[ntp:questions] ntpd: time reset problem

David Lord snews at lordynet.org
Thu Sep 24 22:00:35 UTC 2009

Unruh wrote:
> David Lord <snews at lordynet.org> writes:
>> James Browning wrote:
>>> On Sep 17, 10:52 pm, "David J Taylor" <david-tay... at blueyonder.not-
>>> this-part.nor-this.co.uk.invalid> wrote:
>>>> Don't worry about trees - a modern GPS such as the GPS 18x LVC is very
>>>> sensitive, and can work with a lot of obscuration.  Although my GPS 18 LVC
>>>> is sitting on the roof, the GPS 18x LVC works just as well sitting in a
>>>> room on the top floor of the house.
>>> I have a SiRF star III based USB dongle (US GlobalSat BU-353)</plug>
>>> that I got a location off of in a basement room about 6 feet from the
>>> window. currently it's attached to box whos' ntpd crashes every time
>>> I
>>> mention it. I think it's talking in the binary mode and the kernel
>>> device
>>> is tuck at the wrong speed. (I probably just need to restart it.)
>> You can check with minicom or similar as to output. I used
>> Telix from DOS with my BR304 to set to just give a single
>> RMC sentence per second output. The codes needed to switch
>> modes were in the br304 manual I downloaded.
>> Having said that the br304 is now in pieces to be explored
>> with scope probe in case it's possible to locate any signal
>> usable for ntp. Best I had was +/- 0.5 sec but it is much
>> less sensitive than the Garmin GPS18x-LVC that early in year
>> maintained an offset within 4us (that's until sats in view
>> reduced by tree growth so it became unusable).
> Since the usb line has nothing to which a PPS interrupt could attach
> AFAIK, you are not going to get even ms timing from a usb dongle. You
> should certainly get better than .5 sec however.


sorry I missed that the BU-353 was usb. The BR-304 I have is rs232
same as Garmin GPS18x-LVC but no pps line. The BR304 is also
SiRF II rather than SiRF III.

Below are values for offset and jitter from polls at
30 min intervals.

           BR-304         gps18x-lvc         gps18x-lvc
              rmc                rmc            rmc+pps
    polls  offset      polls  offset      polls  offset
   %     n   (ms)     %     n   (ms)     %     n   (ms)
  50    90   9.23    51    27   0.07    54    76  0.002
  95   170  53.02    92    49   0.16    99   140  0.005
  99.5 178 435.84   100    53   0.20   100   141  0.006

Problem with BR-304 was lack of sensitivity and being
unable to keep sufficient satellites in view, even
though positioned with slightly better view of sky than
the Garmin.


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