GPS module wanted for timing.

Started by Clive Arthur July 11, 2018
On Wed, 11 Jul 2018 17:50:54 +0100, Clive Arthur
<cliveta@nowaytoday.co.uk> wrote:

>On 11/07/2018 17:04, Jeff Liebermann wrote: >> On Wed, 11 Jul 2018 10:02:22 +0100, Clive Arthur >> <cliveta@nowaytoday.co.uk> wrote: >> >>> 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 >> >> The 1PPS and serial (4800 baud) outputs are easy. The 10 MHz clock >> output is not. For that, you will probably need to build or buy a >> GPSDO (GPS diciplined oscillator). There are designs for a GPSDO >> available that are really crude and simple if you don't need the >> ultimate in stability, and others that take a week to stabilize and >> offer more accuracy and stability than you could possibly need. I >> built an OCXO (oven controlled crystal oscillator) with a 1, 5, and 10 >> MHz outputs to run my pile of test equipment. >> >> You can buy Chinese OCXO based GPSDO bricks on eBay for about $120 and >> up: >> <https://www.ebay.com/sch/i.html?_nkw=10+mhz+gpsdo> >> >> As others have mentioned, the place to get accurate info is the >> Time-Nuts mailing list: >> <http://leapsecond.com/time-nuts.htm> >> Archive: >> <https://lists.febo.com/pipermail/time-nuts_lists.febo.com/> >> >These look like they might do it well enough for me... > >https://www.u-blox.com/en/product/neolea-m8t-series > >...what do you think? > >Cheers
From what I read on the ublox 10mhz output is that there is quite a bit of jitter, hence the GPSDO with the OCXO Cheers
On Wed, 11 Jul 2018 15:19:37 -0700, Jeff Liebermann <jeffl@cruzio.com>
wrote:

>On Wed, 11 Jul 2018 22:37:08 +0300, upsidedown@downunder.com wrote: > >>On Wed, 11 Jul 2018 08:24:01 -0700, John Larkin >><jjlarkin@highlandtechnology.com> wrote: >> >>>On Wed, 11 Jul 2018 12:23:34 GMT, Johnny B Good >>><johnny-b-good@invalid.ntlworld.com> wrote: >>> >>>>On Wed, 11 Jul 2018 02:51:02 -0700, jack4747 wrote: >>>> >>>>> On Wednesday, July 11, 2018 at 11:02:27 AM UTC+2, Clive Arthur wrote: >>>>>> Hi >>>>>> >>>>>> I need a GPS module which will give me: >>>>>> ...an accurate 10 MHz clock output ...an accurate 1 second pulse output >>>>> >>>>> What do you mean by "accurate"? >>>>> >>>> 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. :-) >>> >>>The fundamental GPS output is the 1 PPS pulse. >> >>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. > >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.
1 kHz x 1023 = 1.023 MHz chip rate.
> >>>It jitters and wanders >>>around a lot, as satellites come and go and the atmosphere changes. >> >>As long a the receiving station remains in a fixed position, this >>should not really be an issue. > >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.
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.
On 12/07/18 00:23, Martin Riddle wrote:
> On Wed, 11 Jul 2018 17:50:54 +0100, Clive Arthur > <cliveta@nowaytoday.co.uk> wrote: > >> On 11/07/2018 17:04, Jeff Liebermann wrote: >>> On Wed, 11 Jul 2018 10:02:22 +0100, Clive Arthur >>> <cliveta@nowaytoday.co.uk> wrote: >>> >>>> 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 >>> >>> The 1PPS and serial (4800 baud) outputs are easy. The 10 MHz clock >>> output is not. For that, you will probably need to build or buy a >>> GPSDO (GPS diciplined oscillator). There are designs for a GPSDO >>> available that are really crude and simple if you don't need the >>> ultimate in stability, and others that take a week to stabilize and >>> offer more accuracy and stability than you could possibly need. I >>> built an OCXO (oven controlled crystal oscillator) with a 1, 5, and 10 >>> MHz outputs to run my pile of test equipment. >>> >>> You can buy Chinese OCXO based GPSDO bricks on eBay for about $120 and >>> up: >>> <https://www.ebay.com/sch/i.html?_nkw=10+mhz+gpsdo> >>> >>> As others have mentioned, the place to get accurate info is the >>> Time-Nuts mailing list: >>> <http://leapsecond.com/time-nuts.htm> >>> Archive: >>> <https://lists.febo.com/pipermail/time-nuts_lists.febo.com/> >>> >> These look like they might do it well enough for me... >> >> https://www.u-blox.com/en/product/neolea-m8t-series >> >> ...what do you think? >> >> Cheers > > From what I read on the ublox 10mhz output is that there is quite a > bit of jitter, hence the GPSDO with the OCXO
From memory, it has a 48MHz clock, and the output is synchronous with that. Plus the 1pps output deviation from nominal is a sawtooth wwveform with a 21ns step.
Clive Arthur <cliveta@nowaytoday.co.uk> writes:

