Ok, if you really want to know, I finally found a (somewhat data) document which describes this in semi-understandable terms.
And yes, the real time does fall out of the equations (see watch error - which is how fast or slow your reference clock is). http://www.maa.org/sites/default/files/pdf/cms_upload/Thompson07734.pdf What I'm hearing from my GPS module vendor is effectively that since they don't really have to do any additional work to output a 1PPS signal from a 3d lock, they feel comfortable in doing so. Adding the complexity of surveying an location to an useful accuracy and then using that to compute the time is a lot of additional work with a lot of variability they don't want to try to deal with without additional demand. I do know that a while back we tried some shortcuts to get there, but they were not all that useful. -forrest On Tue, Aug 11, 2015 at 12:25 PM, Sean Heskett <[email protected]> wrote: > 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.)? >>> >> >> > -- *Forrest Christian* *CEO**, PacketFlux Technologies, Inc.* Tel: 406-449-3345 | Address: 3577 Countryside Road, Helena, MT 59602 [email protected] | http://www.packetflux.com <http://www.linkedin.com/in/fwchristian> <http://facebook.com/packetflux> <http://twitter.com/@packetflux>
