I have studied that the main three systems for performing Precision Approaches (PA) are the ILS (Instrumental Landing System), PAR (Precision Approach Radar) and MLS (Microwave Landing System).

However, in most of the airports the ILS is implemented as the main PA aid; while the others are often developed, but not fully implemented or unused.


What's the reason that the ILS is the most popular PA system?

What are the benefits and disadvantages of the other systems for not being as much considered?

How do they compare regarding approach performances?

  • 1
    $\begingroup$ ILS is significantly cheaper than PAR (in time and aircraft that can land per hour) and than MLS (in hardware). Anything ground- or controller- based is going to be expensive. You should be asking why LPV or other precision GNSS (satellite-based) approaches are not more popular than ILS! $\endgroup$
    – egid
    Jun 2, 2015 at 14:25
  • $\begingroup$ Also, this is three questions in one, and it reads a bit like a (slightly outdated) homework assignment. Are you still in flight school, by any chance? :) $\endgroup$
    – egid
    Jun 2, 2015 at 14:26
  • 3
    $\begingroup$ Well it's not homework. Just I'm interested in knowing that. I did three questions because they help compare the systems, sorry if it sounded like an assignment. And I know that LPV or GNSS based approaches cannot attain a higher level of precision (maximum precision is ILS CAT-I) that's why I asked the third question. $\endgroup$
    – Airman01
    Jun 2, 2015 at 14:29
  • 1
    $\begingroup$ Ok! Wasn't accusing, just curious. You are right that LPV is not certified to CAT-I standards but it's certainly more precise (and more common) than PAR, and I have only heard of one MLS approach existing in the world, a curved one in Alaska. There may be more, but it is ultra rare. $\endgroup$
    – egid
    Jun 2, 2015 at 14:46
  • 1
    $\begingroup$ Okay thank you for the info :) I was asking about those systems because appeared on my book and it didn't provide a lot of information and I wanted to know the differences. That must be the reason, that they are more rare to find. $\endgroup$
    – Airman01
    Jun 2, 2015 at 14:51

1 Answer 1


At least in the U.S., ILS is popular because it is popular. It's relatively cheap to install and operate, the receivers share technology with VOR, and it is already installed everywhere. Momentum (and cost of replacement) has a lot to do with the failure and success of navigation systems.

MLS requires new hardware on the ground and in aircraft. The FAA tried to roll it out in the U.S. in the 80s, with a goal of replacing ILS by 2010, and utterly failed. The equipment was expensive for aircraft and ground facilities, and the cost of developing the approach procedures was high, as well as each approach needs tailored antenna programming (ILS is reasonably standardized). The adoption rate was low and the benefits (better arrival area guidance, better accuracy in some cases) were duplicated by the much cheaper GPS.

Precision GPS (LPV or LNAV/VNAV approaches) only appeared with the development of the Space Based Augmentation System (SBAS), also known as the Wide Area Augmentation System (WAAS). Augmented GPS has higher vertical precision and allows a single navigation system to be used for enroute, terminal, and precision approach guidance, a huge cost savings. The relatively new Ground Based Augmentation System (GBAS, originally called LAAS) adds high precision approach capability bringing GPS approaches closer to ILS and MLS.

PAR exists, but you may misunderstand what it is: it is a controller staring at a radar scope and issuing pilots instructions. It's stressful for pilots and controllers and very time consuming. Outside of military facilities you would be hard pressed to find a place to practice a PAR these days, at least in my experience.

In order of precision, high to low:

  1. MLS
  2. GPS with GBAS (uncommon)
  3. ILS
  4. GPS with WAAS (common, aka LPV)
  5. PAR

In order of (estimated) total cost, high to low:

  1. MLS (very expensive install + unique approach design + maintenance)
  2. PAR (relatively cheap install + costly workforce; educated guess on my part, could be cheaper than ILS?)
  3. ILS (expensive? install + simple approach design + maintenance)
  4. GPS with * (no install, simple approach design, no obvious maintenance)
  • $\begingroup$ Thanks so much for the answer, and yeah I was misunderstanding what a PAR was, now it's much clear and I can see that is stressful and time-consuming. Good answer! $\endgroup$
    – Airman01
    Jun 2, 2015 at 15:33
  • $\begingroup$ I would think that PAR is cheaper then ILS... As far as I know it is "just" a "more sensitive radar", and those are installed at bigger airports anyway (for example for ground operation and guidance)... In other words, PAR approaches (as I was told) are in most cases only a by product and the equipment has another function it is used for as well. Good comparison though! $\endgroup$ Jun 3, 2015 at 15:58
  • 1
    $\begingroup$ @Maverick283 It's not just more sensitive radar; it's significantly more staffing and reduced airport capacities. I've never seen simultaneous PAR approaches discussed, let alone multiple going on at once. Compare with airports running simultaneous parallel ILS approaches, often being managed by one or two controllers. Multiple PARs would require one controller per airplane landing. $\endgroup$
    – egid
    Jun 3, 2015 at 16:00
  • $\begingroup$ Oh yeah I agree, as you were talking a lot about installing costs I assumed that list would be a lost of installing costs... But in running costs, of course the PAR is highly expensive just because of the staff, totally agree! I have heard a PAR approache being flown once on the radio, that poor guy had an receiver failure and couldn't get down in the ILS. I believe that is about the only time they really do PAR approaches - emergency... $\endgroup$ Jun 3, 2015 at 16:03
  • $\begingroup$ I'll update my answer to make it obvious that I'm talking cost over time. I don't think the install + mx on an ILS for, say, 20 years would exceed the cost of constant PAR approaches for 20 years, but obviously that's conjecture. $\endgroup$
    – egid
    Jun 3, 2015 at 16:04

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .