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 17:00:41 +0100
  • Message-id: <3751421.TAvaFMoj1b@acme>
On Wednesday, January 15, 2014 04:19:40 PM jdd wrote:
Le 15/01/2014 15:52, Roger Oberholtzer a écrit :
meters a second), an error of 0.1 seconds produces an error of 2.5 meters.

any correct clock is better than 1s for a hole month!

The problem is that the times in records from a GPS are when that record
was calculated. It is not the time when I see it.

?

two things.

* if the gps clock is really unusable, read this:

http://tf.nist.gov/service/pdf/computertime.pdf

for some very good solutions (two last paragraphs)

* I do not own a gps like yours nor have ever tried to connect the gps clock
to ntp, but I wonder how the gps internal clok is advertised to the world.

The problem is not the accuracy of the GPS clock. Let's pretend it is perfect.
That is only internal. It must communicate this information externally
somehow. This is via serial, USB or ethernet. All three of those are non-
deterministic in that you do not know how long it takes for the data to get
from the GPS into your system. That jitter is a source of error. With the
serial port, the GPS can toggle a line that interrupts the client device,
allowing improved interpretation of the time when, eventually, it arrives.

I mean, what is the gps saying when there are no satellite visible? is it
transmitting anything? don't it transmit at least the time?

This is not standard. Some report whatever their clock says. Others transmit
nothing. We use the NMEA ZDA record for this. It seems to be transmitted even
when there are no satellites. Until you see something like a GGA record, you
have to assume there has not been a sat fix and thus the time in the ZDA record
is not known to be current UTC time. The NMEA protocol is highly bad. Read
Eric Raymond's assessment at http://esr.ibiblio.org/?p=801



--
Yours sincerely,

Roger Oberholtzer

Ramböll RST / Systems

Office: Int +46 10-615 60 20
Mobile: Int +46 70-815 1696
roger.oberholtzer@xxxxxxxxxx
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Ramböll Sverige AB
Krukmakargatan 21
P.O. Box 17009
SE-104 62 Stockholm, Sweden
www.rambollrst.se
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