[ntp:questions] GPS 18 and PPS

Tom Campbell tom at cosmosx.com
Sat Feb 26 04:58:14 UTC 2005

I'm trying to hook up a recently aquired Garmin GPS 18 (LVC), and am 
having some 'difficulties'.

NMEA data is working fine, both in mode one and mode two. (I switched 
from 1 to 2 due to a sample config I received. the wiring appears fine, 
and including a continutiy test from DCD, to my last solder point.

>From it's position on the roof, both indicate a good fix, however I 
can't seem to get PPS working.

When I query the unit, via $PGRMCE I get:

Which tells me PPS is on, with a 100ms duration. Power has been dropped 
from the unit, so I assume the setting is 'active'.

The unit attached to a linux 2.6 kernel, and since PPSKit lags, it's 
not available. But, from everything I read, driver mode PPS should be 
available, at a cost of jitter and resolution, which I'm ok with until 
I can put together a FreeBSD box, and port my monitering tools.

The current config in ntp.conf is:

#enable pps
server mode 2 version 4 prefer minpoll 4 maxpoll 4 # 
Generic NMEA Device

Other datapoints I can think of (these are just after an external sync):

# ntptime
ntp_gettime() returns code 0 (OK)
  time c5ca801c.ae4fd000  Fri, Feb 25 2005 23:57:00.680, (.680905),
  maximum error 36723 us, estimated error 3824 us
ntp_adjtime() returns code 0 (OK)
  modes 0x0 (),
  offset -103.000 us, frequency -10.348 ppm, interval 4 s,
  maximum error 36723 us, estimated error 3824 us,
  status 0x1 (PLL),
  time constant 0, precision 1.000 us, tolerance 512 ppm,
  pps frequency 0.000 ppm, stability 512.000 ppm, jitter 200.000 us,
  intervals 0, jitter exceeded 0, stability exceeded 0, errors 0.

ntpdc> clockstat
clock address:
clock type:           Generic NMEA GPS Receiver (20)
last event:           0
current status:       0
number of polls:      41
no response to poll:  0
bad format responses: 0
bad data responses:   0
running time:         660
fudge time 1:         0.000000
fudge time 2:         0.000000
stratum:              0
reference ID:         GPS
fudge flags:          0x0

Should this configuration work? (No PPS in the kernel), if so, is there 
something I'm just missing?

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