[time-nuts] Leap seconds and POSIX
joegwinn at comcast.net
Fri Jan 2 11:31:51 EST 2009
At 8:39 AM +0000 1/2/09, time-nuts-request at febo.com wrote:
>Date: Fri, 2 Jan 2009 14:00:56 +1300
>From: "Steve Rooke" <sar10538 at gmail.com>
>Subject: Re: [time-nuts] Leap seconds and POSIX
>To: "Discussion of precise time and frequency measurement"
> <time-nuts at febo.com>
>2009/1/2 Joe Gwinn <joegwinn at comcast.net>:
>> Platforms vary because NTP is at the mercy of the kernel developers.
>> From the standpoint of the average user, there is a transient error.
>> Not that many average users will notice, so long as nothing crashes
>> or hangs.
>> In systems where the transient error and possibility of a crash or
>> hang cannot be tolerated, one common dodge is to lie to NTP by
>> configuring the GPS receiver and NTP timeserver to emit GPS System
>> Time, and live with the fact that the local computer clocks are ~14+
>> seconds off of UTC. Purpose-built user displays are programmed to
>> compute and use the correct time.
Examples of what?
The systems of my direct experience are radars and the like. Such
systems always include trackers, and having a time step or a re-lived
second can really destabilize things. To avoid all such problems,
one common approach is to create a uniform and smooth timescale for
use by the radar software.
This was done long before GPS was invented, or NTP for that matter.
Now days, the same smooth and uniform timescale is often implemented
using a GPS receiver set to emit GPS System Time and NTP to
distribute this time to the rest of the radar system.
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