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