hi david, i'm not an expert in atmospheric delay correction and gps, but if you are interested, i think there are several papers about what corrections gps can do and what it can't do. some references are listed at the end of: https://en.wikipedia.org/wiki/Error_analysis_for_the_Global_Positioning_System
the best GPSDO's on the market provide errors of around 10 ns RMS wrt UTC. i think most of this error is due to variable atmospheric delays that can not be removed (eg: dispersion errors can be measured and removed, but other propogation delay errors can not be removed). best wishes, dan On Fri, Mar 8, 2019 at 11:25 PM Forbes, David C - (dforbes) < dfor...@email.arizona.edu> wrote: > That's an interesting question, Dan. You seek a low cost oscillator that > follows GPS without following it too closely. > Do you have a plot of a typical day of GPS atmospheric disturbance? > > > On Mar 8, 2019 2:55 PM, Dan Werthimer <d...@ssl.berkeley.edu> wrote: > > hi robert, randall, dale and casperites, > > thanks for your time/freq standard suggestions. > > our problem to get accurate 1 PPS wrt UTC is not limited by internal > oscillator stability. > the problem is mostly about GPS atmospheric delay corrections. > i don't thinks one needs an ultra stable oscillator in a GPSDO time/freq > standard if one is only interested in 1 PPS accuracy wrt UTC. > there's no need for rubidium, hydrogen, or microsemi's atomic gizmo for 1 > pps accuracy > -- an OXCO is fine. > > the typical GPS disciplined oscillators (eg: srs and trimble) output 1 > PPS that have 100 ns errors from UTC. > the best ones we have found so far are made by endrun technologies (< 10 > ns error wrt UTC). > > endrun claims to have unique algorithms for GPS atmospheric correction. > although one can do next day GPS atmospheric correction for atmospheric > delays > (the next day, there are correction tables available for the previous day > that are location/satellite dependent), > endrun technologies can do some fairly accurate measurements and > atmospheric delay corrections in near real time. > we'd prefer not to use next day correction tables in our application. > > has anybody used GPSDO time/freq standards that have < 10 ns accuracy wrt > UTC? > has anybody used the endrun technologies standards ? > has anybody used GPSDO products from microsemi? > > https://www.microsemi.com/product-directory/clocks-frequency-references/3826-gps-disciplined-oscillators-gpsdo > the GPSDO's listed at the bottom of that page have 10 ns wrt utc. > > best wishes, > > dan > > > > On Thu, Mar 7, 2019 at 8:57 AM Robert F. Jarnot < > robert.f.jar...@jpl.nasa.gov> wrote: > >> Dan, >> >> I looked into GPS disciplined oscillators for a project, and ended up >> using atomic clocks instead, as they are now very small, can be flown, and >> have remarkably low power consumption. We have 2 of them, $7500 each. See >> https://www.microsemi.com/product-directory/clocks-frequency-references/3824-chip-scale-atomic-clock-csac >> >> I suspect that a good choice of GPS disciplined oscillators would >> work pretty much as well, and be cheaper. >> >> Bob >> On 3/7/19 8:51 AM, Dan Werthimer wrote: >> >> >> >> in a somewhat related question. >> >> can anybody give us advice about GPS disciplined oscillators time/freq >> standards that are very accurate wrt UTC? >> we don't want to buy a hydrogen maser (too pricy). >> we have been looking at a company called endrun technologies that sell >> time/freq standards accurate to about +-10 ns wrt UTC. >> they might be able to match a pair of them that track each other +- 3ns >> RMS. >> we need a pair of well matched time/freq standards for coincidence time >> stamping/correlation between two observatories for our panoseti experiment. >> (the two optical/IR observatories are 500 km apart, and don't have >> masers). >> >> thanks for any advice on this. >> >> btw, we are using white rabbit for time/frequency distribution over 1 Gbe >> bidi fiber, >> and we put the white rabbit hardware (VCO and DAC chips) and software on >> our FPGA boards for this project. >> (we made our own FPGA boards with white rabbit and kintex7 because we >> need a few thousand boards) >> white rabbit does sub-ns accuracy in timing distribution - some white >> rabbit users have measured 30 ps RMS. >> >> best wishes, >> >> dan >> >> >> On Thu, Mar 7, 2019 at 12:05 AM Michael Inggs <miki...@gmail.com> wrote: >> >>> Hi Franco >>> >>> Simon Lewis in the RRSG at UCT has White Rabbit hardware and expertise >>> (PhD incubating). Snag is that it runs on 1GE Fibre. We also have a GPS >>> version. The former gives sub ns precision, the latter about 4 ns rms. Send >>> me a message off line and I can link you. We also have a scheme of aligning >>> a trigger to both a local MHz clock and the 1 pps. This is all open source >>> hardware and software. >>> >>> Regards >>> >>> On Thu, 7 Mar 2019 at 08:52, James Smith <jsm...