Sorry, meant 10ppm. Not too stringent but already rules out typical crystal oscillator-based designs.
I have an old cheap Global Specialties counter, the one with the Intersil chip and LED display. It has a trim cap for the crystal which I guess was set once. It's always better than 1 PPM.
Those old HP OCXOs are usually within 100 PPB, even from ebay gear. I guess your problem is to verify whatever you buy. Used to be that you could use the color burst or the horizontal oscillator of a TV set tuned to a color broadcast, but LCD TV's don't let you get at them. I guess GPS is the way to go now.
John
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