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I understand that cell base stations transmit very accurate time signals among other things. Do you need a SIM card and an account to receive this information, or is it something you can "leech" with a GSM/CDMA module without establishing a connection?

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    \$\begingroup\$ It sounds easier to do that with a GPS receiver, gives more accurate time as well \$\endgroup\$ – ratchet freak Nov 2 '18 at 17:03
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    \$\begingroup\$ @ratchetfreak in many urban locations mobile network signals are far more available, especially indoors, than GPS. GPS really needs to see four satellites to accomplish much; a raw time transmission from a single mobile tower would be more useful than hearing just one GPS satellite. It seems like using a mobile network as an alternative to something like WWVB for human-scale time fixes could be an interesting idea, if it is workable. \$\endgroup\$ – Chris Stratton Nov 2 '18 at 17:08
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    \$\begingroup\$ Indeed the context of the question is a fallback form GPS. Indoor applications have non-deterministic acquisition times. The accuracy I'm looking for is on the order of 10us within a local area. \$\endgroup\$ – iter Nov 2 '18 at 17:16
  • \$\begingroup\$ Get a USB gps reciever, they are 10-20$, 10us should be way doable with GPS, because it has to know the TOF of the signals to within ns to calculate position \$\endgroup\$ – Voltage Spike Nov 2 '18 at 17:29
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    \$\begingroup\$ The context is a microcontroller device, not a USB host. In any case USB GPS is useless for timing. You don't know the delay from TOW to serial transmission, and you don't know USB latency. \$\endgroup\$ – iter Nov 2 '18 at 17:34
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Heh, very precise timing "on the order of 10us" can only be given to you by an Internet connection through which you then connect to the closest NTP server you can find. The GSM functionality for broadcasting time data is called NITZ and according to its Wikipedia article it's anything but precise (or reliable). Moreover you can't "leech" this data off of any BTS because the very reference specs of the NITZ standard (published by ETSI, the GSM standardization body) say that the data is obtainable only "upon registering on the network" (if you don't believe it, check out also this, this or even this). Seriously just get a GSM module and put a SIM card with some cheap plan into it instead.

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