[time-nuts] GSM/GPRS module timing accuracy

Tom Harris celephicus at gmail.com
Tue Nov 11 16:52:46 EST 2014

How are you getting time from the GSM network? If you are using the NITZ
(look it up) service then you can use an unactivated SIM for free, but the
accuracy is woeful, it can be seconds out.

Else you will have to have a live SIM and use NTP to a server. If you make
any progress please post to the list. I have not heard of being able to
access the clock in the tower over the network.

The beauty of phone network is that it works in buildings, where GPS does
not reach.

Tom Harris <celephicus at gmail.com>

On 12 November 2014 01:24, Tony <tnuts at toneh.demon.co.uk> wrote:

> How accurately can you set, and keep in sync a local clock (in the UK) by
> getting timing from a cellular network using a very low cost GSM/GPRS modem
> module such as a SIM900? Is it possible to get better than hundreds of
> microseconds for example, perhaps even a few tens of us? How frequently can
> the clock be synced (to avoid having to have a high performance local
> clock) and would it incur network charges?
> Does it depend mostly on the network, the module,  location or the
> algorithms employed?
> I am thinking of a fixed location application but presumably couldn't
> guarantee that connections will always be to the same base station so is it
> possible to estimate the distance to the base station to compensate for the
> time of flight of the radio signal? I guess it should be possible to read
> the signal strength from the radio module to get a crude estimate.
> It should be obvious that I know next to nothing about GSM standards other
> than that the base stations have access to a high quality clock.
> If its not possible to do this reliably, are their any low cost (< $20)
> combined GSM/GPRS + GPS modules which provide timing accurate to a few 10s
> of us or so?
> Thanks
>    Tony H
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