It depends on what you mean by accurate. I use the 1 second output of
 a GPS receiver as a cheap off-the-shelf source. Depending upon your
location
 the VLF Time Signals offer an even cheaper source but normally require some
 work either building a receiver or adapting a Radio Clock.

 Long Term accuracy of both is to Atomic Standards but beware the
 instantaneous accuracy might be out by as much as +-50nS on GPS. The
 instantaneous accuracy of VLF signals depends upon your location with
 respect to the transmitting station.

 Regards
 Chris
>

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