How does it work? (GPS approach)

[khaos1ft]

I have a question about the principles that stand behind the vor/loc/gs indication provided by some GPS systems. Also, I want to learn more about the GPS approach methods...

What I need to know is how indication gets to the vor/loc/gs indicator...
That's general !

More specific, it's about the Garmin 500 with the Garmin GI-106A VOR/LOC/GS/GPS Course Deviation Indicator (CDI). First, I thought that this indication is coming as a parallel indication from the VOR/ILS receiver (KURS MP 2), but it's not.

I think (but it's more a guess and I didn't find any documentation at the first research) that the GPS being all about the positions, it could compute an ideal landing track, knowing of course the position of the runway (at least two points of interest).

About the precision, I personally think you cannot compare this kind of indication with the one provided by a classic VOR/ILS receiver.

[Kevin Horton]

Originally Posted by khaos1ft

More specific, it's about the Garmin 500 with the Garmin GI-106A VOR/LOC/GS/GPS Course Deviation Indicator (CDI).

Do you have the Garmin Aera 500 handheld GPS, or the Garmin GPS 500 panel mount IFR approved GPS?

[pierre smith]

Originally Posted by khaos1ft

..........About the precision, I personally think you cannot compare this kind of indication with the one provided by a classic VOR/ILS receiver.

With a Garmin 430W, the precision is incredibly good and it auto-zooms as well, during the approach.
It will show the lines on the runway, plus you can manually zoom some more on the ground and it'll show you EXACTLY where you are in relation to the center lines!

I don't see any advantage of using the old ILS system compared to the LPVS we have now.

Best,

[Sig600]

Originally Posted by khaos1ft

About the precision, I personally think you cannot compare this kind of indication with the one provided by a classic VOR/ILS receiver.

CAT 1 GPS approaches are just as accurate, if not more so than ILS. GPS non-precision approaches are WAY more accurate than a VOR approach. If you look at the actual numbers on both (glide slope, localizer beam width, tolerances, vor accuracy over distance) you'll see that GPS (especially WAAS) is light years ahead of the old stuff.

[Brantel]

The GPS has a database of approach parameters. It knows where you are in 3D space. It crosses that position with the database and calculates the amount of error there is for where you are vs where you are suppose to be in the approach. It can use the 106A CDI and GS needles to display this error.

VOR/LOC/GS indicators are a fixed scale with the variable being in the signal. The closer you are to the transmitter, the more sensitive the needle is.

GPS indicators have variable scaling since the navigation source is always the same sensitivity. Approach certified GPS's automatically change the scale of the indicator as you progress on the approach to make it more sensitive close in than farther out.

If your indicator is indicating GPS data, what you see on it has nothing to do with the VOR/LOC/GS (or Nav) side of the NAV/GPS radio. Same is true when it is indicating NAV data, what you see has nothing to do with the GPS side of the radio.

[aerhed]

That was very succinct Mr. Brantel.

[breister]

There is one additional esoteric difference I have read about between a precision GPS and an ILS approach, but it is mostly unnoticeable from a flying perspective. An ILS uses a radio beam which guides the aircraft vertically down a straight line through space. But since the earth curves, over a long distance your vertical speed (feet per minute) to stay on the beam will vary slightly as you approach the runway. For a precision GPS approach, the vertical aspect will lower you a certain number of feet per nautical mile from the FAF to the DH, so your vertical speed should remain constant (no wind, no vertical shear conditions).

Some pilots on the GNS 480 forum discussed this at length, with a few of them flying "20 mile straight ins" and comparing the ILS and GPSS vertical steering to confirm this. The GPS vertical steering will, I believe, trend you initially above the ILS with a constant VVS while ILS will require an initially higher VVS to "ride the beam," dishing out a bit near DH.

Just more interesting and virtually useless information...