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Today I Learned: Default CZML "great cirlce" aerial paths can be made straight line with "arcType: NONE

 I spent a little bit of time doing math debug this weekend, but in the end it turned out the QSO mapping app had a visualization issue, not a math issue.

It was fun to get to look at the math for calculating the apparent launch angle of our antenna using F2 height data and rx/tx station locations. I wouldn't have thought to do the review except I had data that didn't match the maps I was getting back. According to the launch ange calculations made by our, (mine and KO6BTY's), QSO mapping app, the launch angle for the signal from our QTH was 0.00227 degrees. The map however, showed the path of the signal soaring over the very nearby Bay Bridge.


The angle shown is much larger than 0.00227 degrees. Here's a picture of the nearby Bay Bridge with our antenna in the foreground.


After completely reviewing the underlying math, it occurred to me that CZML likes to make lines that follow great circles. To make something that approximated a circle out of a path with a very low launch angle, the software might 'lie' about the launch angle. So, I asked ChatGPT 

"My polylines from the surface of the Earth into the sky are curved along great circles. I'd like for them to be straight lines because the represent light beams. How can I make them be straight? What should I specify in a czml file to make them straight lines?"

And it promptly replied that I should change the arcType setting of each aerial path line to NONE. A few minutes later, I had the correct, very believable map result.


Where the Earth starts to curve away, (KO6BTY's observation), from the almost tangent line somewhere out by Tracy, CA. Of course, that's probably not what the signal really did, it probably really did something like one of thse paths, number 4 or 5 to be specific.


but now, the map and the math agree.

Figure was taken from



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