Unruh wrote:
> David Lord <sn...@lordynet.org> writes:
>
>> James Browning wrote:
>>> On Sep 17, 10:52 pm, "David J Taylor" <david-tay...@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.
Oops
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.
David
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