Clouds. (Radar sats can see through clouds but can also be jammed.)

But even on a clear day, most of the people looking to target a carrier these days (Iran/hamas etc) don't have their own satellites. But a real-time GPS position accurate to few meters? That could be tactically useful to anyone with a drone.

An active fitness tracker might also give away the ship's readiness state, under the assumption that people aren't going to be doing much jogging while at battle stations.