> These look like they might do it well enough for me... > > https://www.u-blox.com/en/product/neolea-m8t-series > > ...what do you think?
At least Lea-6T had horrible jitter at 10MHz. Average frequency was ok, though. If I remember correctly, the output was derived from 48MHz clock and anything that is a direct 48MHz/N division result was clean. I ended up using 4MHz output as reference to my project at that time. -- mikko
upsidedown@downunder.com wrote in news:nfqdkdhi2gci3iu2btbk73cqvo783serpg@
4ax.com:

> Is the receiver locking only to 4 satellites at a time ? >
It does not "lock" to ANY. It takes readings of the arrivals of timestamped pings which the satellites constantly produce.
torsdag den 12. juli 2018 kl. 16.35.25 UTC+2 skrev DecadentLinux...@decadence.org:
> upsidedown@downunder.com wrote in news:nfqdkdhi2gci3iu2btbk73cqvo783serpg@ > 4ax.com: > > > Is the receiver locking only to 4 satellites at a time ? > > > > It does not "lock" to ANY. It takes readings of the arrivals of > timestamped pings which the satellites constantly produce.
which it does by "locking" onto the frequency of the satellite and the phase of the prn sequence
DecadentLinuxUserNumeroUno wrote:
> >> Is the receiver locking only to 4 satellites at a time ? >> > > It does not "lock" to ANY. It takes readings of the arrivals of >timestamped pings which the satellites constantly produce.
It would be good if you stopped spouting all sorts of ideas, and studied the subject. The GPS receiver searches for the correct position in the Gold code, From: http://lea.hamradio.si/~s53mv/navsats/theory.html <quote> The filtered IF signal is then used for several purposes. First, it is used to acquire and maintain synchronization of the locally generated code. Dithering the locally-generated code back and forth by a fraction of the bit period generates an amplitude modulation on the filtered signal. The phase of this modulation contains the information required to keep the synchronization of the local code generator. <end quote> So sync LOCK is needed to decode. That link http://lea.hamradio.si/~s53mv/navsats/theory.html is the best GPS description I have ever come across, with hardware the guy designed before the Sirff-3 chips appeared, to make his own GPS / GLONASS receiver. There is a lot to learn there. I suggest you read it, then build it, and then .. There are more interesting math links and experiments on the subject, for example receiving GPS with a rtl_sdr stick: http://michelebavaro.blogspot.com/2012/04/spring-news-in-gnss-and-sdr-domain.html Some mathlab, a rtl_sdr stick and GPS mouse required. Been there done that. Time flies.... 2012
<698839253X6D445TD@nospam.org> wrote in news:pi80tp$pr5$1@gioia.aioe.org:

> It would be good if you stopped spouting all sorts of ideas, > and studied the subject.
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".
On 2018-07-12 21:29, DecadentLinuxUserNumeroUno@decadence.org wrote:
> > Fuck you. It does not "lock on" to a satellite. It synchronizes itself > with their streams, and switches birds constantly.
You might need to read up on how CDMA works.
+++ATH0 <news@ringpiece.local> wrote in
news:5d2dnW9pjdazsdXGnZ2dnUU7-d-dnZ2d@supernews.com: 

> On 2018-07-12 21:29, DecadentLinuxUserNumeroUno@decadence.org wrote: >> >> Fuck you. It does not "lock on" to a satellite. It synchronizes >> itself >> with their streams, and switches birds constantly. > > You might need to read up on how CDMA works. >
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.