They are based upon a telco surplus "UCCM" GPSDO board. Both Symmetricom and Trimble made (mostly) compatible units. I prefer the Trimble units... they store their settings in EEPROM. The Symmetricom units don't... and have to do a long re-survey every time you power them up. Plus the firmware in the Symmetricom units has a couple of minor glitches (occasionally drops a time stamp output message and can send the time stamp message in the middle of other message outputs). That said, the Symmetricom is rumored to have a slightly better oscillator)... but it is driven by a 6V to 12V DC-DC converter that is known to be somewhat unreliable. I have not measured the output ADEVs, etc. A little shopping on Ebay (search Trimble GPSDO) will turn up some units for less money. I bought one of these and it arrived quickly and safely in good condition (BTW, you used to be able to get them for $50): http://www.ebay.com/itm/Trimble-GPS-Receiver-GPSDO-10MHz-1PPS-GPS-Disciplined-Clock-/121966793020?hash=item1c65c9913c:g:TTgAAOSwVllXHYRU There are unscrupulous sellers shipping damaged units. This thread on EEVBLOG covers the units pretty well: http://www.eevblog.com/forum/projects/a-look-at-my-symmetricom-gpsdo-(ocxo-furuno-receiver)/ I now have Lady Heather working with both models.