General > General Technical Chat
Cheapest way to get date/time from GPS
5U4GB:
--- Quote from: tom66 on February 27, 2024, 11:47:49 am ---
--- Quote from: peter-h on February 27, 2024, 11:46:52 am ---To get date/time.
Or to periodically sync a standard 32768Hz xtal RTC.
--- End quote ---
...to what required accuracy? Is there a good reason strobing power to the GPS module as has been proposed here not practical?
--- End quote ---
Or just do an HTTP GET to a local web site and read the date and time from the header? There's still nowhere near enough information to provide a useful response. In particular the two examples above are so different it sounds more like "I want to play with GPS, and I'll figure out what to do with it eventually". Which is perfectly fine, but you then need to say so.
peter-h:
Guys, I am asking to learn, not because I am developing a product. This is such a generally applicable issue... I've been looking at this for 30-40 years.
I have indeed developed a product just recently but that uses GPS time, using a U-BLOX GPS module (as I posted). It is 12/24V DC powered so power is not an issue, and actually neither is the £5-10 cost of that module.
--- Quote ---Actually, using a mobile network as a time source is also an option. Unless a device is really installed in a remote location, you'll have some coverage. The pro is that a mobile network has a much better indoor coverage.
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I was about to post that I read somewhere years ago that one can get time from GSM (no need for GPRS/3G/4G/5G) without needing a paid-up SIM card, and now I see that Someone has posted something that :) I slightly doubt the assertion in that PDF that no SIM card is required, but (obviously) if you emulate one (how much is that module??) then you don't need to buy one :)
One great thing about GPS is that you get UTC, always. I am not sure if GSM tower time is UTC; it may just be local time... otherwise how does your phone switch to local time (it definitely uses GSM; this worked with phones 20+ years ago) when it doesn't know its location i.e. the time zone? Maybe the tower sends out the TZ too.
If you can get to some web server then you can do NTP, so that doesn't help.
5U4GB:
--- Quote from: peter-h on February 28, 2024, 08:07:27 am ---Guys, I am asking to learn, not because I am developing a product.
--- End quote ---
Actually if you did want to work on something specific, because having a concrete goal is always a motivator :-), you could look at an update to this which seems to be along the lines of what you've been talking about.
nctnico:
AFAIK the time is in de mobile signal. No need for a sim card.
shapirus:
Receiving time over GSM without even a SIM card (as, apparently, the time information is available in the broadcasted signal) is very interesting. Are there any existing modules that could be used for this?
I have an idea of taking an old nixie tube clock that is quite inaccurate to be used comfortably as it is, and replacing the internals with a decent crystal oscillator and RTC (+MCU) that would be periodically auto-adjusted, including initial setup, based on precise (and +/- 0.5 s is precise enough!) time acquired one way or another. This could be: a) NTP over WiFi; b) GPS; c) GSM (?).
WiFi requires initial setup (and reconfiguration when the connection settings change). GPS has poor indoors reception. GSM, if at all feasible, sounds like a perfect solution that would allow to build a clock with zero effort setup: just plug it in, wait for the time sync, done. The signal is available almost everywhere and can be received indoors, save for places like cellars etc. Well, maybe a button to set the time zone offset. And DST.
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