I'm familiar with the combined effects of special and general relativity making clocks on GPS satellites tick faster than those on Earth by about 38 microseconds per day. The clocks on the satellites are adjusted to stay in sync with Earth clocks, and all is fine an dandy in that regard. However, there is an element of this I cannot reconcile on my own. It's clear the clocks will stay in sync with Earth clocks by this means, but I'm wondering how GPS clocks stay in sync with each other. They're all at the same altitude so there is no GR effects between them, but they are all moving relative to each other. These motions are, as I understand it, inertial from the perspective of general relativity (they are following curved inertial paths in spacetime, experiencing no actual acceleration). It would seem then to me that they should all see each other's clocks ticking more slowly and end up way out of sync with each other. A more simple example I can think of is two satellites in identical orbits, except one is inclined 90 degrees from the other. They both pass the intersection points at nearly the same time (so as to just miss colliding). When they pass one of those points, their clocks are synced. Now, like in the twin paradox, they move away from each other, and each should notice the other's clock ticking slower than its own. But because of curved spacetime they end up moving back to meet up again without any acceleration. Since there was no acceleration, and thus no change of reference frame occurred, both should see the other's clock having passed less time when they meet again, an obvious contradiction. Now, I realize I must be wrong about this. It seems to obvious a flaw to go unnoticed if it were real. I just don't quite see why its wrong. I'm sure it has to do with the fact that I don't have as deep an understanding of general relativity as I do special relativity. So, I'd love to know what I'm missing in this scenario. It's especially frustrating because I ended up in an argument with this individual who has published a paper claiming to disprove special relativity, and one of his main talking points is how GPS satellites cannot possibly stay in sync with each other under the model of SR, and since they obviously do, SR must be wrong. Based on his other arguments its clear he doesn't fully understand SR, but I don't have any compelling rebuttal to that particular argument and I wish I did.