GPS, I already said that in the link I gave you, which you obviously didn't read.

And please, saying GPS isn't a reliable source is just digging the already pretty deep hole even deeper.

It would be fairly simple to calculate the altitude from the radio signals as there's usually 5 or 6 amateur trackers receiving the signal

And of course GPS does not really exist, it's all faked since, for it to be real there must be satellites in geosynchronous orbit--in space. Space flight as you know, along with every scientific observation that contradicts "flat earth," was faked.  This also makes me wonder how I get my satellite television.

PS. You FE guys are the best metatrolls on the internet. 10/10