ILS vs LPV

[EricJ]

I think there are fewer potential impairments to the reception of LPV than ILS.   The FAA says a GPS glidepath is more stable than an ILS glideslope.   Personally (radio comm engineer here), I have more confidence in LPV guidance than ILS, although there's really nothing wrong with either.   About the only thing that impairs the GPS signal is intentional jamming, while ILS (which is also subject to jamming, but probably less likely to be jammed), has some other impairments, like snow and range sensitivity, etc., that can be problematic at times.   

https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/library/factsheets/media/WAAS_QFSheet.pdf

 

[kpaul]

56 minutes ago, EricJ said:

has some other impairments, like snow and range sensitivity, etc., that can be problematic at times.

Yep, like someone holding short and not knowing the difference between the hold short and instrument hold lines, or on days when the weather is above 800/2 and the ILS critical area is not protected.

[jaylw314]

14 hours ago, ilovecornfields said:

I’ve noticed a lot of airports have ILS and LPV approaches to the same runway with similar minimums. I usually pick the LPV approach but I’m wondering what other people do and the reasons for it.

My thoughts are that the LPV approaches have better monitoring and the missed approach is usually easier. Not sure if there are any advantages to the ILS.

This varies from airport to airport, but one trend seems to be that the transitions and feeders into ILS approaches are shorter than those of LPV approaches.  This may or may not be significant if you're trying to save time, but I imagine controllers might prefer the ILS if it's faster to give them more options.

For example, at my home base at KCVO, the ILS 17 approach has feeders into LWG, about 7 nm from the runway.  You do the procedure turn and you're about 10 miles out.  The corresponding RNAV 17 feeds you into INNOP, about 12.5 nm from the runway.  You do the 4 nm hold-in-lieu-of-PT, then you're about 16 nm out on final.

Of note, yesterday I was doing some approaches.  We had a fantastic cloud layer from 400 AGL up to about 1200 AGL (my wife thinks it strange I get excited about "perfect" bad weather).  I did an approach elsewhere, then came back to do the ILS 17 at KCVO.  I hadn't picked up the ILS yet, when approach cleared me when established and sent me to the CTAF.  However, I flew right through the approach course without picking up the ILS.  I hesitated for a second, then called approach back and let them know I hadn't picked up the ILS and wasn't sure why.  I asked for the RNAV, and he said sure and gave me vectors to final.

I tuned and double-checked the radios while circling back, but I kept convincing myself I had simply must have tuned the wrong frequency (since I flew a different approach earlier).  As I got busy I just gave up and figured I'd check it on the ground.

The RNAV and ILS approaches have the same DA's, and the cloud bases was more like 300-350 AGL, but still a pretty easy transition.  Once on the ground, I heard approach talk to a Baron asking for the ILS approach, and he mentioned my problems.  I double-checked my second NAV radio and the ILS frequency, and sure enough, neither were picking up the ILS signal.  I called approach to cancel IFR, and mentioned I was not picking up the ILS on the ground with two different radios.  At that point, the controller mentioned he found a NOTAM that the ILS was OTS, and had been for a couple days.  I looked at the NOTAMS before and after, and I still can't find that NOTAM, though.

Take home story is that if you can do both, it's probably best to practice both...

[jaylw314]

1 hour ago, EricJ said:

I think there are fewer potential impairments to the reception of LPV than ILS.   The FAA says a GPS glidepath is more stable than an ILS glideslope.   Personally (radio comm engineer here), I have more confidence in LPV guidance than ILS, although there's really nothing wrong with either.   About the only thing that impairs the GPS signal is intentional jamming, while ILS (which is also subject to jamming, but probably less likely to be jammed), has some other impairments, like snow and range sensitivity, etc., that can be problematic at times.   

https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/library/factsheets/media/WAAS_QFSheet.pdf

 

I imagine it is relatively easy to jam ILS although not by jamming the source.  If you install a transmitter along the final approach path about 5 miles out, you're about 10 times closer to aircraft on approach than the ILS, so a transmitter 1% of the power could in theory spoof aircraft, at least temporarily until the aircraft gets closer to the ILS.  Anyone know how many watts the ILS transmitters are?

