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<div dir="ltr">> Date: Tue, 8 Apr 2014 19:15:46 -0500<br><div>> From: dabigboy@cox.net<br>> To: simpits-tech@simpits.org<br>> Subject: Re: [simpits-tech] GPS craziness and "desired track"<br>> <br>> On Sat, 5 Apr 2014 20:13:02 +0200<br>> Jeroen Huijben <jeroen_huijben@hotmail.com> wrote:<br>> <br>> > Your method of finding where you should be sounds right.<br>> > But if you want to fly straight to this point you will have to fly<br>> > perpendicular to the desired track and if you do this until you reach<br>> > it you will overshoot and end up snaking along the desired track. So<br>> > you should add something to start the turn before reaching the<br>> > desired track and make a smooth transition.<br>> <br>> This "intermediate" point isn't something I would navigate to, its sole<br>> purpose would be to find a bearing solution from this point to my next<br>> waypoint, so that I would get the correct desired-track value for that<br>> point in the Great Circle route.<br><br>Picture: <a href="http://imgur.com/gEmLOo7" target="_blank">http://imgur.com/gEmLOo7</a><br>Plane P flies from A to B.<br>All possible courses are divided into five sectors:<br>A course in sector a will be the best option as this will bring you to the desired track before reaching the destination.<br>In sector b you will fly towards the desired track but you will pass by the destination before reaching the desired course.<br><br>Courses in sectors c, d and e will take you away from the desired track, the destination or both.<br><br>Which specific course you take depends on what you want most.<br>If you want to stay on the desired track then the course a/e will take you there quickest.<br>This is the perpendicular intercept course.<br>If you want to get to the destination the quickest then the course a/b is a direct route.<br>Any course between a/e and a/b is a compromise between the two.<br><br>> > You could add a warning to start a standard rate turn when you are<br>> > one turn radius away from the desired track. That should place you on<br>> > the track while flying along it. Then you can switch to a course<br>> > deviation indicator to show how far away from the track you are.<br>> <br>> I am actually tied to the operation of the old 1990s Trimble GPS units,<br>> as I am basically re-implementing the Trimbe "OS" in Python (so that<br>> it can be interfaced to X-Plane, or even FSX/FlightGear/et al).<br>> The CDI is always available. I'm pretty sure the only "help" it's<br>> suppose to give the pilot is a fly-to heading, which I already have<br>> written into my software. I do not yet have it set up for course<br>> corrections, but I'm thinking I will simply make it a percentage of the<br>> course error.<br>><br>> One thing I'm not sure about is if the CDI/heading corrections are<br>> based on bearing, or cross-track error. I'm assuming it's bearing, as<br>> this is how a VOR works. Otherwise, you could be almost past the<br>> waypoint, but have only a small CDI deflection because your actual<br>> cross-track distance might be fairly small (a couple miles or so, for<br>> instance). But then maybe a real GPS works this way, I'm not sure.<br><br>This is what I would do, without knowing the methods used by any particular real GPS.<br><br>Another picture: <a href="http://imgur.com/7C6Vr8J" target="_blank"></a><a href="http://imgur.com/Fykzipp" target="_blank">http://imgur.com/Fykzipp</a> <br>Again, P wants to fly from A to B and wants to intercept AB at point I.<br>To do this it flies intercept course PI. (see above)<br><br>First the GPS will indicate the distance from the line PI on the CDI.<br>By keeping the CDI centered the plane will fly along PI towards I.<br>The course to steer is the course from P to I.<br><br>When the plane gets within distance d from the line AB the CDI will start indicating the distance from the point T.<br>By keeping the CDI centered the plane will fly an arc around point T and transfer from the course PI to the course AB.<br>The course is perpendicular to the direction of T and will rotate as you fly along the arc.<br><br>When the plane gets beyond distance d from the line PI the CDI will start indicating the distance from the line AB.<br>By keeping the CDI centered the plane will fly along AB towards B.<br>The course to steer will any course in sector a, as explained above.<br><br>As an extra you can calculate the difference between the heading and the groundtrack (=wind drift) and add that to the course to steer to get the right heading that will yield the right ground track.<br><br>Turns when flying through a waypoint can be handled in a similar fashion.<br>Picture: <a href="http://imgur.com/Flx2Vub" target="_blank">http://imgur.com/Flx2Vub</a> <br>First the CDI indicates the distance from one leg, then the distance from a turning point T and then the distance from the next leg.<br>Except here the GPS would switch between the three phases when it flies to specific radials from point T.<br><br>> I'm tempted to yank the Trimble antenna off the Sabreliner, rig up a<br>> portable power unit for the GPS, and take it up in a real plane so I<br>> can see how it behaves in the air. :)<br><br>Or reprogram it's database with the local roads and put it in your car :)<br><br>> > You could even seamlessly integrate each part and present only a CDI<br>> > to the pilot. To reach the desired track you will first fly a great<br>> > circle perpendicular to it. Then you fly along a small circle that<br>> > touches the intercept track and the desired track. Finally you fly<br>> > along the great circle that is the desired track. In each case the<br>> > CDI would show the distance from the active great or small circle.<br>> <br>> By "small circle" are you referring to something other than a Great<br>> Circle, or do you mean just a shorter Great Circle? :)<br><br>A small circle is any circle on the surface of a sphere that is not a great circle:<br><a href="http://www.mathworks.nl/help/map/great-circles-rhumb-lines-and-small-circles.html" target="_blank">http://www.mathworks.nl/help/map/great-circles-rhumb-lines-and-small-circles.html</a><br><br>> Matt<br>> _______________________________________________<br>> Simpits-tech mailing list<br>> Simpits-tech@simpits.org<br>> http://www.simpits.org/mailman/listinfo/simpits-tech<br>> To unsubscribe, please see the instructions at the bottom of the above page. Thanks!<br><br>I hope that clarifies things.<br><br>Regards,<br>Jeroen.<br><br></div></div>
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