My example- I am flying a visual VFR conditions and using a published RNAV. i would like to use the three points along the way as points to reach a certain altitude 2700/2000/900 and finally if desired, the runway. As mentioned previously I do this today and it works, it is not IFR and I would not expect that from a non WAAS certified GPS, but I sure would like to have that line reference on my HSI that makes it so much easier to look at the screen as if flying a virtual glideslope.
Everything I would want is already in place today, I just would like the glideslope line in additional to the arc on the MAP, by having this I can see the location on the MAP I want to reach at a certain altitude, can program that VSI than remove the MAP screen to allow for reaching the current arc point without a need for a MAP to be displayed to see if I remain on target.
There is a huge difference between a line on the glideslope, which indicates if you are above or below a virtual line in the sky which is at a specific angle out of the runway touchdown point, which does not move as the airplane moves, and a VSI command which takes where you are right now and tells you how fast to descend to be somewhere, but does not define any specific path in the air.
We already have VSR (vertical speed required). Give SkyView a flight plan to an airport, and look on your VSI bar. There's a pink line, which is the speed you need to descend at to be 1,000' above the airport 1NM before it. You can adjust the height and distance in setup. This will take you from wherever you are to that point and adjust for your ground speed as it changes.
Of course, this guidance shuts off when you get 1,000' above the airport, well within VFR minimums, but not quite useful for IFR.
If you want a non-precision step- down, what do you expect the guidance to be in-between points? For it to build a line between the altitude points and drive the HSI to this? Or to drive some VSI indicator at the VSR between points? What do you expect it to do at the last point to the runway? If you want guidance from the last point to the runway, isn't that a precision approach?
When people say "Synthetic Approach", generically I think:
1) Horizontal guidance on runway course out from the runway (like a Localizer), which can drive the CDI on the HSI.
2) Vertical guidance out of the touchdown point on the runway (not the runway end) at a specific angle that provides terrain and obstacle clearance. This can drive the GS needle on the HSI. This guidance stays active until the plane touches the ground. Exactly like a glideslope.
3) A method to build this out of any point in the world as defined by the user, not just for built-in airports.
4) A UI to activate and terminate this guidance