LPV approaches ... GPS altitude?

Discussion in 'Cleared for the Approach' started by alanbreck, Jun 24, 2019.

  1. alanbreck

    alanbreck Pre-Flight

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    Please help me understand something about LPV approaches. They are flown like an ILS, and often allow the same minimums as ILS, but there is no ground infrastructure (all good stuff).

    But what is the altitude source driving the glideslope indicator during an LPV approach? Is it purely GPS altitude? I'd always heard that GPS altitude was not real accurate.

    Thanks.
     
  2. MauleSkinner

    MauleSkinner Final Approach

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    LPV requires WAAS...a Wide Area Augmentation System that basically makes the GPS signals accurate enough, both laterally and vertically, for the approaches.

    Keep in mind that you’re still using barometric altitude to determine DA...GPS altitude still can’t do that.
     
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  3. PaulS

    PaulS Final Approach

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    LPV glideslope is driven by the gps, but the decision height is based on your altimeter.
     
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  4. chemgeek

    chemgeek Cleared for Takeoff

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    WAAS is required for LPV approaches, which is a protocol whereby ground stations monitoring GPS signals broadcast correction factors to GPS satellites to increase the 3D fix accuracy for WAAS-enabled GPS units. The improved fix accuracy is sufficient to provide both lateral and vertical position integrity sufficient to fly a precision-like approach. Your WAAS receiver checks for adequate positional integrity prior to commencing an LPV approach.

    In short, your WAAS GPS is not so much providing an "altitude" as it is defining a 3D position in space to follow. While you will identify the DA with your barometric altimeter, it should be noted that WAAS vertical accuracy (95% envelope) is around 4 meters, which is pretty darn good!
     
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  5. Let'sgoflying!

    Let'sgoflying! Touchdown! Greaser! PoA Supporter

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    The GPS Story (free Prime video) link I posted recently in Book Club explains a lot of the GPS error corrections made, it's pretty incredible stuff.
     
  6. benyflyguy

    benyflyguy Pattern Altitude

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    It always blows my mind to start thinking of how the calibrate and account for the crazy Doppler effect created by these satellites whirling around
     
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  7. Let'sgoflying!

    Let'sgoflying! Touchdown! Greaser! PoA Supporter

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    They listed at least 4 things I had never heard of or considered. These guys were mathematical geniuses.

    So. Back to the OP's Q.

    The GPS+W system can make that electronic, descending, localizer-like line in space - and send that info to the airplane.
    However it cannot do the same for the airplane itself (determine it's 3-d location in space because it cannot figure the airplane's altitude with precision).

    Right (or close) so far?

    When the coupled approach tracks that precise GPS-derived GS info, it is tracking it very precisely, using the air data to do so.
     
  8. luvflyin

    luvflyin Final Approach

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    I think it can figure the airplanes altitude with precision. Otherwise, how could it tell you and/or the auto pilot whether you are on/above/below Glidepath? WAAS is what allows it this precision.
     
  9. dmspilot

    dmspilot En-Route

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    That's the whole point of WAAS—to increase accuracy. With WAAS the aircraft's altitude can be determine accurately enough to determine its position above or below the glideslope, which for LPV, is GPS-based, not baro-based. The problem with GPS altitude is primarily error due to the shape of the earth. However, when an LPV approach is designed, the data to correct GPS altitude with respect to the reference ellipsoid to true MSL altitude becomes part of the approach and part of the GPS database.
     
  10. aterpster

    aterpster Pattern Altitude

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    Wide Area Augmentation System. Augmentation is accomplished by both satellites separate from GPS, and includes ground reference systems. The augmented vertical component is more critical than the augment horizontal component. If either is out of tolerance there is no LPV or LP.
     
  11. Let'sgoflying!

    Let'sgoflying! Touchdown! Greaser! PoA Supporter

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  12. aterpster

    aterpster Pattern Altitude

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    The attached is the final approach segment "path record" for an LPV approach. An LP approach has a similar path record. These data have to be precisely coded into your WAAS navigator when you load such an approach from your database. None of this is required for an LNAV approach.
     

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  13. Velocity173

    Velocity173 Touchdown! Greaser! PoA Supporter

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    We used to use a tactical approach in Y code (encryption) that was non WAAS GPS altitude. Not as good as LPV but was still pretty accurate for government work.

