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DIY ILS app (GPS-ILS)

It would be wrong for a threshold position, but it belongs to a point along the centreline 250m inbound of the threshold. A 3 degree glideslope projected from this point would cross the threshold at approx. 50ft which is consistent with a normal approach. Unfortunately the manual does not say which point the coordinates actually designate.

…. and there lies a problem. The given point is actually an arbitrary position in the touchdown zone roughly abeam the PAPIs, (to be a little more precise 348.17m from the threshold). The selection of such a location for the ‘aiming’ point and generation of a notional GP clearly demonstrates a lack of understanding regarding instrument approaches and their design – to speculate that touchdown should occur abeam the PAPIs and then engineer a glideslope from this point is naive.

The manner in which IAP are created, validated and calibrated can only be done with reference to the runway threshold and TCH/MEHT. The ‘touchdown’ point isn’t a specified location and can only be reverse engineered from the TCH and the threshold location. In this particular circumstance, the TCH is defined as 51ft and the GP angle measured as 3.01 degrees. Do the maths with those numbers and you get a theoretical ‘touchdown point’. You also need to take into account differences in elevation between threshold (31.205ft) and Touchdown Zone (31.489ft).

Whilst all of this is rather academic, there’s a need to understand that playing around with GPS ILS (other versions are available) brings risk. Sure, following it to CAT I minima probably isn’t going to cause a problem.

PS. My business is flight calibration. :)

Last Edited by Dave_Phillips at 24 Jul 11:20
Fly safely
Various UK. Operate throughout Europe and Middle East, United Kingdom

…(to be a little more precise 348.17m from the threshold)

You are right, I made a typing mistake when I wrote 250m. My calculator also showed 350m.

The selection of such a location for the ‘aiming’ point and generation of a notional GP clearly demonstrates a lack of understanding regarding instrument approaches and their design…

I think it’s a good approximation if you have nothing but the runway coordinates. Probably rounded several times during conversion from HMS to decimal format and back, so only accurate to three or four decimal places anyway…
The only way to know for sure what he bases his calculation on will be to ask the developer (who, by matter of coincidence, learned to program in FORTRAN using punch cards together with me ) . He flies since 1985 or so, holds a ph.d in aerospace engineering and has a brother who captains A380s. I strongly doubt that “lack of understanding” is the underlying problem here.

The ‘touchdown’ point isn’t a specified location and can only be reverse engineered from the TCH and the threshold location.

Yes sure. But on the other hand, TCH is not easily available, at least not from machine-readable sources in the public domain, so all he can use are the published coordinates.

Whilst all of this is rather academic, there’s a need to understand that playing around with GPS ILS (other versions are available) brings risk.

A lot of risk, same as with all homegrown approach procedures. But as a backup if everything else fails, following this App is surely better than just cutting the power, closing your eyes and wait for the impact.

Last Edited by what_next at 24 Jul 11:50
EDDS - Stuttgart

So, let me get this right, should GPS-ILS show the ILS correctly (or close to) when you are tracking a real ILS?

And should it show an “ILS” correctly when you are tracking the LNAV+V advisory glideslope?

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

So, let me get this right, should GPS-ILS show the ILS correctly (or close to) when you are tracking a real ILS?

And should it show an “ILS” correctly when you are tracking the LNAV+V advisory glideslope?

Sounds like it should be close but on the second question it will depend on whether the angle of the GP aligns with the setting in the database in the app. Overall a good product though and well designed. As said earlier though as it uses a track based-HSI, don’t allow for drift – align course pointer straight up.

EGTK Oxford

Peter wrote:

So, let me get this right, should GPS-ILS show the ILS correctly (or close to) when you are tracking a real ILS?

And should it show an “ILS” correctly when you are tracking the LNAV+V advisory glideslope?

There’s no guarantee of correlation. An ILS GP is derived from the TCH and GP angle. There is no ILS system on this planet which provides (sensible) GP information beyond TCH. It looks as if GPS ILS has basically picked a point on the runway abeam the PAPIs and then driven a GP out from here. Clearly they will be pretty close to each other but the baseline assumption is wrong.

The other issue is that ILS GPs are rarely exactly 3.00 deg – there is an allowable fudge factor of 0.02 deg on a Cat III and far more on Cat I/II and these figures drift with time. If the GPS GP has been set at 3.00 deg (which is mathematically quite easy to do ) it will not necessarily correlate, with the error clearly being more noticeable at range.

LNAV+V – this is designed in the same way as an ILS in that it uses the TCH/Threshold as the reference point.

The numbers are small and to the average GA pilot largely irrelevant. The problem is that if someone wants to develop pseudo ILS capabilities (even with the “guidance only” health warning) they need to understand how everything else works.

I’m off to a regional airport with Cat III in a couple of weeks; I’ll take GPS ILS with me and compare it against real data.

Last Edited by Dave_Phillips at 24 Jul 17:39
Fly safely
Various UK. Operate throughout Europe and Middle East, United Kingdom

Just tried it out on an ILS approach into my homebase at around 11PM. Coudn’t pay too much attention to the app though, because there were big thunderstorms with lightnings uncomfortably close by… and ATC and other aircraft asking about it and whether the departure routes were still clear (only to the north, to the south the WX radar showed solid red).

I never saw the computed glideslope needle within range, so at some point I switched the correction on (Samsung Galaxy Prime phone), which must be the dip at around 4,5NM. This brings the (now corrected) GPS altitude closer to the computed glideslope, but the offset is still big enough to get full GS deflection. The autopilot was flying and could hold the glideslope very well despite the turbulence. The slope of the computed GS and that of the real one are identical, but there is an offset of something like one hundred feet between them.

EDDS - Stuttgart

Very interesting!

100ft above the real GS would be fine…

I am also concerned that there is no obvious indication of a GPS fix loss. Yesterday I was testing it at the office and found that even the accuracy figure just freezes. Maybe an android app really has no means of checking the GPS fix via the API provided?

We had a debate on that here, in a different but similar context.

Administrator
Shoreham EGKA, United Kingdom

I sent the GPS-ILS log to the developer who says I was above the glideslope and should have used the altitude correction set to ON.

I wish the ON setting could be saved. The app loses it when it exits.

Administrator
Shoreham EGKA, United Kingdom

There is a workaround for saving the alt correction: edit the state.txt file directly and change the last entry on the 1st line from 0.0 to 1.0.

I have it by email from one of the project participants (not the developer of the app, Dierk) that the app was “not trying to mimic existing procedures”. I suggested that he posts detail here but he has refused, describing the people here who were critical of it in less than polite terms.

I still think the app has a lot of potential, especially for the purpose I mentioned above (flying nonprecision approaches, clearing the SFDs on the way down) but it appears clear to me that one needs to do some database verification to see what kind of “glideslope” was actually coded.

Logically, where there is an ILS, the ILS GS should have been coded. Where there is a GPS/LNAV approach with a LNAV+V “advisory glideslope” coded, that glideslope should have been coded. That much is just obvious. Why code anything else? You may as well take advantage of the obstacle surveys which have been done, checked and published.

If you are doing a DIY ILS into some grass strip, then all bets are off, of course. I would still code a 3 deg ILS, but that might be prevented by obstacles.

Administrator
Shoreham EGKA, United Kingdom

There was some pretty harsh (and borderline personal) negativity earlier in the thread which was allowed to go unchecked – what do you really expect in return?

The root problem is trying to compare this with a conventional ILS (or indeed a surveyed LPV approach) – it is neither. It’s a free app running on a bit of cheap consumer electronics.

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