Rubidium OCXOs keep coming down in price. If you really need good holdover, I think you could stay synched for several days if you used one of those.
But do people really want to add $200 for this feature.... From: Forrest Christian (List Account) Sent: Wednesday, August 12, 2015 4:34 AM To: af Subject: Re: [AFMUG] GPS Timing 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) Sent: Tuesday, August 11, 2015 11:39 AM To: af 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