[M016576]

13 minutes ago, jaylw314 said:

I imagine it is relatively easy to jam ILS although not by jamming the source.  If you install a transmitter along the final approach path about 5 miles out, you're about 10 times closer to aircraft on approach than the ILS, so a transmitter 1% of the power could in theory spoof aircraft, at least temporarily until the aircraft gets closer to the ILS.  Anyone know how many watts the ILS transmitters are?

Not sure- but they are only spec’d to the feather- so probably about 2-3x less than a low vor?  Just guessing

Edited December 9, 2018 by M016576

[carusoam]

Here goes an attempt at a summary....

1) Choices... good to have two.  Local ground based vs. satellite based.

2) Linear math and rectangular coordinates, measured in tenths of miles vs. angular math and things measured in degrees... in the end, we are following needles over a range of dots... going off-scale initiates the same thing...

3) Identifying the ILS still requires patience and the knowledge of Morse’s dots and dashes... vs. a RAIM check...

4) Scalloped lines and mountain or building shadowing vs. blocked signals and magenta lines...

5) Nobody mentioned using their WAAS technology to simulate performing an ADF based approach... one thing for sure, you can’t use your ADF to simulate a WAAS approach.  But, you can receive broadcasts from many AM radio stations... 

6) Greatest Gen, Boomer Gen, Gen X, Gen Y, Millenial.... Analog needles vs. digital displays... the tough part is the narrow range in the middle... where their digital nav equipment is driving analog needles.... 90s Vettes and Mooneys are full up with this GenX specific stuff!

7) Thanks to the military aviators for offering the great details on various guidance systems.  We probably wouldn’t have civilian GPS without the military leading the way...  and of course somebody in government not wanting to allow civilian use because that would allow evil-doers to target buildings in NYC...  the evil-doers used rental trucks and street maps, or hijacked planes on a VFR day...

8) The coolest response to this discussion... the MSers that independently tune both the WAAS and the ILS on independent units...

9) The most uncool failure of the dual GPS boxes... dueling GPSi, and antennae induced interference... early antenna swaps to GPS capable antennae, re-used mounting locations and accidentally placed two antennae too close together.  The interference was somewhat random and completely blocked out the nav capability of both devices.... on many occasions...

10) The coolest part of MS... no matter what Gen you are physically associated with... you get to compare and contrast with the other Gen’s experience... there is greatness in all Gens!  it is still people that make these systems work.

PP summary of what I think I read...

Best regards,

-a-

 

[jaylw314]

2 minutes ago, carusoam said:

Here goes an attempt at a summary....

1) Choices... good to have two.  Local ground based vs. satellite based.

2) Linear math and rectangular coordinates, measured in tenths of miles vs. angular math and things measured in degrees... in the end, we are following needles over a range of dots... going off-scale initiates the same thing...

3) Identifying the ILS still requires patience and the knowledge of Morse’s dots and dashes... vs. a RAIM check...

4) Scalloped lines and mountain or building shadowing vs. blocked signals and magenta lines...

5) Nobody mentioned using their WAAS technology to simulate performing an ADF based approach... one thing for sure, you can’t use your ADF to simulate a WAAS approach.  But, you can receive broadcasts from many AM radio stations... 

6) Greatest Gen, Boomer Gen, Gen X, Gen Y, Millenial.... Analog needles vs. digital displays... the tough part is the narrow range in the middle... where their digital nav equipment is driving analog needles.... 90s Vettes and Mooneys are full up with this GenX specific stuff!

7) Thanks to the military aviators for offering the great details on various guidance systems.  We probably wouldn’t have civilian GPS without the military leading the way...  and of course somebody in government not wanting to allow civilian use because that would allow evil-doers to target buildings in NYC...  the evil-doers used rental trucks and street maps, or hijacked planes on a VFR day...

