UTC Time from NMEA receiver one second behind DCF?

UTC Time from NMEA receiver one second behind DCF?

Post by Harald Bri » Mon, 11 Aug 2008 22:20:26


This is my setup:

I am using a Navilock NL-320U connected to a small Linux box running ntp
4.2.4p4-44.1 that came with the openSuSE 11.0 distribution. This is
supposed to supply a time service to the local network without the use of
external network ntp servers purely from the received GPS signal.

I know that using a USB connection is not optimal, but the achieved accuracy
is fine for my needs.

Last Saturday (2008-08-02) I noticed for the first time that the time off
the GPS unit was one second behind the DCF time, which I monitor on a
separate radio clock. A reboot of the system did not help. On Sunday
(2008-08-03) everything was back to normal. I noticed the same effect again
on Friday evening (2008-08-08) and through yesterday (2008-08-09), but
everything seems fine today (2008-08-10).

I added a network server to my ntp configuration to double check the effect.
This is how the output "ntpq -p" looks like when everything is fine:

remote refid st t when poll reach delay offset
jitter
==============================================================================
LOCAL(0) .LOCL. 10 l 19 64 377 0.000 0.000
3.906
*GPS_NMEA(0) .GPS. 3 l 22 64 377 0.000 14.902
3.906
xptbtime2.ptb.de .PTB. 1 u 422 1024 377 66.375 -8.106
5.216

And this is the output when I observe the one second lag:

remote refid st t when poll reach delay offset
jitter
==============================================================================
LOCAL(0) .LOCL. 10 l 33 64 17 0.000 0.000
3.906
*GPS_NMEA(0) .GPS. 3 l 30 64 17 0.000 -978.27
11.515
xptbtime2.ptb.de .PTB. 1 u 31 64 17 67.160 1.002
3.906

Looking at the raw NMEA output, the UTC info in there also seems to be one
second slow.

In all of this I presume the PTB time to be correct.

My question is, has anyone else observed this and how can I fix this?

Thanks

Harald
 
 
 

UTC Time from NMEA receiver one second behind DCF?

Post by Richard B. » Tue, 12 Aug 2008 01:36:34


It sounds as if the GPS receiver you have was designed for navigation
rather than timing!

Timing receivers typically have a Pulse Per Second (PPS) output and use
a binary protocol rather than NMEA to transmit the time to a serial port.

Using the proper tool for the job should solve many of your problems.

 
 
 

UTC Time from NMEA receiver one second behind DCF?

Post by Harald Bri » Tue, 12 Aug 2008 04:37:52


==============================================================================
==============================================================================

Point taken. Didn't I mention I am a cheapskate? ;-)

What flummoxes me is that I occasionally (and only in the last couple of
days) have observed this offset of almost exactly one second. If this would
happen to a receiver with PPS, the result would then be exactly one second
off. I was hoping for some educated guesses how this might have happened.
Maybe a GPS receiver bug in connection with the upcoming leap second?
 
 
 

UTC Time from NMEA receiver one second behind DCF?

Post by Unru » Tue, 12 Aug 2008 09:25:38

arald Brinkmann < XXXX@XXXXX.COM > writes:




The Garmin 18LVC reveiver is about $60, and has a pps and serial output--
some wiring is required.

It is not clear what is happening. what GPS receiver is it? What are you
running to get the time off of it?






 
 
 

UTC Time from NMEA receiver one second behind DCF?

Post by Harald Bri » Wed, 13 Aug 2008 05:06:43


The only European source for the Garmin 18LVC I found asks for 160 Euro.



(Please note the corrected transposed digits in the receiver type.)

I think the point I am interested in is still not yet quite clear to those
to have been kind enough to reply. I have had this setup up and running for
about 18 months. The achieved accuracy has always been better than 100
milliseconds, which may sound awful to most of you, but is good enough for
my needs. Now, all of a sudden, and only on some days, do I get an offset
of more or less exactly one second. I haven't yet observed any offsets of
less than one second. I also haven't observed any bigger offsets. It is
always either this one second offset or normal operation with relatively
small offsets that I don't worry about. All of this only began to happen in
the last week or so. Apparently the Navilock uses the Sirf III chipset,
which seems to be in widespread use. If there is a bug in there, it should
have happened to someone else as well, right?
 
