View Full Version : Microwave Landing System

18th Apr 2004, 02:01
Hi all,

first of all, Im not exactly sure if this is the right forum for my question, but since it is somewhat technical... here goes:

We all know what an ILS is, and we all know how useful it can be. Its a proven system, rather easy to comprehend and pretty much straightforward.
So last week I had a day off, and since it was raining I decided to look through old issues of "Flug Revue" (a german aviation magazine) for grins. In several issues of the 1985-1990 range there was a talk about a MLS (Microwave Landing System), which was supposed to replace the ILS system by the mid-90s.

Well, seeing as it is 2004 now and there is no MLS to be seen (at least not anywhere I have ever been to), Im wondering what happened to it? Why has it not been implemented? Is it still being tested somewhere? And also, what are the major advantages and disadvantages of an MLS (apart from the obvious things like costs by retrofitting aircraft etc.), what makes it better than the proven ILS?

18th Apr 2004, 06:42
as you know it was initialy developed for the military quick installations.operated in the uhf range and didn't suffer the terrain concerns as did ILS glide slope signals.In Canada there were 2 installations one on the prairies, one north of Vancouver in the mountains.Both worked well,but as the original promise of low cost dematerialised ,and the concern of frequency congestion/FM station interference was overcome the Industry saw little benefit with the retooling costs.Now GPS has pushed the use/interest of MLS out of the picture..:O

There was also installations in YUL/YYZ with the downtown intercity project.These worked well wth twinotters and a 'steep' glide slope.After proving its sucess this project was discontinued by the Federal Govt...

18th Apr 2004, 12:33
ILS has some limitations:

* only capable of a single, straight line approach for each installation, typically requiring ~7-10nm. Geography can limit the available options. MLS can do multiple curved approaches for a single installation.

* ILS requires a fairly large protected areas around the largfe antenna arrary & in the transmission direction to avoid signal interference. Less of a problem with MLS.

* ILS freqs. are adjacent to FM broadcast freqs. When ILS was first developed there were few, if any, FM stations. Now they're the medium of choice for broadcast & are fighting for frequency capacity. MLS is in different freqency band & not as subject to interference.

* was supposed to be cheap(er). Ignoring re-equipment costs, of course.

GPS augmented with LAAS/WAAS looks to be able to be developed to provide the advantages of MLS & cheaper. A GPS solution also removes the need for an installation at each location where you wish to provide an approach so now an approach can be devised for anywhere. Care for one in the mid-Atlantic? No worries...

GPS can be used for other navigational tasks eg en-route & terminal ie it's not a single task device so eliminates multiple boxes in the a/c.

Effectively the potential of GPS has stopped MLS implementation.

18th Apr 2004, 21:06
I am not so sure that GPS has killed MLS.

Cat 1 GPS is no prob, but I understand that 3B GPS at the same safety level as ILS is still a long way off. Thats before you talk about GWB running all the world app aids (unless AQ hack the GPS system!!!).

18th Apr 2004, 21:34
Here at Heathrow we're getting all four ends of 27L/09R and 27R/09L fitted with MLS.

I think the UK CAA (and Europe in general) has some misgivings about GPS based tools for precision approaches.

I believe that the fitting of MLS is actually due to pressure from the airlines (for us at LHR read BA), who want a much better landing rate in low vis.

19th Apr 2004, 00:14
Then perhaps it's more accurate to say that GPS has profoundly affected the original plans for MLS implementation?

19th Apr 2004, 09:14
oldebloke is right when he says...
Now GPS has pushed the use/interest of MLS out of the picture..
The fact is that there's a few places other than the UK that have found the benefits of GPS to be a VERY compelling argument against MLS. Yes, GPS (and all related satellite-based systems, such as Galilieo) does have a long way to go, but it will get there.

If the reported activity at LHR is correct, I can see some logic in it, tho I have to wonder how airlines other than BA will react to it. The equipment costs for those airlines will be very high, for the sake of just that one airport, so maybe the BAA is allowing BA to extract a commercial advantage? I could be wrong about that, of course.

For many countries, however, as tinny has said, GNSS offers much greater flexibility at less cost. Significantly less cost than that of MLS and, in the highly competitive world of commercial aviation, anything that reduces costs is a winner. If it improves safety, it's a significant bonus.

I thought that I'd read a report, a while back, which said that the UK CAA was starting to play catchup on GNSS due to the advent of the Galileo system. Maybe I was mistaken in that...

20th Apr 2004, 10:42
As far as I know there are some European airports with MLS, but the system implementation has constantly been delayed.

In Europe, Galileo will be the future system (in general, the GNSS-2 systems), but the date it is still far away.

23rd Apr 2004, 15:01
Apparantly my grandfather was involved in fitting the first MLS into West Drayton years ago when he worked for Marconi.

Was he having me on?