    D33B63CC-C4CA-4153-A971-7D55A38CCFAB.jpeg
     
  14. luvflyin

    luvflyin Final Approach

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    Wow, Lat Long to 4 decimal places. That’s like about an inch I think
     
  15. aterpster

    aterpster Pattern Altitude

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    The performance of LPV is better than the typical Category I ILS.

    GLS (local augmentation) is better than a CAT III ILS. GLS is the future for the airlines. That's why they are not inclined to equip with LPV.
     
  16. luvflyin

    luvflyin Final Approach

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    Were those Numerous aircraft on the cratered 14R in one piece?
     
  17. Velocity173

    Velocity173 Touchdown! Greaser! PoA Supporter

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    At the time that’s where we parked our aircraft. After about a year, they improved the ramp and moved them there. They finally repaired the bomb craters and my last day there I witnessed the first FW landing (IL-76).

    There were several MIGs, F-1s, L-29s and Jet Provests lying around in various states of damage though. Would have been great war trophies to ship home!
     
  18. luvflyin

    luvflyin Final Approach

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    What were you doing there? Were you Army, if so what came first, Army or Marines?
     
  19. Velocity173

    Velocity173 Touchdown! Greaser! PoA Supporter

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    Army. 4th ID (Army Infantry) took the airfield. Old friend of mine flying a Black Hawks actually arrived just after the fighting and some infantry LT literally turned the airfield over to him. One of the first tasks for the company / battalion instrument examiners, is designing an emergency GPS recovery. After that, ATC rolls in with portable NDBs and PARs which were still used for either emergency IMC or VFR training during my time there. Over the next year the Al Sahra name was dropped in favor of Camp Speicher, an F-18 pilot KIA from Desert Storm.
     
    Last edited: Jun 25, 2019
  20. olasek

    olasek Pattern Altitude

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    With probability 95% the GPS-WAAS vertical error is less than 2 m so clearly it isn't that bad.
     
  21. flyingron

    flyingron Touchdown! Greaser!

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    I should clarify that the "satellites seperate from GPS" don't really provide any role other than broadcasting the WAAS correction data to your GPS.
     
  22. aterpster

    aterpster Pattern Altitude

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    That role is what makes LPV and LP possible.
     
  23. flyingron

    flyingron Touchdown! Greaser!

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    Of course. I was just clarifying that the other (INMARSAT C) satellites in the system are there purely for communicating data from the ground stations. The other key in addition to the ground reference update is that the military promised to not mess with the phase on the L1/L2 phase which gives the reasonable atmospheric propagation correction.
     
  24. aterpster

    aterpster Pattern Altitude

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    You are very knowledgeable. Not a trick question: Why does Hawaii have inadequate vertical coverage even though they have a ground station at PHNL?
     
  25. N1120A

    N1120A Line Up and Wait

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    Actually, most newer commercial aircraft and those with newer GPS systems are WAAS equipped, but they are not authorized to fly LPV approaches to LPV minimums. They can fly them to LNAV/VNAV minimums. Its kinda stupid.

    Current GLS approaches are only authorized to CAT I, though they will ultimately be certified to the same standards as CAT IIIA/B/C ILS approaches. According to a friend who regularly flies GLS approaches with Lufthansa, they are significantly easier to fly and more accurate than even an ILS.
     
  26. flyingron

    flyingron Touchdown! Greaser!

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    Dunno, they specifically provided for everything with 30 miles of shore as its own WAAS zone there. Originally, we used Inmarsat 3-F3 to serve the Pacific, but that's no longer in use. SES 15's footprint includes Hawaii, but I don't know if it's turned up yet. It was scheduled for IOC in 2019. The other one out there is Galaxy 15, which is having its own problems. It has only marginal coverage for the Hawaiian islands. it's sweetspot is several hundred miles north of the islands. Galaxy 15 will be pulled from WAAS when SES 15 is up.
     
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  27. olasek

    olasek Pattern Altitude

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    They may be WAAS "equipped" but are not certified for WAAS approaches. It may not take a lot to have them WAAS certified but there is probably little interest from airlines for extra expense and dubious utility.
     
