[ntp:questions] GPS reception problem in UK

David Lord snews at lordynet.org
Sun Apr 18 23:54:30 UTC 2010


unruh wrote:
> On 2010-04-18, David Lord <snews at lordynet.org> wrote:
>> Hi
>>
>> I just noticed that the GPS used as reference for my xtal oscillator
>> clock source has from midnight last night been losing reach. Previous
>> 24 hr to that logging at 6 minute intervals had all 240 entries
>> at 377 but now they are mostly at 0 or start increasing for a short 
>> period, last up to 76, then drop back to 0. Any chance this is due
>> to atmospheric volcanic ash?
> 
> I sort of doubt it, unless the ash is conductive. 
>> David

I'd had 48hr with all sources at 377 throughout since last
change. I withdrew that post within a minute or so of posting
once I saw reach on my xtal clock had also been lost at about
same time. I think there is some interaction (bug) between
the different refclock sources that's been fixed in more
recent sources (I can't so far get these to compile on NetBSD).

I had this setup
	GPSb 127.127.20.0 using gps0 => tty00
	PPSb 127.127.22.2 using pps2 at ppbus
	OSCc 127.127.22.0 using pps0 => tty00

I've restarted with xtal oscillator OSCc moved to parallel port
and PPSb from GPS moved to serial port.

	GPSb 127.127.20.2 using gps2 => tty00
	PPSb 127.127.22.2 using pps2 => tty00
	OSCc 127.127.22.0 using pps0 at ppbus

and it looks ok again:

      remote       refid    st t when poll reach   delay   offset  jitter
========================================================================
+GPS_NMEA(2) .GPSb.         0 l   63   64  377    0.000    0.004   0.004
oPPS(2)      .PPSb.         0 l    6   16  377    0.000    0.001   0.004
  PPS(0)      .OSCc.         0 l    3   16  377    0.000  -186.94   0.088
-k6x400      158.152.1.76   3 u   12   64  377    1.215    0.144   0.517
+me6000g     .MSFa.         1 u   63   64  377    0.754   -0.103   0.560
-p4x2400c    192.168.59.64  2 u   24   64  377    1.247    0.841   0.540

I'm busy at moment getting circuit for xtal oscillator oven
worked out. Best open air xtal has done is 10ms change over
24hr but tonight it's drifting about 6ms per hour. Temperature
has varied since 11th from mid 20s to 12C or below so I'm
reasonably confident I'd get better than 1ms/day holdover
with even a crude temperature control. I'm using much same
circuit as for my beer brewing box but that has a 500W heater
element pwm via an opto isolated triac.


cheers

David




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