the satellites are constantly moving tho and since they are moving faster in orbit than we are here on earth you need to account for relativity. knowing where you are doesn't give you enough information to know where the satellite is and therefore you can't accurately calculate the relativity offset. once you have 3D lock with 4 satellites you can accurately calculate the relativity offset and therefore calculate the accurate time for where you are on earth.
shoulda taken the blue pill ;-) -Sean On Tue, Aug 11, 2015 at 12:08 PM, Bill Prince <[email protected]> wrote: > That's what I thought too. Once one of these little beggars has been > online for a half hour or more, the location should be "set" so to speak. I > would then expect them to hold time sync even with 1 satellite in view. > Knowing that the location is static and unmoving, I would expect that > maintaining time lock would be gravy. > > Sadly, this does not seem to be the case. > > bp > <part15sbs{at}gmail{dot}com> > > > On 8/11/2015 10:48 AM, Chuck McCown wrote: > > Interesting, I guess you need to know where you are to calculate the > delay. Had not considered that. But if you know where you are and have > ephermis data, you should be able to calculate the delay and arrive at a > pretty accurate timing pulse with one satellite. > > *From:* Forrest Christian (List Account) <[email protected]> > *Sent:* Tuesday, August 11, 2015 11:39 AM > *To:* af <[email protected]> > *Subject:* Re: [AFMUG] GPS Timing > > > You need an accurate 3d position to get accurate timing. To have an > accurate 3d position using GPS alone, you need four satellites. Three > only gets you a 2d lock, and less than that you don't get a lock at all. > > There are receivers out there which will survey a position and then use > that position to be able to continue to provide a timing signal if you > subsequently lose lock but still have sats in view. As far as I know, > this type of receiver is not in use in any commercially available timing > product for the cambium radios. In fact I think we've almost all ended up > using the exact same GPS modules, at least for any recently designed > product. > > Some of the earlier products would attempt to preserve the sync signal > across a GPS lock loss with various levels of success. For instance the > cmm micro in early releases provided a wildly incorrect sync pulse even > without a lock. Same with early syncpipes. The CTM has a holdover > timer. And so on. I think most of us have moved away from this in newer > designs. > On Aug 11, 2015 8:36 AM, "Dan Petermann" <[email protected]> wrote: > >> What is the minimum amount of satellites needed for a proper GPS sync >> pulse? >> >> And does that differ across products (CMM, CTM, SyncPipe, etc.)? >> > >
