[lxgr]

Satellite-based time signals also seem easier to spoof, given the inherent power constraints of the transmitters.

That said, newer ones can use authentication, which (together with a reasonably accurate local oscillator) can prevent at least trivial spoofing.

[WorldMaker]

The most common satellite-based time signal today is GPS and global navigation system signals are possible to spoof, but it is harder than "trivial" at least.