[time-nuts] GSM/GPRS module timing accuracy

[Tony]

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