8) The coolest response to this discussion... the MSers that independently tune both the WAAS and the ILS on independent units...

9) The most uncool failure of the dual GPS boxes... dueling GPSi, and antennae induced interference... early antenna swaps to GPS capable antennae, re-used mounting locations and accidentally placed two antennae too close together.  The interference was somewhat random and completely blocked out the nav capability of both devices.... on many occasions...

10) The coolest part of MS... no matter what Gen you are physically associated with... you get to compare and contrast with the other Gen’s experience... there is greatness in all Gens!  it is still people that make these systems work.

PP summary of what I think I read...

Best regards,

-a-

 

Anthony, do you take a vitamin for "positive-thinking" daily?   

[carusoam]

Jay,

Watch how a good message can get completely lost,  when the positive attitude gets left out...

Of course, i’m Happy to be alive every day too.

Best regards,

-a-

[David Lloyd]

9 minutes ago, carusoam said:

..

5) Nobody mentioned using their WAAS technology to simulate performing an ADF based approach.

 

Actually did that in VFR conditions a couple months back just to see if I could still remember how.  Yep.

[jaylw314]

Just for S&G's, here's a video of my approach yesterday.  Thanks to @gsxrpilot for suggesting neutral density filters, I think I might have gone too dim so I'm going to try the next darkness up next time...  Yes, I know, it's not my best landing ever 

 

[Bob - S50]

Mostly already covered here, but given the same minimums I'll take the LPV and dial up the ILS on the KNS80 for the backup CDI.

I prefer the LPV because I do not have to worry about beam bending due to terrain and traffic.

I'm not afraid to ask either.  If they tell me to expect the ILS and I prefer the RNAV, I'll ask for it.

[Piloto]

One issue specially in winter is ice on the LOC and the GS antennas on the ground stations. It can change the ratio of the 150/90hz antennas and cause the signal ground monitor to shutdown the transmitters momentarily.

Unlike CAT II ILS (CAT III have DME), all the LPV approaches have distance to threshold and accurate altitude even if you forgot or if not available to set the baro altimeter. I found this helpful to achieve the proper speed at touchdown.

José 

Edited December 9, 2018 by Piloto

[ArtVandelay]

Just for S&G's, here's a video of my approach yesterday.  Thanks to [mention=11970]gsxrpilot[/mention] for suggesting neutral density filters, I think I might have gone too dim so I'm going to try the next darkness up next time... 

I would add a 2nd camera (interior view), makes video more interesting to switch between different angles.

[jaylw314]

1 hour ago, teejayevans said:

I would add a 2nd camera (interior view), makes video more interesting to switch between different angles.

I'm struggling to find a second place to mount a camera.  I've tried the side windows, but my (cheapo) suction cup mounts keep falling off once I climb over 4000'.  Why the windshield suction cup sticks but the windows won't I cannot understand.  I've tried adhesive mount on the headliner, but anything on the interior plastics gives too much jello.  

[Wayne Cease]

Whichever ATIS says is for the active. Or whichever is set-up for an easier approach from the direction I'm coming from.

Only time I really care is if one get me lower and the ceiling requires that little bit lower.

[toto]

I don't *think* this specific point has been raised (apologies if so), but one minor advantage to the LPV is that the pilot must take a positive action to select a GPS approach using any equipment.

For many ILS-capable radios, there's no visual confirmation of a selected approach. You listened for an audio ident, well, you're pretty sure you did, but you also flipped back and forth to verify the stepdown fix etc. Are you really sure?

Especially in congested airspace with a lot of localizer frequencies within range, it's awfully easy to choose the wrong one. But the GPS is going to make you read the approach name on the screen and confirm. 

[gsxrpilot]

I'll just mention here (not to call out @chrisk and the GTN650) but my IFD540 loads the ILS or the LP/LPV approach exactly the same. All I have to do is select the ILS off the list of available procedures, it sets up the radios appropriately and flies the approach. So from a button pushing perspective, there is no difference for me between an ILS, VOR, or any GPS approach.

