GPS module wanted for timing.

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Hi

I need a GPS module which will give me:
...an accurate 10 MHz clock output
...an accurate 1 second pulse output
...a serial interface to read UTC.

(Position isn't important.  I know where I am.)

I know there are probably dozens of modules which will do this, but I  
have no experience in this area, so any advice would be welcome.  UK  
supplier preferred but not strongly.  Will need only a few.

Cheers
--  
Clive

Re: GPS module wanted for timing.
On Wednesday, July 11, 2018 at 11:02:27 AM UTC+2, Clive Arthur wrote:
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What do you mean by "accurate"?

Bye Jack

Re: GPS module wanted for timing.
On Wed, 11 Jul 2018 02:51:02 -0700, jack4747 wrote:

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 Normally, in the context of GPS, the word "accurate" would normally be  
regarded as being somewhat redundant. Presumably an accuracy measured in  
nanoseconds is being sought. :-)

--  
Johnny B Good

Re: GPS module wanted for timing.
On Wed, 11 Jul 2018 12:23:34 GMT, Johnny B Good

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The fundamental GPS output is the 1 PPS pulse. It jitters and wanders
around a lot, as satellites come and go and the atmosphere changes.
It's accurate long-term, so you can lock a very good oscillator to it,
slowly, and get a good 10 MHz. The slower the lock loop and the better
the oscillator, the better 10M. A rubidium is good.

Cheap GPS receivers can have 1PPS jitter in the ballpark of a
microsecond.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: GPS module wanted for timing.
On Wed, 11 Jul 2018 08:24:01 -0700, John Larkin

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What is so "fundamental" about the 1PPS output. After all the GPS
signal repetition rate is 1 kHz, so a 1 kHz output would be more
useful for faster locking.

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As long a the receiving station remains in a fixed position, this
should not really be an issue.

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You need weeks to lock to 1PPS signals.

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Or even worse. The 1PPS is often software/firmware derived.


Re: GPS module wanted for timing.
4ax.com:

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It is fundemental inasmuch as it is the standard adopted by the scientific  
community and the military for time synchronisation of equipment.

Re: GPS module wanted for timing.
On Wed, 11 Jul 2018 22:37:08 +0300, snipped-for-privacy@downunder.com wrote:

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There is no "repetition rate".  The C/A codes are sent at rate of
1.023 Mbit/sec which is modulated by navigation messages at 50
bits/sec.

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Ummm... The satellites are moving.  However, even if both the
satellites and receiver are stationary, atmospheric delays are quite
erratic, unpredictable, and noisy.  From my experience living in a
forest, where foliage blockage by trees is a problem, when the data
source moves from one satellite to a different satellite, there's a
small but noticeable phase glitch.  If my GPSDO loses lock, the OCXO
clock oscillator drifts off frequency for the duration of the outage
and then takes about the same amount of time as the outage to return
to a stable lock condition.  However, when I played with a Cesium
secondary clock GPSDO, where I could theoretically get to 1 part in
10^14 accuracy (about 4 nsec/day), it did take days for things to
settle down.  I never got close to this level of accuracy because 60Hz
power line and 120Hz power supply noise was wrecking my measurements.
If it had works, it probably would have taken several days to
stabilize.

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Locking to 1pps was the way GPS time and freq clock receivers worked
before built in higher frequency clock outputs were available.  The
time to gain lock was roughly proportional to how far out of phase the
PLL clock was with the 1pps output.  I don't recall the exact time,
but I think it was less than 2 mins if the oscillator had warmed up,
and maybe 20 mins if it was from a cold start.  These use the 1pps
output:
"1pps locking module"
<http://www.quartzlock.com/product/timing-modules/1pps-locking-modules/A6-1PPS
"GPS-Locked Frequency Standard"
<http://www.techlib.com/electronics/GPSstandard.htm

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Not a problem because it has 1 full second to do the computation
necessary to run whatever DSP filter algorithm (usually Kalman) is
needed to reduce the noise and speed up locking by predicting the
Doppler shift from each satellite.

--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: GPS module wanted for timing.
wrote:

Sigh.  I found my notes.  Looks like my memory is failing.  Some
corrections.

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With a Cesium clock, possibly.  With an OCXO, much less.

