[ntp:questions] almost decided which new PPS GPS to buy
Ron Frazier (NTP)
timekeepingntplist at c3energy.com
Mon Mar 5 15:40:14 UTC 2012
On 3/4/2012 5:51 PM, David Lord wrote:
> Ron Frazier (NTP) wrote:
>> Hi all,
>> Some of you know from my prior threads that I've been debating what
>> GPS to buy to upgrade from my USB based BU-353. I think I've just
>> about nailed it down, based on a number of conflicting factors.
>> I'm getting + / - 6 ms accuracy now with the BU-353. My goals for
>> the new device are: 1 ms if using USB-serial interface, and 100 us if
>> using a real serial interface. One problem with the BU-353, is that
>> it's average offset from the internet servers seems to wander around
>> up to 60 ms or so. When that happens, NTPD sometimes marks my GPS as
>> a falseticker. ESR, the guy mentioned in another thread who wants to
>> connect GPS to 100 routers, called this "wobble", and said it's
>> related to the SIRF chipset. I haven't done any other research to
>> confirm that.
> There is sawtooth error when you get to the nanosecond level
> and some timing GPS can output a correction factor for this.
> I suspect you mean the uncertainty in timing of the NMEA
> I have added a couple of extra parameters to my ntp.conf
> # mindist allows sources with large offsets
> tos mindist 0.3
> # mode 18 for the Sure
> server 127.127.20.2 mode 18 prefer
> # time2 to give offset around zero
> fudge 127.127.20.2 time2 0.410 refid GPSb
> server 127.127.22.2
> fudge 127.127.22.2 flag2 0 flag3 1 refid PPSb
> With both ntp-4.2.6p3 and p5 I found that PPS had an "x"
> flag rather than "o" when I used minpoll and maxpoll
> although PPS was conditioning the clock and PPS offset
> was around 0 usec.
Hi David L.,
Thanks for the info.
I'm already using fudge time 2 to correct the offset in the NMEA data
I'm getting on my USB GPS.
I looked up the data for mindist here
http://doc.ntp.org/4.2.6/miscopt.html . I couldn't find a similar page
for NTP 4.2.7. I'm assuming they're very similar. I'm starting to
realize there is potentially a LOT to know to configure NTP beyond the
simple basic case. The page said this:
quote on ->
Specify the minimum distance used by the selection and anticlockhop
algorithm. Larger values increase the tolerance for outliers;
smaller values increase the selectivity. The default is .001 s. In
some cases, such as reference clocks with high jitter and a PPS
signal, it is useful to increase the value to insure the
intersection interval is always nonempty.
< - quote off
I'm still not totally sure what it does, but it says it increases
tolerance for outliers and specifically mentions reference clocks and
pps, so I might try the default value you suggested
I couldn't find the data for flag2 and flag3, and I was wondering if
they're different for different drivers.
>> I'm pretty sure I'm going with the Sure board (pun intended) for the
>> following reasons:
>> A) Low cost for a NEW product.
>> B) Antenna and USB cable included.
>> C) No need to add connectors to the board, or a power supply if
>> powered by USB.
>> D) Good timing performance.
>> E) High sensitivity for potential indoor operation.
>> F) David Taylor's excellent tutorial on setup:
>> From my reading, it appears that I need to add the following to the
>> Sure Electronics package of parts:
>> 1) DB-9 Male - DB-9 Female serial cable
>> 2) Motherboard header - DB-9 port adapter
>> 3) Plastic box for mounting. Misc hardware like screws, standoffs, etc.
>> (David T. or others, I don't suppose you can suggest any specific
>> part numbers for those hardware items from Radio Shack [do you have
>> those in the UK], Mouser, or DigiKey. Just thought I'd ask.)
>> My plan of attack is as follows, although the sequence may vary:
>> A) Get the unit working "at all", using USB NMEA data only. I would
>> hope to see performance equal to my BU-353.
>> B) Get the unit working with NMEA data from the RS-232 port, going
>> through a Trendnet TU-S9 USB - Serial converter, based on the
>> Prolific chipset. Performance should, again, be similar to the
>> BU-353 (which also uses the Prolific chipset internally).
>> C) Get the unit working with NMEA data from the RS-232 port, going to
>> a real serial port. I have to tinker with the motherboard on another
>> computer to do that.
>> D) Get the unit working with NMEA + PPS, with data from the RS-232
>> port, going through the TU-S9, which claims to pass all the
>> handshaking signals. I'll have to do the mods that David recommends
>> on the board for this. Hopefully, the same procedure will still work
>> on the recent boards. This is where I'm hoping to get + / - 1 ms
>> E) Get the unit working with NMEA + PPS, with data from the RS-232
>> port, going to a real serial port on the other computer.
>> This should be an interesting adventure, hopefully ending with a time
>> server running at 100 us accuracy or better.
(PS - If you email me and don't get a quick response, don't be concerned.
I get about 300 emails per day from alternate energy mailing lists and
such. I don't always see new messages very quickly. If you need a
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timekeepingdude AT c3energy.com
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