 
 

UTC Time from NMEA receiver one second behind DCF?

Post by Venu Gopa » Wed, 13 Aug 2008 14:09:03


I have a couple of similar experiences !
I observed that the NMEA sentences are not generated in sync with PPS.
Theres lot of jitter in these sentences resulting in 1 second offsets.
Its fine if the jitter is within few milliseconds. But sometimes it
exceeds a second and thats really painful.

This observation was discussed earlier and the solution is to go for
a better GPS receiver!
 
 
 

UTC Time from NMEA receiver one second behind DCF?

Post by David J Ta » Wed, 13 Aug 2008 16:34:24


[]

Look harder, then!

http://www.yqcomputer.com/

GBP 66.95 including tax => Euro 85.

Cheers,
David
 
 
 

UTC Time from NMEA receiver one second behind DCF?

Post by David J Ta » Wed, 13 Aug 2008 16:35:55


Use the shortest GPS sentences, and the highest baud rate, to keep the
total message time as short as possible.

David
 
 
 

UTC Time from NMEA receiver one second behind DCF?

Post by Venu Gopa » Wed, 13 Aug 2008 18:46:00


The issue is not with the length of the messages and the baud rate.

Venu
 
 
 

UTC Time from NMEA receiver one second behind DCF?

Post by David J Ta » Wed, 13 Aug 2008 20:49:48


Well, I found that if you had a bad configuration (sentences too long or
baud rate too low), the sentences could exceed one second, and therefore
be useless for NTP.

However, I have not made any measurements showing jitter versus sentence
length, so I would appreciate any pointers to measurements you have made
or know about.

Cheers,
David
 
 
 

UTC Time from NMEA receiver one second behind DCF?

Post by hal-usene » Thu, 14 Aug 2008 05:22:28


[big snip]


I've seen similar quirks from GPS units using the SiRF chip set.
http://www.yqcomputer.com/ ~hmurray/ntp/leap-gps3.gif

It started about the time the leap second was announced. (Otherwise,
they work, but not very well. They have a 100 ms jitter/drift.)

It looks like a weekly pattern. It will be interesting to see what happens
during the next week.

--
These are my opinions, not necessarily my employer's. I hate spam.
 
 
 

UTC Time from NMEA receiver one second behind DCF?

Post by Harald Bri » Thu, 14 Aug 2008 05:50:28


Well, I *looked* harder and all German sources I found ask for well in
excess of 100 Euro. In addition many of them state that the Garmin is out
of production or out of stock. The Navilock starting at just over 30 Euro
is pure cheapskate heaven by comparison. But I am tempted by the U.S.
prices. I will have to sleep over whether I'm a bigger cheapskate or a
bigger nerd.

Thanks

Harald
 
 
 

UTC Time from NMEA receiver one second behind DCF?

Post by Harald Bri » Thu, 14 Aug 2008 06:02:30


But even problems caused by a slow data transmission wouldn't explain why
the observed offset is small compared to one second most of the time (and
in my case continuously for well over a year) and suddenly and only on
certain days there is an offset of almost exactly one second, unless the
messages are suddenly and only on those days extraordinarily long and just
happen to delay everything by one second. I'll have a lookout for the raw
NMEA messages the next time this happens. *If* I were using a PPS signal, I
could understand the system being confused about which NMEA message belongs
to which PPS pulse, so one would get an offset of (maybe multiples of) one
second. But that's not the case here.

Harald
 
 
 

UTC Time from NMEA receiver one second behind DCF?

Post by Harald Bri » Thu, 14 Aug 2008 06:09:31


Yes, that fits my observations well, including the weekly pattern...so far.
I suppose you don't know of any workaround for ntp purposes?

Cheers

Harald
 
 
 

UTC Time from NMEA receiver one second behind DCF?

Post by David J Ta » Thu, 14 Aug 2008 15:25:35


[]

No, indeed. Hal's graph shows it nicely. I have a SiRF III chip set
equipped GPS (Garmin GPS60 CSx), but it doesn't display the seconds (or is
there a screen where it does?). One casual check suggested that it
changed from 06:20 to 06:21 UTC at about the expected instant, using the
clock on the map display screen, but I've done no systematic checks.

I do hope you get to the bottom of this.

Cheers,
David