@ska.ac.za> wrote: >>> >>>> Hello Franco, >>>> >>>> As I understand it, PTP wasn't terribly useful in our application >>>> (though I wasn't involved with this directly). You can probably sync the >>>> little Linux instance that runs on the ROACH2, but getting the time >>>> information onto your FPGA may prove somewhat tricky. >>>> >>>> Are you using an ADC card in the ROACH2? Or is the data digitised >>>> separately? >>>> >>>> What we've done with ROACH and ROACH2 designs in the past is more or >>>> less this: >>>> >>>> - FPGA's clock comes from a timing & frequency reference (TFR). >>>> - ROACH2 gets a 1PPS input from the same TFR. >>>> - In the FPGA logic there's a counter which is reset as part of the >>>> initialisation, and some logic that starts the counter going after a set >>>> number of 1PPS pulses (two to three, I forget exactly now). >>>> - The output of this counter is pipelined along with the data and >>>> then sent out as part of the SPEAD data on the 10GbE network. >>>> >>>> The idea here being that you know with a fairly high degree of >>>> precision which pulse your ROACH was initialised on. The counter that comes >>>> through on the SPEAD packet counts in FPGA clock cycles (or multiples >>>> thereof, perhaps you might want to count in spectra), and then you can use >>>> the start time to calculate the timestamp of each packet (Unix time, MJD, >>>> whichever your preferred reference is). >>>> >>>> Hope that helps. >>>> >>>> Regards, >>>> James >>>> >>>> >>>> On Wed, Mar 6, 2019 at 7:41 PM Franco <francocuro...@gmail.com> wrote: >>>> >>>>> Dear Casperiites, >>>>> >>>>> I was given the task of timestamping ROACH2 spectral data in a >>>>> telescope that uses PTP (precision time protocol) as a synchronization >>>>> protocol. I understand that ROACH's BORPH come preloaded with NTP (network >>>>> time protocol) libraries/daemos, but PTP is preferred because is already >>>>> in >>>>> use in the telescope, and it achieves greater time precision. >>>>> >>>>> Does somebody know if it is feasible to compile/install PTP libraries >>>>> in BORPH? >>>>> >>>>> Alternatively, we have though of sending the ROACH the current time >>>>> through a GPIO pin using IRIG-B timecode standard. Has anybody done >>>>> something similar in the past? >>>>> >>>>> Thanks, >>>>> >>>>> Franco >>>>> -- >>>>> You received this message because you are subscribed to the Google >>>>> Groups "casper@lists.berkeley.edu" group. >>>>> To unsubscribe from this group and stop receiving emails from it, send >>>>> an email to casper+unsubscr...@lists.berkeley.edu. >>>>> To post to this group, send email to casper@lists.berkeley.edu. >>>>> >>>> -- >>>> You received this message because you are subscribed to the Google >>>> Groups "casper@lists.berkeley.edu" group. >>>> To unsubscribe from this group and stop receiving emails from it, send >>>> an email to casper+unsubscr...@lists.berkeley.edu. >>>> To post to this group, send email to casper@lists.berkeley.edu. >>>> >>> >>> >>> -- >>> Michael Inggs >>> 10 Devon Street, Simon's Town, South Africa. Tel: +27 21 786 1723 Fax: >>> +27 21 786 1151 Skype: mikings Cell: +27 83 776 7304 >>> "Ex Africa semper aliquid novi" >>> -- >>> You received this message because you are subscribed to the Google >>> Groups "casper@lists.berkeley.edu" group. >>> To unsubscribe from this group and stop receiving emails from it, send >>> an email to casper+unsubscr...@lists.berkeley.edu. >>> To post to this group, send email to casper@lists.berkeley.edu. >>> >> -- >> You received this message because you are subscribed to the Google Groups >> "casper@lists.berkeley.edu" <casper@lists.berkeley.edu> group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to casper+unsubscr...@lists.berkeley.edu. >> To post to this group, send email to casper@lists.berkeley.edu. >> >> -- >> * Robert F. Jarnot, M/S 183-701 | robert.f.jar...@jpl.nasa.gov * >> * Jet Propulsion Laboratory, | Google: (657) 229-5204 * >> * 4800 Oak Grove Drive, | Cell: (818) 653-9266 * >> * Pasadena, CA 91109-8099, USA | FAX: (818) 393-5065 * >> >> -- > You received this message because you are subscribed to the Google Groups " > casper@lists.berkeley.edu" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to casper+unsubscr...@lists.berkeley.edu. > To post to this group, send email to casper@lists.berkeley.edu. > > -- > You received this message because you are subscribed to the Google Groups " > casper@lists.berkeley.edu" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to casper+unsubscr...@lists.berkeley.edu. > To post to this group, send email to casper@lists.berkeley.edu. > -- You received this message because you are subscribed to the Google Groups "casper@lists.berkeley.edu" group. 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