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Cheapest way to get date/time from GPS

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fourfathom:

--- Quote from: EPAIII on April 19, 2024, 02:20:49 am ---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.
--- End quote ---
There are several reasons why you might not want to use the WWV (and WWVH from Hawaii, and CHU from Canada, and a few others) broadcast GPS time/frequency signals:
* Propagation can be world-wide, but usually isn't. 
* Even with good propagation, this is usually only on some frequencies (WWV broadcasts on 2.5, 5, 10, 15, 20 MHz), and which frequencies you can receive depends on time of day and location.
* Ionospheric propagation (skip) causes of course delay, and this delay varies hourly, daily and seasonally, causing Doppler shift of the received signals.  This shift can be several Hz.  Not a big deal for general time-of-day stuff, but if you need better accuracy this matters.
* Antenna size, available receivers, etc.

GPS is not without faults and vulnerabilities, but it is usually the better solution.

Peabody:
In the US you would use WWVB, which is the low-frequency transmission.  It transmits the coded "exact" date and time every minute.  You can use the $7 eBay receiver module and an Arduino decoder, so the cost would be very modest.  But you still have the propagation delay problem, so the short-term accuracy isn't good.  But over the long term, as long as you can receive the signal, you would always be within one second of the correct time.

nctnico:

--- Quote from: dietert1 on April 19, 2024, 09:32:59 am ---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.

--- End quote ---
200km is pretty close. But the 10 ** -11 is compared to what? Does the lab you work at have a direct (land line) connection to the clock driving the DCF77 transmitter for comparison purposes?

dietert1:
There is a german paper here: http://cadt.de/dieter/dcf/Praezisionsfrequenzmessungen.pdf
In Figure 12 you can see the 10 ** -11. This doesn't mean there was a determination of time to 10 psec, but the clock speed (in that case the rubidium oscillator) could be calibrated to that precision using DCF77. In my tests i actually saw agreement between a GPS and DCF77 (resp. the disciplined oscillators) at the 2E-12 level. Maybe they are using a GPSDO at the DCF77 station. Nowadays we better say GNSS.

Regards, Dieter

tom66:
DCF77 uses a Caesium atomic reference.  It will be pretty close in frequency accuracy to GPS, with the only differences being due to special and general relativity (I assume the correction for this is done in any module for both time and position, but I do not know for sure.)  GPS also has ionospheric correction via the almanac data.  Of course, DCF77 is not particularly subject to relativity, but it doesn't provide any ionospheric correction, so the absolute accuracy of time information cannot be as good as GPS.  But for applications outside of metrology, it's perfectly fine.

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