Cambium is using a new receiver on the 450APs that does GPS+GLONASS. I
assume it's from Global-Top, but I haven't opened up a new AP to look.
I'm not real excited about using the Russian signals, but with so many
satellites available, it does acquire lock very fast. Have you thought
about doing the same for your 'Pipes? I think it would be beneficial.
On 8/12/2015 5:34 AM, Forrest Christian (List Account) wrote:
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]
<mailto:[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]
<mailto:[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)
<mailto:[email protected]>
*Sent:* Tuesday, August 11, 2015 11:39 AM
*To:* af <mailto:[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]
<mailto:[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] <mailto:[email protected]> |
http://www.packetflux.com <http://www.packetflux.com/>
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