I already spend a lot of time to find a working solution, how to follow the moving marker of a recorded GPX track resp. GpxDataSource by the camera, but I didn’t succeed.
It’s such a simple desire, but nothing works.
Perhaps I have to add another helper entity synchronized to the position property of the data source to be finally able to utilize the viewer’s trackedEntity method, or something similar…
I would be really happy, if anybody here could give me a hint, how to realize this kind GPX driven object and camera motion in the most simple manner like in the flight path interpolation sandbox examples.
Hi @mash-graz
I created a Sandcastle demonstrating how this can be currently achieved. It’s not ideal, but I think it achieves the result you’re looking for. Let me know if this solves your problem.
Thanks, @Gfycat.com for your very useful suggestions.
Yes – your sandbox example is indeed much closer to the desired result as any of my previous attempts (…which in fact didn’t work at all, because I couldn’t figure how to access the changing position information).
Nevertheless, I’m still not perfectly satisfied by your proposal, because this kind of changing the camera position in the tick-callback doesn’t support those relative free view angle modifications via GUI user interaction, which are provided in case of tracking by the trackedEntity feature.
But thanks to your suggestions, I could finally figure out another workaround by utilizing a constantly repositioned helper object, which seems to support the trackedEntity mechanism.
Here is a simple demonstration:
Cesium Sandcastle.
Thanks again for your encouraging help! 
I’m glad you were able to resolve the issue! Thank you for sharing your solution!
I’m also struggling with this. It’s easily done using czml. Do you think it makes sense to convert the gpx to czml to accomplish this?