What Happened to the Microwave Landing System?

[aviationluver]

I remember reading about the Microwave Landing System (MLS) years ago in aviation textbooks in the states. Supposedly, (back then), the MLS was going to be all the rage and replace most ILSs. What happened? I don't know of any MLS approaches? Was it a funding issue as to why the MLS never went forward or did the GPS do in the MLS?

Thanks.:confused:

[zlin77]

It was a victim of GPS technology, lots of dollars spent developing it in many countries, have a quick google of MLS, INTERSCAN, Wiki links contain some concise info.

[DaveReidUK]

I remember reading about the Microwave Landing System (MLS) years ago in aviation textbooks in the states. Supposedly, (back then), the MLS was going to be all the rage and replace most ILSs. What happened? I don't know of any MLS approaches? Was it a funding issue as to why the MLS never went forward or did the GPS do in the MLS?

From a response on PPRuNe to a similar question re MLS in the UK :

MLS is not supplanting ILS, in fact MLS will be dead and buried within a few years. It's not that funding is going to run out, it's that nobody manufactures the system any more, and the current one is about to reach it's end of life. The main benefits of MLS (smaller sensitive area in CAT II/III ops) have largely been replicated by the new 32-element array ILS localiser antenna on the market now anyway.

A standard RNAV GNSS (APV Baro VNAV) approach can't get to the same decision height as a CAT I ILS, so that won't do it either.

GBAS will, especially when CAT III certification is granted.

However, even new aircraft coming out of the factories now are noit fitted for GBAS, so ILS will be around for another 30 years at least.

[Alpine Flyer]

While I have flown with aircraft having MLS buttons on the guidance panel, I have never seen an MLS receiver or an airport equipped with MLS.

Much like airline STOL, MLS was required for too small a niche and provided too little benefits to be successful. With airlines having very long life cycles it may take decades to equip a fleet with new technology if that technology is not mandatory equipment.

[Metro man]

There is a further level in RNAV approaches called LPV which provides a D/H below LNAV/VNAV minimums, typically around 200' which is ILS CAT 1.

MLS simply got overtaken before it got started.

[Art E. Fischler-Reisen]

I've often wondered about MLS. Ever since we picked up our company S-76B from maintenance and found shortly afterwards that some "helpful" avionics engineer had been fiddling and had changed the ILS receiver frequencies to MLS. We found out in solid IMC whilst being vectored for the ILS. We had no idea this was possible and had no idea how it had been done, or how to change it back again. That caused a few more grey hairs.

[STBYRUD]

Just flew into LHR , nearly all the BAs flew the microwave 27R...

[wiggy]

FWIW most of BA's Shortfall Airbus fleet is MLS equipped (but not the 744/777s, not sure about their 787s).

As a result of the above LHR offers published MLS approaches down to CAT 3b no DH limits (typically 75 metres RVR) for all runways ....so I'm not sure where this idea that MLS is already a victim of GPS or that it has been overtaken already by RNAV approaches has come from. As I understand it MLS is still technically more capable than RNAV/GPS when it comes to genuine low vis ops, but I'd agree that unlike GPS/RNAV it's a technology that's not going anywhere.

As for the "why?" and the original question:as I recall it during the introduction MLS it was being pushed as having lots of advantages verses an ILS: for example curved approaches to low minima - which in reality was not going to happen at many airports e.g. due noise routings; OTOH it provides reduced spacing down the approach- which I believe (an ATCer no doubt will confirm or deny) is a facility LHR ATC use to their advantage to improve the flow rate during Low Vis ops - but whilst it's all good stuff as I understand it unlike GPS/RNAV MLS basically doesn't provide enough commercial advantages 24/7 to have make it a "killer app" vs ILS.

[DaveReidUK]

Just flew into LHR, nearly all the BAs flew the microwave 27R...

BA's shorthaul (Airbus) fleet has MLS.

AFAIK, it's not fitted to their Boeing 747/767/777/787 fleets, not sure about the A380s.

[peekay4]

some "helpful" avionics engineer had been fiddling and had changed the ILS receiver frequencies to MLS
There has to be more to the story?? VHF/UHF vs. Microwave bands...

[Alpine Flyer]

for example curved approaches to low minima
Can MLS really do curved? I thought it can do offset only. As there is no "center" beam on an MLS but a fan-shaped area in which the onboard equipment measures where you are in relation to the edges you can select an azimuth and glide angle. For a curved approach you'd need to change these during the approach which seems awkward to tricky.

A nice writeup is here (https://www.aircraftspruce.com/catalog/pdf/13-08721.pdf).

[peekay4]

Can MLS really do curved?
Yes. Within the service area, MLS allows the aircraft to determine 3D position with very high accuracy & precision. The onboard FMS can then compute curved trajectories between waypoints -- including curved glide slopes if desired. The FMS database would contain all the information required to construct each curved segment (e.g., center of the turn circle, arc length of the turn, radius of the turn, etc)

In principle this is not very different from a GPS approach.

