Skip, If you are setting against GPS then you are done. For most applications either Zeeman frequency- synthesizer setting will be fine. What the Zeeman adjust lets you do is set the frequency (via the C-field) to within the accuracy specs for the instrument without having to use another reference to do the setting. That's why it's a "primary" frequency standard. If you want to use GPS to tweak it as close to possible go ahead. Just ignore all the Zeeman stuff! Cheers, Corby