  28. luvflyin

    luvflyin Final Approach

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    Is that some FAR 121 or other FAR thing? Or do the airline companies just not want to do it?
     
  29. chemgeek

    chemgeek Cleared for Takeoff

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    I think the commercial operators have to conform to their SOPs, which may or may not include GPS approaches. It is indeed ironic that I could fly the LPV approach into our field immediately after it was published--we were among the first few airports to get one commissioned--but the turbine operators were stuck flying our crappy VOR-A approach due to their SOP limitations.
     
  30. olasek

    olasek Pattern Altitude

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    It has nothing to do with "turbine", latest Gulfstreams for example can fly LPV approaches. Also many Alaska's 737s can fly very advanced RNP approaches which are probably the 'holy grail' of GPS approaches, it is a matter of what is needed by the operator and what makes financial sense in terms of aircraft certification and pilot training.
     
  31. aterpster

    aterpster Pattern Altitude

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    WAAS has to be integrated into their FMS. Those types of modifications are $$$$ for a large fleet. Keep in mind most modern air carrier airplanes have approach-certified Baro VNAV so they can fly to LNAV/VNAV minimums provided the airport temperature is within the limits stated on the approach chart.
     
  32. aterpster

    aterpster Pattern Altitude

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    Yes, but RNP AR does not require WAAS. It does require a whole lot of other things well beyond basic RNAV approach capability.

    Alaska's RNP AR approaches are all specials, so the procedures aren't publicly available. But, there are many FAA-developed RNP AR approaches that provide significant access advantage for other RNP AR equipped operators, such as KPSP, KGUC Rwy 24, KEAT Rwy 30, KLWS, KGPI Rwy 20, KMSO Rwy 29, and the list goes on.
     
  33. JeremyW

    JeremyW Filing Flight Plan

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    Oh cool I didn’t know this. Scott Speicher went to Florida State (my alma mater) and they named the tennis complex in his honor. I also was at a football game where there was an F18 fly over in his honor with one jet leaving the formation in a vertical climb with afterburner representing him going to heaven. Gave me goosebumps. I think my video of this is still on youtube.


    Sent from my iPad using Tapatalk
     
  34. Velocity173

    Velocity173 Touchdown! Greaser! PoA Supporter

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    Yeah there was a lot of controversy surrounding his death. There were rumors that he might have survived in an ejection. Think at the time he was still listed as MIA then ultimately changed to KIA. Either way, an appropriate honor to have the base named after him.
     
  35. N1120A

    N1120A Line Up and Wait

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    Pretty sure the LPV unavailability is linked to 121. 135 operators in heavy corporate jets can use LPV.
     
  36. aterpster

    aterpster Pattern Altitude

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    Pretty sure the 787 has LPV.

    I know Northern Air Cargo has had WAAS since 2013 (see attached).
     

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  37. MauleSkinner

    MauleSkinner Final Approach

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    121 operators typically fly to airports that have ILSs to most, if not all, runways. LPV capability would add significant expense in training and operations without much return on investment.
     
  38. N1120A

    N1120A Line Up and Wait

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    The 787 most definitely has the capability, but they can't use LPV minimums. The E175 has WAAS and LPV and the pilots can't use it.

    Most RNAV GPS equipped 121 aircraft have LPV capability, they just don't use it. I don't see where training would be significant, seeing that LPV is ILS accuracy without the radio sensitivity issues.
     
  39. MauleSkinner

    MauleSkinner Final Approach

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    Just to put round numbers to it...

    Let’s say they can train it with half an hour of ground and half an hour of simulator.

    Let’s say the airline has 50 airplanes, and is staffed at 4 crews per plane.

    That’s 400 hours of training per year, 200 of which is simulator time.

    That’s over a quarter million dollars a year just for the sim time, plus probably 40% of an instructor salary, plus whatever they pay pilots during training.

    Let’s say it adds up to half a million dollars a year, and they get two approaches a year where the ILS is out of service.

    ROI doesn’t justify it, and that’s just direct training costs. That doesn’t include administration, courseware development, proving runs, or any of the other stuff that lives in the background.
     
  40. luvflyin

    luvflyin Final Approach

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    Would they have the LPV capability 'disabled,' making it unusable? Or would they just say you are not allowed to use it?