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Faulty memory.  About 14 hrs from a cold start for the OCXO oscillator
using a 1pps reference.  This probably varies radically depending on
ambient temperature and equipment temperature, but I only did one run.
About 30 minutes if the OCXO had warmed up and had been locked for a
while.  Sorry about the memory failure.

Drivel:

Visual GPS Software:
<http://www.visualgps.net

NMEATime2 - PC GPS Time Synchronization
<http://www.visualgps.net/#nmeatime2-content
   $20.48USD, 30 day free trial.

--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: GPS module wanted for timing.
wrote:

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1 kHz x 1023 = 1.023 MHz chip rate.

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Is the receiver locking only to 4 satellites at a time ?

At least with slightly higher antenna position, there should be much
more than 4 satellites visible at a time. Of course, if you calculate
the time solution from different combinations of 4 satellites, you
then have the problem, which solution to believe. To be sure about a
measuring result, measure only once :-).

In practice, most timing solutions from different combinations of
satellites are quite close together, so select the median or just
average the best solutions.

I do not know, what kind of foliage you have, but it shouldn't
attenuate the 1.5 GHz GPS signal too much.


Re: GPS module wanted for timing.
4ax.com:

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  It does not "lock" to ANY.  It takes readings of the arrivals of  
timestamped pings which the satellites constantly produce.

Re: GPS module wanted for timing.
torsdag den 12. juli 2018 kl. 16.35.25 UTC+2 skrev snipped-for-privacy@decadence.org:
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which it does by "locking" onto the frequency of the satellite and  
the phase of the prn sequence

Re: GPS module wanted for timing.

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  Fuck you.  It does not "lock on" to a satellite.  It synchronizes itself  
with their streams, and switches birds constantly.

  It synchs to their streams and reads their arrival times with respect to  
each other.

  Since I have yet to see tiny dishes on any GPS antenna, I feel relatively  
certain that a GPS device reads and synchs with the data streams, NOT  
"locks on to" a satellite's position.

  "Lock on"  would more closely match the characteristics of an earth based  
dish positioner and a geostationary satellite.  THAT "locks on".

Re: GPS module wanted for timing.
On 2018-07-12 21:29, snipped-for-privacy@decadence.org wrote:
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You might need to read up on how CDMA works.

Re: GPS module wanted for timing.

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I worked in Sorrento Valley for nearly two decades.

  You know...  companies like Qualcomm...  the inventor of it (modern  
CDMA link topologies.

  General Instrument...  The makers of MCPC.  (that one went over your  
head).

  ViaSat, the masters of Link 16.


  No, fuckhead...  I do not know anything about it...  Yeah, right.

  Like I said before...  fuck you.

Re: GPS module wanted for timing.
wrote:

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At least when correcting some of the atmospheric errors in position
fixing, differential-GPS is used. In this, the DGPS station calculates
a pseudo code distance correction from the DGPS station to each
visible satellite individually. A distance correction of 3-30 m
corresponds to a 10-100 ns time variation.

Now that the atmospheric error is similar to other nearby GPS
receivers, the other stations measures the pseudo code distances then
apply the DGPS correction to each measurement individually, before
calculating the position fix.

Couldn't these DGPS corrections be used for accurate timekeeping ?

Of course, if the clock receiver is in a fixed well known location, it
can generate its own GPS correction.


Re: GPS module wanted for timing.
On Friday, July 13, 2018 at 1:16:47 AM UTC-4, snipped-for-privacy@downunder.com wrote:
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This is a bit of a cyclic analysis.  The position can only be found by getting the corrected time from all the sats factoring in the path delay.  So time and location are all solved together.  

GPS is used for surveying where time averages are used to get very accurate locations which in turn requires much more accurate time keeping.  

Rick C.  

