[time-nuts] Symmetricom TimeSource 2700

Peter Bell bell.peter at gmail.com
Wed Nov 30 05:42:39 EST 2011

It's been a while, but from what I remember the sync channel message
does indeed include the system time (which is the same as GPS time
with a UTC offset) and also the PN code offset that this cell is
using.  This leaves the only remaining unknown as the path delay to
the cell and the possible error in the local clock on the BTS.

The other possible source of error is that if one of the sites loses
GPS lock, it will flywheel - this will generate a yellow alarm, but
this is not communicated over the air interface - I suspect that the
largest component of that stated 10uS maximum timing error is based on
worse-case accumulated phase error.  I also suspect this is why that
Symmetricom box is tracking multiple pilots, so it can isolate and
discard any that appear to be significantly out.



On Wed, Nov 30, 2011 at 1:37 PM, Dennis Ferguson
<dennis.c.ferguson at gmail.com> wrote:
> 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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