The gps18lvc is an OEM model of the Garmin 12-channel/WAAS receiver,
packed together with an antenna into a small puck device. The PPS signal
is specified as "< 1 us", which means that the stability of the server
system clock will determine the actual performance.

At the other end of the (default) 3 m cable there's just a set of
colour-coded wires, carrying rs232-level send/receive/ground & pps
signals, as well as a pair of (slightly thicker) red/black power supply
wires.

I got the gps in a couple of days from a local distributor here in Oslo,
and picked up a 9-pin female rs232 connector with housing and a 1 m
small-size USB cable: Total cost was less than 1000 NOK, in the US I'm
guessing you could get the same for approx $100.

(Google... Yes, it seems like the gps itself is available for about $80!)

I then cut the USB cable and identified the +5V power wires (also
red/black coded!), I cut off the USB signaling wires with a small offset
between them to avoid any risk of a short circuit.

The only remaining task was to solder the signaling wires onto the rs232
connector:

pin     signal  colour
----------------------
1       PPS     yellow
2       TX      white
3       RX      green
5       GND     black

and join the remaining red/black wires to the corresponding pair from
the USB cable.

After wrapping some electician's tape for insulation around the power
lead joins, I carefully fit everthing inside the 9-pin rs232 cap and
closed it up.

Total time was less than 2 hours, including the trip to pick up the
parts, and it worked immediately:

I'm currently gathering clockstats info on the unit, my current
guesstimate would be something like ~5 us RMS time offset.

I used Garmin's SNSRCFG_280.exe program to setup the gps to work in NMEA
mode, sending out the most common time/position reports (including
GPRMC, which is preferred by the ntpd NMEA driver) once per second.

Using a spare USB plug as the power supply is a particularly nice
option, since any PC used as an ntpd server these days will have at
least one usch port available!

With temperatures stable, jitter can be as low as 0.001ms.

Thank you !

     remote           refid      st t when poll reach   delay   offset  jitter
=========================================================================== ===
+GPS_NMEA(0)     .GPS.            0 l   25   64  377    0.000   -0.036   0.020
oPPS(0)          .PPS.            0 l   43   64  377    0.000   -0.046   0.030
...

configured as:

server 127.127.20.0 mode 2 prefer
server 127.127.22.0
fudge 127.127.22.1 flag3 1

I am not sure about the PPS kernel stuff. ntptime gives me

ntp_gettime() returns code 0 (OK)
  time c735804b.b2cb2000  Mon, Nov 28 2005 14:10:35.698, (.698412),
  maximum error 28509 us, estimated error 16 us
ntp_adjtime() returns code 0 (OK)
  modes 0x0 (),
  offset 5.000 us, frequency 6.200 ppm, interval 256 s,
  maximum error 28509 us, estimated error 16 us,
  status 0x107 (PLL,PPSFREQ,PPSTIME,PPSSIGNAL),
  time constant 2, precision 1.000 us, tolerance 512 ppm,
  pps frequency 0.000 ppm, stability 0.000 ppm, jitter 2.000 us,
  intervals 4299, jitter exceeded 12, stability exceeded 12, errors 67.

is this really the "true" PPS, or is there more to it?
This is ntp 4.2.0 on Solaris 8, UltraSPARC

the connections go like this:

Garmin                  RJ45                    DB25f-Adaptor   Sun DB 25

Vin     Red             7,5     ws/br,ws/bl     gn,br           (+5V)
Gnd     Black           8,1     br,ws/or        bl,ws           (Gnd)

PPS     yellow          2       or              or       8      DCD (PPS)
Gnd     black           4       bl              rt       7      Gnd
Tx      white           3       ws/gn           sw       3      Rx (in)
Rx      green           6       gn              ge       2      Tx (out)

(color codes:
ws=white, sw=black, br=brown, bl=blue, or=orange, gn=green, ge=yellow,
rt=red)

the power lines are not connected to the DB25, but to an external power
supply. The RJ45-DB25 adaptor is item 12.03.8030 from rotronic,
<http://shop.rotronic.ch/>

For setting up the Garmin, the DB25 is conntected via an adaptor to the DB9
port of a laptop running Garmin's SNSRCFG tool. This is also how I updated
the firmware.

John (also using a GPS18LVC)
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.1 (MingW32)

The NMEA driver assumes that a PPS signal will be on the DCD pin (if
available), and uses it automatically:

ntp1# ls -la /dev/pps*
ls: No match.
ntp1# ls -la /dev/gps*
lrwxr-xr-x  1 root  wheel  10 Nov 23 14:07 /dev/gps0 -> /dev/cuad0
ntp1# ntpq -p
     remote     refid    st t when poll reach   delay   offset  jitter
=========================================================================== ===
+ntp9.hda.hydro. .GPS.   1 u    7   16  377    0.673    0.018   0.013
-nontp1.hydroisp 136.15  2 u   22   64  377    1.398    0.537   0.202
-nontp2.hydroisp .GPS.   1 u   53   64  377    1.513    0.353   0.053
+nontp8.hydroisp .GPS.   1 u   46   64  377    8.108    0.230   5.128
*GPS_NMEA(0)     .GPS.   0 l    3   16  377    0.000   -0.004   0.002

ntp9 is located on the same LAN segment, while the other servers are
behind firewalls, and (as seen from the delay values) even on the other
side of the country.

Terje