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

[Matt Bailey]

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