[Martin Lorton] acquired a GPS-disciplined oscillator. He wasn’t quite sure what to do with it, so he did a little research and experimentation. If you have about two hours to spare, you can watch his videos where he shares his results (see below).
The unit he mainly looks at is a Symmetricom TrueTime XL-DC, and even on eBay it ran over $500. However, [Martin] also looks at a smaller unit that is much more affordable.
So what do you use something like this for? The idea is simple. A high-quality crystal (or rubidium) oscillator is disciplined to agree with the timing signals from GPS or another type of navigation satellite. Because the navigation satellites are kept very precise, the crystal oscillator can be accurate down to the nanosecond level.
In the military and commercial world, the timing from such an oscillator can synchronize multiple receivers which is necessary for applications such as passive RADAR. GPS timing by itself has excellent long-term stability, but since the unit of timing is one pulse per second — combined with things like multipath and other propagation effects — the short-term stability isn’t always great. A crystal oscillator, however, has great short-term stability. They tend to drift with temperature (mitigated by ovens) and simple aging of the crystal and other components. By combining both, you can obtain excellent short-term performance that holds over long periods, too.
Depending on how it is set up, such an oscillator can be accurate to within a few parts per trillion very shortly after turn on. Even low-end devices will be able to operate in the parts per billion range. Of course, you are going to need an antenna that can see the sky, too.