Re: GPS module wanted for timing.
On Fri, 13 Jul 2018 08:16:47 +0300, snipped-for-privacy@downunder.com wrote:

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DGPS can produce postion accuracies down to the centimeter level as
long as the distance between the DGPS receiver is fairly small.  The
local station is at Pigeon Point, about 30 miles away from Santa Cruz.
I was getting about 5 meters accuracy without DGPS, and 2 or 3 meter
accuracy using DGPS from Pigeon Pt.  I think I could have done better.
<http://www.gpsinformation.org/dale/dgps.htm

However, the USCG is turning off their beacon band DGPS system, which
has been replaced by WAAS:
<https://www.navcen.uscg.gov/?pageName=dgpsMain

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That's the way it's done.  Back in the stone age of DGPS, it was just
a log file of positions from a terrestrial DGPS receiver.  The
assumption was that the errors for the users GPS receiver and the
static DGPS receiver were the same at any given instant of time.  So,
we logged the RTCM SC-104 position records from the GPS receiver, and
compared it with data from a DGPS receiver.  The difference produced a
much more accurate position fix.  With a good view of the sky,
accuracy down to the width of the antenna were fairly easy.

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I don't think so.  Since the differential correction system required
time syncing each recorded position report, this would create a
circular reference, where the uncorrected time would be used to
correct itself.  I don't think that will work.

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Same problem as before.  You can't use the current time to correct the
current time.  

Incidentally, before WAAS, I was looking into building a DGPS station
and transmitter.  For a time, I had all the necessary equipment and
associated licenses.  It was suppose to eventually be a free service
to police and fire to improve their GPS location accuracy to the where
they could quickly and reliably locate their officers and vehicles
down to about 1 meter or better.  The idea was to broadcast the DGPS
corrections on a commercial VHF or UHF frequency, similar to what
MBARI is doing to accurately locate its vessel and buoys in Monterey
Bay.  The stumbling block was when I discovered that the site and
antenna needed to be surveyed to an accuracy of at least 1 order of
magnitude better than the accuracy we expected, and that it needed to
re-surveyed regularly to deal with tectonic movements.  Using the GPS
constellation to do the survey was not good enough.  I don't recall
the price, but it was more than I wanted to pay.  End of project.

Today, GPS correction services are quite common and the data is easily
obtained on the internet.  However, that is only useful for doing
post-processing of the data and not for real time.
<http://gpsworld.com/sources-of-public-real-time-high-precision-corrections/

For some application, DGPS ground stations have been replaced by
pseudolytes, which are essentially GPS satellites on the ground.
<https://en.wikipedia.org/wiki/Pseudolite

My apologies for my fumbled around.  I haven't done anything with GPS
for about 15 years.

--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: GPS module wanted for timing.
fredag den 13. juli 2018 kl. 18.38.56 UTC+2 skrev Jeff Liebermann:
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afaik every cell phone basestation has a gps reciever for timing and  
phones already download info from them to get a quicker GPS fix (A-GPS)  

I don't know if they also use it for D-GPS, but it seems like all the bits  
and pieces are there




Re: GPS module wanted for timing.
On Fri, 13 Jul 2018 11:45:47 -0700 (PDT), Lasse Langwadt Christensen
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DGPS is sometimes used, but not in all systems.  Someone had the
bright idea of time synchronizing all the GSM and CDMA2000 cell sites
to make handoffs between sites much easier and seamless.  That's a
good idea, but requires that all the sites in the system synchronize
their clocks to a common source such as GPS.  I don't recall the
timing accuracy required, but I'm fairly sure it doesn't require DGPS.

AGPS (assisted GPS) is a different nightmare.  
<https://en.wikipedia.org/wiki/Assisted_GPS
Basically, the GPS receiver in the typical handset does not provide a
sufficiently accurate location fix to satisfy the FCC and the alphabet
soup of served agencies.  So, some providers use TDOA (time difference
of arrival) information from cell sites near the user to improve the
location accuracy, and to shorten lock time.  

The data from the cell site is sent over the internet to a location
services server.  The data from the handset it sent to the cellular
service provider and then forwarded to the location services provider.
The two (or more) data sets are conglomerated into lat-long-altitude
fix, which is then forwarded to the PSAP (public safety answering
point).  Note that the data sent by the handset is NOT it's
lat-long-altitude, but rather the raw time delays from each satellite.
That allows for applying DGPS corrections at the location services
provider.  The only parts of the system that requires a GPS receiver
is the handset, and the location services server for applying DGPS
corrections.

Again, I'm far behind on the location technology in use today because
I haven't been involved in GPS for a long time.



--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: GPS module wanted for timing.

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ts  
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I'd would have thought it was mostly a matter of not wasting time and batte
ry  
on searching for satellites and downloading almanac at 50bps when you have  
a megabit data connection to a cell tower that has all the info  



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