Re: [Discuss-gnuradio] Time syncing between SDRs on different computers

[Markus Heller]

my experiments with an arbitrary GPS module offering a PPS discipline
showed that it is easily archievable to create a local clock (ntpd
based) with a clock precision of 1 microsecond even with cheap low-power
soc computers: I was using a Raspberry Pi. 

The most interesting finding was the clock drift in correlation with the
air pressure. Air pressure change obviously influences the quarz on the
raspberrypi. 

http://www.dl8rds.de/index.php/NTP-Server_with_Raspberry_Pi_and_Sure_Electronics_GPS_Eval_board

vy73
markus
dl8rds

Am Mittwoch, den 16.08.2017, 22:27 +0200 schrieb Marcus Müller:
> 10 ms timing certainty is actually pretty hard to achieve across USB2!
> But: as far as I can see, it'd be rather easy to say, tune both sticks
> to DVB-T, cross-correlate to find the relative offset, tune to the
> frequency of interest and work from there. That is, if the delay spread
> in the cellular channel < 10ms, but considering that would be equivalent
> to a path length difference of 3000 km...
> 
> I don't think the tuning process itself would interrupt the sampling,
> right (Sylvain)?
> 
> Cheers,
> Marcus
> 
> 
> On 08/16/2017 07:40 PM, address@hidden wrote:
> > Say we need +-10ms accuracy in timestamps. Precision Time Protocol in LAN 
> > should sync PC's clocks with enough accuracy. 
> > And then using USRP-like methods - set current/start time - get first 
> > sample in known time? Of course USB will add
> > undetermined delays but no external GPS required.
> >
> > On Wed, 2017-08-16 at 16:57 +0100, Derek Kozel wrote:
> >> It should be pointed out that the hardware based timestamping is only 
> >> needed if you need time alignment better than a
> >> half second or so. With USB transfers, various buffers, NTP based 
> >> alignment of the host computer's time, and some
> >> extra code on the host side you could do a coarse time alignment, probably 
> >> with less than a half second of error.
> >>
> >> You could also time align the streams if both radios receive the same 
> >> signal and you know the distance (and other
> >> details depending on the precision needed) between the transmitter and 
> >> your two receivers. This is done by many
> >> protocols like the cellular standards to create a distributed timebase, 
> >> but quickly becomes non-trivial.
> >>
> >> The B200 does timestamping in the FPGA and can use a 1PPS signal to align 
> >> to GPS time. It is more expensive than the
> >> HackRF, but much less than the $3000 mentioned above.
> >>
> >> On Wed, Aug 16, 2017 at 4:20 PM, Sylvain Munaut <address@hidden> wrote:
> >>> On Wed, Aug 16, 2017 at 5:11 PM,  <address@hidden> wrote:
> >>>> What type of hardware? I thought from hardware point of view only 
> >>>> precise clock is required and all the other
> >>> things in
> >>>> firmware. I've naively thought i could modify hackrf firmware to get 
> >>>> this feature.
> >>> Mostly a FPGA and a PPS input from a GPS receiver.
> >>>
> >>> Each individual sample has a timestamp of when it has been received.
> >>> And you can also reset the timestamp on the next  PPS edge.
> >>>
> >>> Technically it would be possible to modify the hackrf firmware and
> >>> repurpose some GPIOs and have all samples transmitted to the host be
> >>> in timestamped packets and implements timestamping in the on-board
> >>> ARM.
> >>> For additional hardware you'd only need an external GPS receiver (or
> >>> some other way to have both a single freq reference + single sync
> >>> pulse).
> >>>
> >>> Cheers,
> >>>
> >>>     Sylvain
> >>>
> >>> _______________________________________________
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> >>> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
> >>
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> 
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