[underfire]

Now there is GBAS...even with CAT III autoland. Curved approach, multiple threshold, and able to use the same runway end for both ARR and DEP (not that one would really want to)
System is not complicated, and new ac's already come standard.

[NSEU]

I have never seen an MLS receiver or an airport equipped with MLS.

I dug up some old BA Maintenance Manuals and Schematics which showed MLS circuit breakers, antennas, antenna switching relays and ILS/MLS Switching Relays, but the receivers are marked as "provisional". When I first saw the MLS antennas underneath the nose on BA (many moons ago), it took me a while to figure out what they were.

Do the MMRs have optional modules for MLS?

[B772]

Lax had a MLS in the past. Even though the technology was developed more than 40 years ago I believe the US Military are still a major user with a MLS receiver standard equipment on the C17 and most C130's. I remember seeing the initial test installation at Melbourne in Australia.

[wiggy]

Now there is GBAS...even with CAT III autoland........
System is not complicated, and new ac's already come standard.

What CAT III minima is it currently approved for?

The (albeit dated article) implies autoland off a 50DH, which whilst impressive if true is still not as capable as the MLS at the likes of LHR.


FAA Targets 2018 For GPS-Based Autoland Capability | Commercial Aviation content from Aviation Week (http://aviationweek.com/commercial-aviation/faa-targets-2018-gps-based-autoland-capability)

[safetypee]

The ICAO led initiative for MLS was based on the predicted increase in air transport, the demand for instrument approaches, particularly CAT 3, problems of frequency overlap (runway density), and potential for commercial VHF broadcast interference.
Competing UK and US systems were demonstrated cica 1978/79 with CAT 3 / RNAV performance and integrity which are only just being matched by today's GPS.

The problems were competing political interests in the avionics market, cost, which the operators did not wish to have , and that the perceived threat to ILS was not imminent. The environment / noise lobby was weak, no money, and inability to pressurise the industry.
The GPS card was played as a stalling strategy, but was subsequently mis-interpreted as offering 'the world and more', which influenced the less knowledgable nations during the ICAO evaluation.
The threat faded into history although many of the concepts continued in the military, Madge etc, and specialist private installations e.g. Aspen.

Due to the limited computing power at the time (no FMS), curved approaches used a few 3D waypoints which were linked by existing LOC and GS capture modes in the autopilot. The end result was a very impressive smooth approach which enabled multiple azimuth changes or lateral offsets, and a choice of glide-slope angles and transitions, e.g. 4deg changing to 3deg at 1000ft' and vice versa, as well as straight in steep approach 4 deg.
A 4 deg autoland was demonstrated on a non ILS runway after a 30 deg offset 3 deg GS, transitioning to straight in and 4 deg GS final.

[wiggy]

Safetypee

V. Interesting to hear more of the background story - many thanks.

[Denti]

As far as i know GLS (or GBAS) is only certified for CAT I operation. The company i fly for was the first in europe being certified for operational use of GLS, but only CAT I as well. Autoland and automatic rollout was possible, but not approved. We did however do test flights for CAT IIIb/c trials which were extremely successful, however the system is still not approved for more than CAT I.

Sadly we are not GLS capable anymore as we switched from 737NG where GLS was available at no extra cost from the OEM to airbus where only a retrofit at around 250k € per airframe is available, a cost which is simply to high for no real extra capability.

[underfire]

There are many applications, albeit none commercial for ac, where we are using GBAS for CAT III autoland. While CAT I is approved, other CAT's are in the approval phase with the FAA and others.

Autoland and automatic rollout was possible, but not approved.
Exactly, if one looks at the precision available with GBAS, CAT III autoland is not an issue, you just have to get the regulatory agencies to approve its use.

Just as a turn to final with GBAS is well documented, there are no commercial ac applications. (again, there are a few non-commercial already in use)

We even have RNP AR transition to GBAS final in operation. (military ops)

[peekay4]

While CAT I is approved, other CAT's are in the approval phase with the FAA and others.

FAA/ICAO don't even have all the relevant standards for CAT II/III GBAS (GAST-D) published yet, so it's impossible for them to approve anything. The required standards probably wont be in place until next year at the earliest. Only then can manufacturers have designs evaluated for formal approval, and that process will probably take at least another year.

Also, in the US, currently GBAS is not funded by the FAA so the airlines have to pay for everything (ground equipment, site installation, flight testing, aircraft avionics upgrades, operations, etc.) I think after doing cost/benefit analysis, they are happy to just stick with CAT I GLS for now and are not in a hurry implement GAST-D.

