Oct 2007, 15:42
I think WAAS currently provides Cat I whereas LAAS will give Cat III.

Not currently, but that's supposed to change with the addition of an additional GPS channel in the 2010-2013 timeframe.

25th Oct 2007, 15:51
Of course has GPS its limitations, as has MLS. But you can be sure, if it would have advantages, it would be postulated, classified and certified tomorrow. There is hardly any weather you cannot land with Cat III, so what's the use of another system?


wing tip brakes
25th Oct 2007, 15:54
i read recently that the Americans have now undertaken not to fiddle with GPS accuracy anymore. maybe last weeks Flight International ?

old,not bold
25th Oct 2007, 15:59
have now undertakenThat's only until there's a good reason NOT to allow its use by someone they don't like...problem is it's all or nothing, at least so far as civilian use is concerned.

what's the use of another system?

Better use of airspace, ie less time manouvering (and probably holding) to join a 6 miles straight approach in a landing queue?

ATC Watcher
25th Oct 2007, 17:28
411A , I seem to remember that the principles of ILS were German, by Lorenz if my memory is good, and was used during WWII by them.

CATIII GPS is still a long way away and may cost more than ILS, since the US do no need/want CATIII.
A few remarks :
Will GPS continue to be free of charge once the US military have no more use of it ? Will Galileo ( the European GPS replacement ) be free of charge ?
Accuracy is only one of the pre-requisite for a guidance system. Reliability and integrity are others.
The long-range future xLS may well be an hybrid EGPWS/GPS/ Forward looking IR system.

David Horn
25th Oct 2007, 18:12
Spot on - the ability to turn on Selective Availability was switched off by an Executive Order in May 2000. That doesn't mean that the ability has been physically removed from the current satellites; however any new ones to be launched will be unable to transmit an SA encoded signal.

Check Airman
25th Oct 2007, 18:29
GPS precision approaches will definitely be useful, but I don't think they should or will replace ILS. In addition to many of the valid arguments above, a failure of the system can affect a very large area, whereas if an ILS fails, it's pretty much limited to that airport or runway.

25th Oct 2007, 21:03
Quite simply, the old adage:

'If it ain't broke, don't fix it'



25th Oct 2007, 21:14
Other technology aside, I would have thought the simple answer would have been that ILS is protected by ICAO until 2010 as the primary Precision Approach system (ref. TC AIM COM 3.13)

25th Oct 2007, 21:30
Don't tell the CAA - they still swear by NDB/ADF !

25th Oct 2007, 22:48
Will GPS continue to be free of charge once the US military have no more use of it ?It looks like they will be using for approaches themselves although I'm not sure whether it's ordinary GPS or one of their special military channels:
The Joint Precision Approach and Landing System (JPALS) is a key component enabling U.S. military forces to be highly mobile and capable of "rapid response" on a global basis to a wide range of military scenarios. Similar in concept to the civilian Local Area Augmentation System (LAAS), JPALS will be based on differential Global Positioning System (GPS) technology, and will consist of modular avionics and ground/shipboard components to provide a range of landing minima and system configurations. Aircraft will receive ranging and navigation data from the satellite constellation and differential ranging data or corrections from a ground/shipboard station via a data link

26th Oct 2007, 02:35
411A , I seem to remember that the principles of ILS were German, by Lorenz if my memory is good, and was used during WWII by them.

You could well be right, ATC Watcher, however Sperry/Pigman held the US patents for ILS.
Reed Pigman especially was involved with many navigation systems and was also (later on) the CEO of American Flyers Airline.
He met a rather unfortunate end however when the Lockheed L188 he was commanding crashed just adjacent to Ardmore Oklahoma, many years ago.
Circling with a ceiling of 200 feet doesn't work all that well...:{

27th Oct 2007, 09:37
According to some official U.S. history, namely
work began on an ILS in 1928, and the first successful landing using an ILS was conducted by a certain Lt. James Doolittle on Sep. 23rd, 1929. That makes the idea contemporary with the Radio Range.

It took a little while to refine. The first "modern VHF ILS" installation was demonstrated to the U.S. military and airline industry in 1940 (second ref above).

My copy of Kayton and Fried is at the Uni, so I don't know what they say.