General > General Technical Chat
Cheapest way to get date/time from GPS
EPAIII:
I know that today everything is about satellites and the internet, but is there some reason why you wouldn't use WWV? It has been used for over 100 years as a world time standard.
I have a digital clock sitting here in my office that synchronizes with their broadcast signal. I bought it at Walmart for around US$35. It runs with battery power and always has the correct time. I am sure there are more professional receivers that can easily provide the time in electronic/digital form.
It is my understanding that the WWV signals (they have five transmitters on different frequencies) do reach world wide. The five different frequencies ensure that the signal can be received almost anywhere. They are run by the US Naval Observatory and have an accuracy of 100 ns of UTC. The time is encoded in the signal in a simple manner. They also provide accurate frequency signals.
I don't know if they still do, but in the past they distributed tables of the error at different times and locations so an event that was recorded at different locations could have the time differential calculated to a high accuracy at some time after recording those observations. It is older technology, but it is still there. And it is free to all.
https://en.wikipedia.org/wiki/WWV_(radio_station)
https://www.nist.gov/pml/time-and-frequency-division/time-distribution/radio-station-wwv
https://nortonsafe.search.ask.com/web?q=wwv%20receivers&ssdcat=321&lang=en&source=nag&year=2015&locale=en_us&geo=us&version=22.23.10.10&plang=sym:en&buildname=retail&heartbeatid=4ae3d98c-56e4-4df2-adf1-00b1d085e47e&eapenabled=false&env=prod&vendorid=1014930&plid=866&plgid=41&skup=21389648&skum=21376863&skuf=90001202-fa&endpointid=4ae3d98c-56e4-4df2-adf1-00b1d085e47e&lic_type=2&lic_attr=17059858&psn=7jjxx287wjvq&templatecat=sbu_w_1000_5039_n360dsp_retail&schemacat=sbu_w&schemaver=1.0.0.0&olpchannel=retail&osvers=10.0&oslocale=iso:usa&oslang=iso:eng&os=windows&showuninstallsurvey=1&installstatus=updated&vendorsrc=firefox&machinelocation=us&sw=0&3in1=0&npw=0&hp=0&dsp=0&cdest=nag&annot=false&vendorConfigured=ask&o=APN12178&prt=ngc&ver=3.21.0.6&tpr=111&chn=1014930&guid=4ae3d98c-56e4-4df2-adf1-00b1d085e47e&doi=2023-11-26&browser=FireFox&prod=DS&doi=2023-11-26&installSource=nag&cmpgn=oct23&darkMode=false&sameTabLaunch=false
fourfathom:
--- Quote from: dietert1 on April 07, 2024, 03:03:12 pm ---Recently i commented about a RTC test using a STM32 Nucleo-L476. I estimated the out of the box frequency offset as about 10 ppm.
Meanwhile i connected a ublox 6M GPS module to the board and implemented capture of its one second pulse output using timer 1 running from LSE with a 32768 cycle (channel 3). Then i implemented frequency measurement over spans of 65 seconds (linear regression of captured timings). The diagram shows the measured frequency offsets over about 4 hours in units of the RTC smooth calibration unit = 0.954 ppm. Average offset is 9.764 ppm with a standard deviation of 0.031 ppm.
So the clock noise can be as little as one second per year.
--- End quote ---
I don't know much about the STM32, but FYI I was running a SAMD controller off it's internal oscillator (no xtal) and marveling at how accurate and stable it was.
Uh, no.
I had the controller hooked up to USB, using USB serial to communicate. Turns out that the USB is used to monitor and adjust the internal uC clock. When I unplugged the USB connection the internal clock became pretty awful. It was in spec, no problem there, but the amazing accuracy was no longer so amazing.
dietert1:
In Germany we also have DCF77. The advantage with GPS is availability of cheap and easy to use receivers. To make a good DCF77 receiver with similar accuracy one probably needs a rubidium local oscillator, so it can't be competitive.
I don't have that USB confusion in my results on the STM32L476 LSE crystal oscillator. It's a free running crystal oscillator and it serves as a clock. Typical accuracy i see using the "smooth calibration" RTC feature is 0.1 sec per day and this may still improve after implementing aging and ambient temperature correction.
Depending on required accuracy one could turn on GPS maybe once per week or once per month.
Regards, Dieter
nctnico:
--- Quote from: dietert1 on April 19, 2024, 06:24:54 am ---In Germany we also have DCF77. The advantage with GPS is availability of cheap and easy to use receivers. To make a good DCF77 receiver with similar accuracy one probably needs a rubidium local oscillator, so it can't be competitive.
--- End quote ---
DCF77 will never get you an accurate clock like GPS as DCF77 doesn't / can't compensate for varying propagation delay through the atmosphere and other interferences.
dietert1:
No, sorry, years ago i did some research and using a rubidium clock and only daytime reception i got down to 10 ** -11 using DCF77. Our lab is about 200 km from the station.
Except the receiver wasn't a small module that you order at ebay for under € 7. And there is no redundancy in case they turn it off one day in the future. I didn't propose to use DCF77 instead of GPS.
Regards, Dieter
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