[underfire]

You dont look at a GBAS standard, you look at the accuracy required for CAT III autoland, and meet or exceed that.
That accuracy level is already in place in the standards, it is only a matter of validating the system to meet the standard.
There are many places in the world, and in the military, where the validation does not need to be decades long as with the FAA, but months.
There are GBAS CAT III autoland aircraft and procedures already in operation, much of it government, but there are others...

Also, in the US, currently GBAS is not funded by the FAA so the airlines have to pay for everything (ground equipment, site installation, flight testing, aircraft avionics upgrades, operations, etc.)
a GBAS system is part of the airport infrastructure, same as an ILS, and other NAVAIDS. The airlines do not pay for the system at the airport, the airlines pay for the system on the aircraft.
To outfit an airport with a GBAS system costs about $2 Million, and is good for 26 runway ends and that cost includes maintenance of the system. This cost also includes the approach procedures.
An ILS system costs $500,000 per runway end, and requires about $100,000 a year in maintenance, per runway end. The ILS has all of the multi-path and interference issues, and GBAS has none of these.
The simple costs of it all show that the GBAS is far less expensive, and far more powerful for operations.
If you were an airline paying for the navaid, and the capability... what would you want to pay for?

EDIT: Per the FAA

"The Federal Aviation Administration (FAA) work program is now focused on validating standards for a GBAS Approach Service Type-D (GAST-D) (CAT-III minima) service. The program currently projects a GAST-D GBAS system can be available in 2018."

"ICAO should approve Standards and Recommended Practices for GBAS Category II/III (GAST-D) systems in 2015."

Under SESAR JU, we had GBAS CAT II/III standards approved and validated back in December 2015...

There are a few States which have already approved GBAS for CAT III, and procedures have been designed and are in use.

[peekay4]

@underfire

What you quote there is from an old document. Here's what the current version states:

The FAA expects ICAO GAST-D Standards and Recommended Practices (SARPS) will be approved in 2016 for publication in 2018.

The FAA is currently supporting a non-Federal applicant’s request for System Design Approval (SDA) for a GAST-D GBAS. The FAA currently projects that this SDA will be finalized in 2019.

https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/laas/media/GBAS_FAQ.pdf

So again, the standards have not yet been published, and even design approval isn't expected until 2019. That means an operational GAST-D system will not be available until 2020 or beyond.

Furthermore, as mentioned earlier, GBAS in the US is considered a NON-FEDERAL system. It is NOT like ILS or other NAVAIDs which are paid for and maintained by the FAA.

So although GBAS is part of the airport infrastructure, the funding for GBAS has to come from primarily from the users, i.e., airlines. The airport authority might contribute some seed amount, but at the end of the day, it is the airlines which benefit from the system so if they want GBAS they're expected to burden the bulk of the cost (through fees, etc.)

Extracts from FAA material:

A Category 1 (CAT 1) GBAS system is available and in use in the National Airspace System. While the FAA has indefinitely delayed plans for federal GBAS acquisition, the system can be purchased by airports and installed as a Non-Federal navigation aid. ...

What is a Federal System versus a non-Federal system?

Throughout the National Airspace System, you will find a mixture of Federal and non-Fed navigational aids. The Federal systems are paid for and maintained by the FAA. However, some airports, cities, and/or private entities may purchase and maintain a navigational aid for either private or public use. These navigational aids are known as non-Fed systems because the FAA did not purchase them, nor does the FAA maintain them. However, to ensure public safety, the FAA does retain the power to approve or not approve these non-Fed systems; additionally, they inspect them annually.

If you were an airline paying for the navaid, and the capability... what would you want to pay for?

Considering that currently:


ILS -- paid for by the FAA
GBAS -- paid for by the airlines

It's pretty clear that the airlines will consider cost/benefit of GBAS very carefully. Maybe in the future GAST-D will be part of the Federal system, but right now GBAS is Non-Fed.

[4468]

In BA, only the A320 series aircraft are equipped for MLS. LHR provides MLS approaches to all four runways. BA expect all suitably equipped aircraft to fly MLS approaches where they are available. On the current network this is only LHR.

The reason for this is, MLS allows closer approach spacing (and therefore, flow rate) than ILS in LVPs. All down to the same Cat 3b minima of 75m, no DH.

ATC weren't able to utilise this improved flow rate, as they said they didn't know which aircraft were eligible for reduced spacing. So in order to 'help', BA expect their SIAs to always 'fly the wave'! This was an attempt to satisfy ATC's concern and enable the higher flow rate in LVPs. Sadly this has not happened. Possibly because a few A320 have never been suitably equipped.

So in short, apart from being an additional 'back up', and the fact that microwave is a more solid and reliable signal than ILS, there is absolutely no benefit whatsoever. Just a huge amount of significant expense. I make no comment as to whether this expense, is the only REAL reason BA A320s fly the microwave at LHR.

