Hi The reason people do not routinely jump to number 3 on the list is cost. The only new GPS modules that I am aware of in category 3 are well over $2K each. That is in comparison to Mark’s favorite $5 modules. You can buy eBay surplus older versions of the fancy boards. So far I have not seen one with the “right options” enabled for under $800.
There is another tangent and that is ionospheric correction. It is normally done before you get to option 3. You run some combination of L1/L2/L5 to let the delta frequency enable a real time guesstimate of the ionospheric delay. The cost for that part of it can be in the ~$300 for an older receiver. You also need an L1/L2 antenna. Getting an accurate PPS in or out of a $300 receiver may range from “exciting” to “impossible”. Lots of choices. Bob > On Jul 19, 2016, at 12:32 PM, Michael Gray <[email protected]> wrote: > > Alright, may I vote for tangent #3? I've heard this mentioned in passing > a few times on this list, but never seen it described in detail...so > much so that I have no idea what it means. > > Are we talking an external, steerable LO/clock (similar to #2, except > for the location of the oscillator itself)? An external oscillator > triggering a timestamp against the GPS's internal clock (seems identical > to PPS error-wise, except you can introduce your own dither)? Something > else? Why does carrier phase tracking matter, as long as the GPS is > deriving a time solution through some mechanism that exceeds the > granularity of its CPU clock? > > Forgive my ignorance here, but I'd love to see more of this tangent, > since I've never seen anyone jump into it in detail. > > Michael > >> There are some tangents we could go down: >> 1) There are cases where the inherent dithering you get from sawtooth error >> is actually hugely beneficial to the design of a GPSDO. >> 2) One GPSDO design (Trimble Thunderbolt) is unique in that is has no >> sawtooth problem or TIC or XO or TCXO at all. Instead it directly uses the >> high-quality OCXO as the receiver's LO. They get away with this clean >> solution because they are a company that makes their own receiver h/w. >> 3) Carrier phase receivers with external clock input. >> >> /tvb > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
