[time-nuts] Symmetricom TimeSource 2700

Dennis Ferguson dennis.c.ferguson at gmail.com
Wed Nov 30 05:37:30 UTC 2011


I think they track both the CDMA pilot and sync channels.  The latter
channel sends a message which tells the phone about the cell, and
gives gives the phone enough information to figure out the time of day.

I'm pretty sure CDMA phones have to know what time it is before they
register with the cell.  To receive the paging channel and negotiate a
registration the phone has to receive and send the long code chip sequence,
which I think is 2^40 bits long and takes more than a month to repeat.
The phone has to know what time it is before it has any hope of tracking
that.

I don't know how (or if) they deal with the distance from the cell.  The
accuracy of the PPS signal from CDMA time receivers is usually specified
as no better than 10 microseconds or so, so they may just assume the cell
tower is close enough not to make it worse than 10 microseconds.

Dennis Ferguson

On 29 Nov, 2011, at 18:54 , Peter Bell wrote:
> Assuming it's just tracking the CDMA pilots, the 1PPS output is likely
> not aligned with UTC.  The problem is that the pilot channel is just a
> PN sequence with no modulating data - so when you lock to it you can
> know that your local clock is 19200Hz * 64 chips/bit (1.228MHz) - but
> that's all you know.  Even the code phase doesn't tell you anything,
> since there are two unknowns - the first is the distance to the cell
> and the second is the code phase offset on this specific pilot (each
> BTS has it's modulating sequence offset by an integer multiple of 64
> chips to reduce mutual interference) - the second piece of information
> you can obtain by reading one of the overhead channels, but the first
> is basically not available just using a receiver (your phone can do
> it, since it can ask transmit back to the BTS and measure the round
> trip timing offset).




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