[ntp:questions] GPS Jammers in Use by Criminals - Warping Time for Fraud Suggested
davehart at gmail.com
Thu Feb 23 22:53:56 UTC 2012
On Thu, Feb 23, 2012 at 21:40, unruh <unruh at invalid.ca> wrote:
> On 2012-02-23, Richard B. Gilbert <rgilbert88 at comcast.net> wrote:
>> In the U.S. 60 cycle Alternating Current is the standard and the source
>> of time. It's not going to give you the nanoseconds but very few people
>> could even explain what a nanosecond is let alone needing nanosecond
> It is neither a standard nor is it a "source of time". The requirement
> is that the phase of the 60Hz be the same across the country, so
> electricity can be traded between generation facilities without one
> source shorting out the other.
That sounds a bit oversimplified. In the US, there are three
different grids, with some DC interconnections.
> That does not require frequency accuracy,
> just that the frequency of all the generators be the same and the phase
> difference be less than a ms or so.
> Because the easiest way to ensure phase coherence is to demand frequency
> coherence and use a standard, like UTC, as a reference, they do tend to
> be close.
Load skews frequency, and each grid's frequency runs a bit slow during
peak load times. The generators do not sync to 60 Hz, they sync to a
nearby value that is the grid frequency at any instant.
> But the requirement is phase coherence not time accuracy and
> the latter will be jettisoned in favour of the former.
Yes but only to a limited extent, so far. Because of the prevalence
of clocks that do rely on the grid for timekeeping, each day the
frequency is intentionally boosted on each of the 3 US grids during
non-peak periods to compensate for the missing cycles from the
load-induced slowdown. At the end of the day the total number of
cycles should be near nominal, insulating such clocks from
accumulating offsets day to day due to the interaction of AC generator
load and frequency. Given the dwinding number of such clocks and
relatively arcane grid operational reasons, there's been some movement
to abandon the corrections, which seems to be stalled at the moment.
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