[oneshtein]

Your phone works because engineers did it.

I see strong annual signal in GPS: https://link.springer.com/article/10.1007/s10291-017-0686-6 .

Why it there?

[ben_w]

> Your phone works because engineers did it.

The engineers implemented results from GR.

> Why it there?

Not that it matters, but the answer is in the introduction.