I just read through all the posts to see if we are still discussing frequency offsets or phase variations. My definition of a frequency offset is comparing two oscillators one will over time produce more cycles than the other. If one oscillator is 10MHz exact, and one is 10,000,000.0001Hz, then the second one will create an extra cycle in 10,000 seconds and an extra 8.64 cycles in a day. If the two oscillators are in phase at time t then they are 180 out of phase at t + 5000 seconds.
A phase variation in the short term looks just like a frequency offset. Take the same two oscillators, but after 5000 seconds change the 10,000,000.0001Hz to 9,999,999.9999Hz then after 10,000 seconds they both will have produced the same number of cycles. Flip the frequency every 5000 seconds and the two will produce the same number of cycles (+-half a cycle) over any period of time.
But, at 5000 seconds, and not knowing the control mechanism, one cannot tell if there's a frequency offset or a phase offset (at that moment it is of course both). nctnico started off describing the problem as a frequency offset but since has talked of p-p differences of ns which is more the language of differences of phase. So I am looking for confirmation that nctnico is looking at the dynamic phase differences of oscillators and not long term frequency differences. Long term frequency differences in GPS disciplined systems just don't make sense (except if there is an error in implementation). Long term frequency differences do make sense when compared to rubidium or cesium clocks because they are not as good as the NIST clocks that are used to ensure the long term validity of the GPS system.
I believe all the GPS receivers mentioned are single frequency receivers (the L1 band) and therefore don't have the ionospheric correction capabilities of the dual band receivers. So it is no surprise the oscillators wander a small amount due to this. MegaVolt also mentioned the problem introduced by 'course' ADC control.
Which leads to the question, what is the aim. In developing a cheap GPSDO solution my target was to maintain the OCXO at 10MHz +- 0.01Hz. In discussions with a few people this is satisfactory for a reference for all the local amateur radio enthusiasts. I've achieved that. I believe an order of magnitude better (+-0.001Hz) can be achieved for not much more, but I haven't found anyone that says that is a requirement, just a nice to have. The next magnitude (+-0.0001Hz) is a bit of a hurdle, on paper it can be done for less than $200, but I haven't found anyone who wants it.
So are we really just a lot of aimless tinkerers? [my hand is slowly raised]