[simpits-tech] GPS craziness and "desired track"

[Jeroen Huijben]

Your method of finding where you should be sounds right.
But if you want to fly straight to this point you will have to fly perpendicular to the desired track and if you do this until you reach it you will overshoot and end up snaking along the desired track.
So you should add something to start the turn before reaching the desired track and make a smooth transition.

You could add a warning to start a standard rate turn when you are one turn radius away from the desired track.
That should place you on the track while flying along it.
Then you can switch to a course deviation indicator to show how far away from the track you are.

You could even seamlessly integrate each part and present only a CDI to the pilot.
To reach the desired track you will first fly a great circle perpendicular to it.
Then you fly along a small circle that touches the intercept track and the desired track.
Finally you fly along the great circle that is the desired track.
In each case the CDI would show the distance from the active great or small circle.

Regards,
Jeroen.


> Date: Sat, 5 Apr 2014 11:20:21 -0500
> From: dabigboy at cox.net
> To: simpits-tech at simpits.org
> Subject: [simpits-tech] GPS craziness and "desired track"
> 
> Alright, I think I am sorting out this GPS stuff, but I want to confirm
> something: am I correct in my understanding that once you lay in a
> flight plan/path on a GPS, it will ALWAYS show the DESIRED track,
> which of course is always changing (albeit slightly) because it's a
> Great Circle route?
> 
> This is important because on my GPS software, I was initially showing
> the ideal Great Circle route from the plane's CURRENT position to the
> selected waypoint (like "homing", but as a Great Circle route instead
> of straight-line). Now that I'm digging into the nuts and bolts of the
> routing and flight planning features, it's time to fix my GPS so that
> it actually lays in a set course and keeps it. This makes it necessary
> for me to continually plot the lat/lon of where the plane *should* be,
> if it were still following the planned course exactly, and then find
> the current course it should be tracking at that point in the original
> Great Circle route. "That point" is found by taking the current
> distance of the plane from the selected waypoint, and calculating the
> ideal/desired position of the plane on the desired flight path at a
> distance from final waypoint that is the same as the plane's current
> actual distance from the final waypoint (basically, you could draw a
> circle whose center is the final waypoint, and whose diameter is the
> distance of the plane from the waypoint, then find where that circle
> intersects the DESIRED/original flight path...that would be your
> "imaginary" ideal position).
> 
> Sound about right? I'm pretty sure no aviation GPS shows desired track
> as a course that puts you BACK on the original course, that's why we
> have a course deviation indicator (and possibly a "FLY" heading). And I
> know for sure that my original approach of just "homing" to the
> waypoint was incorrect, even in simple DTO mode.
> 
> -- 
> Matt Bailey
> 
> Keeping It Real:
> Sabreliner 60 Flight Simulator
> Serial # 306-61 - N1JX
> http://sabrelinersim.com
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