Mailinglist Archive: opensuse (1420 mails)

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Re: [opensuse] openSUSE + gpsd + PPS + NTP
  • From: Roger Oberholtzer <roger@xxxxxx>
  • Date: Wed, 15 Jan 2014 08:03:24 +0100
  • Message-id: <1537596.7ZPh1po89l@acme>
On Tuesday, January 14, 2014 11:47:31 AM John Andersen wrote:

NTP will often NOT compensate for several hours of difference.

This is a worry. These systems are in vehicles on the road. We have no control
over the time on the PC clock at boot. We do NOT (repeat NOT) want the users
entering the BIOS and mucking about.

This is by design, but, there may be a switch to over-ride it.
When it does get a connection to NTP, even after weeks of being
disconnected or turned off, it will sync immediately, and adjust
time all at once, provide the clocks are reasonably close.
When I say immediately, I mean as soon as NTP elects a clock
from the list you provide to it.

I don't recall any gradual time shift, but I haven't read the man
page in a while.

Your GPS source should provide you UTC time.
(Well, US GPS sats provide GMT plus an offset to arrive at UTC, but
Glonas and Galileo only provide UTC).

However, I've seen many references on the net about GPS being
a poor source, probably due to the delay in getting time
as you mentioned. It might be wise to not start NTP until
you have believable time from the GPS.

As it turns out, we want to set the time because we time stamp everything. And
the way we hook all the parts together is by location as provided by the GPS.
So, what we really are after is having the PC clock match the GPS time as
closely as possible. How that relates to some global correct time is not
really interesting.

I should add that our GPS are really inertial navigation systems that have the
GPS as one source to maintain location. There are also inclinometers,
gyroscopes and distance measurement devices as well. INS systems use the GPS
time to time stamp locations.

Yours sincerely,

Roger Oberholtzer

Ramböll RST / Systems

Office: Int +46 10-615 60 20
Mobile: Int +46 70-815 1696

Ramböll Sverige AB
Krukmakargatan 21
P.O. Box 17009
SE-104 62 Stockholm, Sweden
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