[ntp:questions] almost decided which new PPS GPS to buy

David Lord snews at lordynet.org
Sun Mar 4 22:51:01 UTC 2012

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 mode 18 prefer
# time2 to give offset around zero
fudge time2 0.410 refid GPSb

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.


> 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

More information about the questions mailing list