[safetypee]

4468, I suspect that BA were given an incentive to fit MLS receivers on a few 757s(?) as part of the MLS trials - UK flag wave. Other operators did not believe that the ground installation would be continued, particularly as the UK submission was rejected. I don't know why the ground system was retained.
Manchester and Gatwick had trial installations for interference tests and Cat 3 evaluation (plus a low approach over Gatwick electric railway). Manchester considered a weaving 'S' approach to 24 avoiding several hospitals, but this was not flight tested.

MLS would have been able to provide a GBAS type of operation with appropriate on board computing, e.g. parallel runway operation to a lower standard; also a computed GS together using the back course (go around guidance) beam to the opposite runway end.
Another consideration at LHR was to use a computed steep approach GS on the front course to a displaced touchdown for 'STOL' regional aircraft to reduce the wake turbulence spacing. (Reconsidered more recently with ILS?)

Overseas MLS trials used installations at Bern and Tehran. Steep approach and segmented approaches were demonstrated at Bern with 4 deg autoland; also take off and GA guidance was tested towards 'the close in hill'. Bern also provided an offset MLS for a cross field grass runway towards the ground station, as well as maintaining the full 'ILS' on the paved surface.

There were associated trials with helicopters, but I don't recall where. Some of the ideas would have enabled multiple approach paths and GS to platforms and one idea was for simultaneous helicopter IFR approaches to LHR from the South with a landing zone where T4 is.

[Victor Mike]

The BA trials at LHR were in the nineties, with one 757 (G-BIKO?) and one 767 (G-BNWB) with a chequerboard painted on the nose cone. The MLS was on 27R and could only be flown manually to CAT 1 minima, the main reason people requested it was when 27L was the landing runway and you wanted to use 27R to park on the old T1 shuttle stands...

[Gonzo]

4468,

ATC weren't able to utilise this improved flow rate, as they said they didn't know which aircraft were eligible for reduced spacing. So in order to 'help', BA expect their SIAs to always 'fly the wave'! This was an attempt to satisfy ATC's concern and enable the higher flow rate in LVPs. Sadly this has not happened. Possibly because a few A320 have never been suitably equipped.

Sorry, this is not quite true. Approach do not have access to the flight plan item where MLS equipment is entered, hence why the crews request MLS approach on first contact. Tower's electronic strip system extracts the item and displays MLS capability on the strip.

As you say, LHR LVP traditionally occurs between 0600-1100ish, so it makes sense that the first inbound wave would give the greatest benefit. However, the higher flow rate has certainly been achieved. BA A320 series aircraft would land using MLS on the departure runway, so the flow rate went from 24-26 in LVP to 28-30 due to MLS. On the arrival runway we would use 5nm gaps ahead of the MLS a/c, compared to 6nm ahead of ILS a/c.

Now we have enhanced ILS systems, which have the same, smaller, sensitive area as the MLS, it's benefits have been equalled: We run with 5nm behind any Medium wake cat aircraft.

[Advance]

Going back to the original thread starter question, there is a reasonably accurate history of MLS development here:
https://en.wikipedia.org/wiki/Microwave_landing_system
At a meeting of the Royal Institute of Navigation at Canary Wharf London in 1994, the USA presented a number of papers defining the GPS system.
Many Europeans present were outraged that the USA had developed this system without their participation or consultation. Seriously, it's true - I was there. The USA members were somewhat taken aback, this was their system designed for their military but with free access available to the Standard Positioning System to any and all users.
Then and there, the European members resolved they were going to have their own Sat Nav system because they did not trust the USA and its total control of GPS. That was the intellectual birth of Galileo back in 1994. How is that going, fully operational yet?

At that time MLS was officially approved by ICAO and was to become mandatory fitment in the future, BUT
ICAO held a special COM-OPS division meeting in Montreal in 1995.

The Australian company I worked for held a reception for all delegates promoting our AWA-Intescan MLS ground equipment.
The next day and at that meeting, the USA delegate read out a letter from President Clinton offering ICAO free use of GPS into the future and guaranteeing at least seven years notice of withdrawal. He also indicated that the "dithering" or positional degradation then operating on the Standard Positional Service (Accuracy CEP of 100 feet) would be turned off thus improving accuracy of SPS by an order of magnitude.
That meeting was the end of the road for MLS as an ICAO mandatory fitment item. And FREE beats COSTLY every time. Precious few operators or authorities were prepared to continue with an MLS program in these circumstances.

Certainly some countries in Europe continued with MLS installation for various local reasons including frequency congestion, interference with ILS, and plain distrust of GPS. A couple of military MLS systems also were installed in Asian countries but general development of the system virtually ceased.

GPS WAAS and its variants plus GBAS systems (Ground Local Area Augmentation Systems) are becoming the future and there are GBAS systems in Air Transport use in various parts of the world.

Hope that answers the original question!