It was also mentioned that the LPV can be followed below minimums whereas the ILS can't? I shot the ILS into KOKC one day and only acquired the lights (barely) at 200 ft. I'm sure the autopilot continued to follow the ILS lower. I didn't punch the red button until I had the runway clear in my sights and was rounding out to land. 

[Jim Peace]

5 hours ago, Bob - S50 said:

I prefer the LPV because I do not have to worry about beam bending due to terrain and traffic.

It may bend due to gravitational lensing...if gravity can bend a beam of photons then I bet it can bend a GPS data....but maybe we all bend with it and its transparent to us?  Thoughts?

 

[Hank]

21 minutes ago, Jim Peace said:

It may bend due to gravitational lensing...if gravity can bend a beam of photons then I bet it can bend a GPS data....but maybe we all bend with it and its transparent to us?  Thoughts?

 

If everything bends the same way, it may as well all be straight. The point here is that ILS can simetines bend differently, and that's a problem. 

[PaulM]

27 minutes ago, Jim Peace said:

It may bend due to gravitational lensing...if gravity can bend a beam of photons then I bet it can bend a GPS data....but maybe we all bend with it and its transparent to us?  Thoughts?

No, all relativistic effects are already compensated for when providing the calculations. (the clocks in the satellites show relativistic time drift due to their orbital speed relative to the ground observer).  The main criteria that WAAS compensates for is transient differences in atmospheric propagation delay. 

The ILS safety area indicates that objects in the field of the GS/LOC antennas can effect the beam path.  We know that there are false lobes in the both the GS and LOC signals.   The result is that the LPV signal is a geometrically smoother line, it has no false paths. 

I suspect like many things in flying the earlier equipment had these effects to a greater degree, and the modern solid state units have less drift, both receivers and transmitters.    30 day VOR checks were necessary with tube radios.  When has any of us failed to have our solid state VOR spot on?. 

[gsxrpilot]

8 hours ago, jaylw314 said:

Just for S&G's, here's a video of my approach yesterday.  Thanks to @gsxrpilot for suggesting neutral density filters, I think I might have gone too dim so I'm going to try the next darkness up next time...  Yes, I know, it's not my best landing ever 

I think the video's GREAT! Watching everything basically white out and then the first thing you see are the ALS lights. The feeling one gets when solo in the airplane breaking out to see those lights right off the nose. Love it!

[M016576]

2 hours ago, gsxrpilot said:

 

It was also mentioned that the LPV can be followed below minimums whereas the ILS can't? I shot the ILS into KOKC one day and only acquired the lights (barely) at 200 ft. I'm sure the autopilot continued to follow the ILS lower. I didn't punch the red button until I had the runway clear in my sights and was rounding out to land. 

I think what the other poster meant was that with an ILS- you aren’t supposed to fly it below the DH, legally, whereas with a LPV you can, assuming you have the runway environment in sight.

personally- I don’t know if this is accurate/true- but that’s what I believe the other poster was saying.

i do know that the ILS signal does not “disappear” right at DH- it continues all the way to the antennas.

Edited December 10, 2018 by M016576

[jetdriven]

You fly both the the DA, which, physically speaking, the pilot decides to miss at the DA, (Decision altitude.. Decide to miss or land)... initiates the missed approach and continues on. Both result in the aircraft continuing below the DA before arresting the descent and climbing. LNAV or LNAV/VNAV are different.

 

DA stands for "Don't add"... airline guys will get this"

(don't add 50' to the MDA for missed transition allowance)

Edited December 10, 2018 by jetdriven

[ilovecornfields]

Thanks for all the responses! Sounds like both are reasonable options and I’ll continue to do what I’m doing which is to fly the LPV approach unless the ILS has lower minimums and it looks like that will be an issue.

[Jim Peace]

24 minutes ago, M016576 said:

I think what the other poster meant was that with an ILS- you aren’t supposed to fly it below the DH,

if there was ever an approach that could be flown below a DH it is an ILS.....below DA or MDA is a different matter....