Actually, everything is orbiting the sun at about the same speed, but
we're digressing...
bp
<part15sbs{at}gmail{dot}com>
On 8/11/2015 11:25 AM, Sean Heskett 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.)?
