[time-nuts] Lost GPS lock or 1PPS recently?

jimlux jimlux at earthlink.net
Sat Sep 8 10:25:30 EDT 2018

On 9/7/18 10:05 PM, John Reid wrote:
> Hi all,
> discussion of how to keep accurate time without access to GPS seems very
> on topic to me.
> These people involved in major catastrophe ('end of the world' as you
> put it) scenarios have a wealth of experience in other ways of keeping
> accurate time.

Actually, they don't necessarily have a wealth of experience, because 
they may have marched themselves down a path where they have a 
*requirement* for much better timing than they realize, because it is so 
easy and cheap to get good time today.

Imagine this scenario - you're a bank, and you batch process checks and 
deposits in one physical location, so you don't much care about when the 
check was written or the deposit made.  Then you move to a distributed 
system across the US, where the reconciliation is done on the basis of 
the date of the transaction - still probably ok, because there are no 
transactions during non-business hours, so as long as you reconcile at 
1AM, if transaction time stamps are off by 5 minutes, it doesn't matter.

Now say "we're going to charge you, the customer a fee, if your balance 
goes negative" and go to 24/7 operations, where transactions are 
journaled immediately, rather than batch processed at night  If a 
deposit that was made at 12:00 (but timestamped 12:05)  is followed by a 
withdrawal made at 12:03 (but timestamped 12:00), you get unfairly 
charged the overdraft fee.

For small problems, banks have ways to "unwind" errors.  But if it 
becomes a systemic thing that's a problem.

So the bank sets up GPSDOs at each transaction point - problem solved.

Until GPS fails.

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