On Fri, 2011-07-15 at 13:41 +0200, Per Jessen wrote:
Probably, but not necessarily. The local clock _could_ be quite accurate by itself. I use ntp with a dcf77 receiver because it's easy and accurate, but I don't actually need the accuracy to more than 1 second, and only between my own machines.
One second accuracy for me would be a disaster. We need to locate things within a decimeter. At 90 km/h, that is 1/250 of a second. So we would like time to be at least 1/500 of a second accurate. By 'accurate' I mean between the systems (the GPS receiver being one of them), not absolute.
I guess it assumes that the error in the real time clock is basically linear.
Or at least predictable.
As long as chrony discovers the method of prediction. The folk on the gpsd list have only good things to say about it.
I define 'seldom' as being when the GPS is on and has a fix. For example, if our vehicle is turned on in a garage, the GPS may not provide PPS signals and a GPS-based time. It too is providing an internal time maintained between satellite access.
So now "seldom" is directly related to how often the vehicle is active? Are we talking a moon buggy or a city tram? :-)
Depending on the season. a bit of both.
On my openSUSE machine, I use NTP and a wireless network enabled by Network Manager (not the traditional if up method). So, when NTP starts at boot, there is no network, and so it never gets the time correct - it stays with whatever the BIOS has. I have to restart NTP by hand after I log in and Network Manager gets me a connection. Seems chrony deals with this better.
I don't know chrony at all, I can't comment. What you're describing about ntp sounds like a bug though. ntp has, in my experience, no problem switching between servers depending on accuracy and availability.
In my local setup, I have one DCF77 receiver which is not always able to receive a good time signal. When that happens my primary ntp server will fall back to stratum 2 and sync with one of two external servers. If those are also unavailable, it defaults to stratum 10 (localhost).
In my laptop case with Network Manager, it moves to localhost at boot because there is no other source available. There is one configured - but it is not available at that time. The problem is that it seems not to re-evaluate the decision. So, when the NTP server is eventually available, it does not care as it is using localhost. So, I run 'rcntp restart' and all is well again. This is not an acceptable solution in our vehicles. Thus chrony. Yours sincerely, Roger Oberholtzer OPQ Systems / Ramböll RST Office: Int +46 10-615 60 20 Mobile: Int +46 70-815 1696 roger.oberholtzer@ramboll.se ________________________________________ Ramböll Sverige AB Krukmakargatan 21 P.O. Box 17009 SE-104 62 Stockholm, Sweden www.rambollrst.se -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org For additional commands, e-mail: opensuse+help@opensuse.org