[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
> outputs.
> I have added a couple of extra parameters to my ntp.conf
> /etc/ntp.conf:
> #######
> # mindist allows sources with large offsets
> tos mindist 0.3
> # mode 18 for the Sure
> server mode 18 prefer
> # time2 to give offset around zero
> fudge time2 0.410 refid GPSb
> server
> fudge 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.
> David

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 ->

mindist /mindistance/
    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: 
>> http://www.satsignal.eu/ntp/Sure-GPS.htm
>>  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 
>> performance.
>> 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.
>> Sincerely,
>> Ron


(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
reply and have not heard from me in 1 - 2 weeks, send your message again.)

Ron Frazier
timekeepingdude AT c3energy.com

More information about the questions mailing list