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## Re: [Paparazzi-devel] Navigation schema ?

 From: address@hidden Subject: Re: [Paparazzi-devel] Navigation schema ? Date: Mon, 16 Jun 2008 10:32:06 GMT

```Famille,

It is important to note that we do not have any "heading" function whatsoever.
The navigation control operates only on bearing and uses one of two schemes:

In a simple "goto wp" the carrot jumps to the wp and the plane just tries to
turn toward that point.  The result is some random path that eventually reaches
the wp (of course it's theoretically possible to end up circling the wp many
times before reaching it).  This is the navigation scheme used between
non-adjacent blocks, when Auto2 is first initiated, or when an active waypoint
is moved.

For a "route" or circle, the carrot exactly follows the line, some fixed
distance ahead of the plane, and the plane always tries to turn toward the
carrot.  This gives a very accurate path as the course is tightly controlled.

>  What do you call a real route ?
Assume the plane flying north (heading 0°). The waypoint you target is
currently 200m east from the current position (bearing 90°). My PID loop has
90 - 0 as input and acts on the rudder. the plane will start flying more to
the right. In the next cycle, I'll have may be heading = 5,  bearing=92, but
that's not something I can really compute in advance. I just let the PID loop
regulate it.

>  It does not make a difference only if you are currently on your desired
> track. Usually you don't want only to go somewhere, you want to follow a
> track all the way until the waypoint is reached.

>  You can compute a carrot position as soon as you are able to project the
> current position of your vehicle on the route you want to follow. Easy with
> straight lines and circles ...
>
> --Pascal

Ach so !
Hmmm. That's not always easy.
Assuming you're close enough to the desired route, and the difference
between
heading and bearing is small, you can place the carrot at the intersection
between the desired route and the "Carrot circle", carrot seconds around the
plane. Correct ?
Figure 1

Now If you're far from the route or your current heading is way off the
bearing to the carrot, you have to change the strategy, and compute a more
complex desired route (ex: figure 2) ? This will typically be the case just
when you switch the autopilot on. Can you do this on the AVR ?

Is this the idea ?

Thanks Pascal
Martial

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