Another variable physical phenomenon that is easy to measure is clock drift.
There are several ways to measure and use clock drift as a source of randomness.
In retrospect it makes sense because this is a time-based token, and it's going to tend to have some clock drift relative to, you know, galactic central.
This will not happen again before the next synchronization due to limits on maximum allowable clock drift.
By contrast, in polar modulation, clock drift simply degrades the phase-modulated signal.
These include the ionospheric effects mentioned earlier, as well as errors in the satellite position ephemeris data and clock drift on the satellites.
On the negative side, clock drift can be exploited by timing attacks.
For instance, in pendulum clocks the clock drift can be manipulated by slightly changing the length of the pendulum.
Atomic clocks are very precise and have nearly no clock drift.
As Einstein predicted, relativistic effects can also cause clock drift due to time dilation.