strat-1 gps

Tue Jun 26 21:34:31 UTC 2012

On Tue, Jun 26, 2012 at 9:11 PM, Majdi S. Abbas <msa at latt.net> wrote:
> On Tue, Jun 26, 2012 at 04:33:35PM -0400, Robert E. Seastrom wrote:
>> Word around the campfire is that the 18x is jittery compared to the 18.
>
>        The 18x is much worse than the 18LVC.  Thankfully I still have
> 2 18LVCs... but that said, given the hockey puck design, and that Randy
> already has an antenna, I wouldn't recommend this approach anyway.  It's
> really only suitable next to a window, or in a short, wooden structure.

I agree the Garmin GPS 18x LVC solution is only appropriate near a
window, or in a short wooden structure.  David J. Taylor's experience
was the older GPS 18 LVC had a substantially less sensitive receiver,
so that his needed to be mounted outside, while his 18x LVC works
inside.

The area where 18x LVC underperforms the 18 LVC is the jitter of the
timing of its NMEA output relative to the leading edge of the PPS.
Configured correctly, this should matter very little, and only
transiently at startup where ntpd first uses the NMEA end-of-line
timestamp (possibly fudged to bring it closer to the top of second) to
"number the seconds" before engaging the PPS.  I don't recommend
attempting to use any NMEA source without PPS.  With it, I don't
understand why Majdi finds the 18x LVC much worse.

The 18x is a number of years old at this point.  Newer GPS receivers
(such as the Sure GPS evaluation kits at around $30) are likely to be
much more sensitive.  I've heard reports of them working in basements
and in office buildings 20 or more feet from a window.

>        Also, we've got a leap second pending, and at least the
> 18LVCs...do not appear to handle those gracefully.  Mine freaked out
> pretty badly in 2008 and had to be reset and reconfigured.  I've also
> seen them lose their configuration (which has to be reset using a Garmin
> utility.)  For this reason I can't recommend running them unattended.

There was a bug in early firmware versions of the GPS 18x LVC that
could result in the device wedging until you either left it unpowered
long enough to drain its battery/capacitor and thereby clear its
configuration, or cracked it open to achieve the same, or (as many
did) exchanged it with Garmin for a replacement.  I believe that bug
was first fixed in the 3.20 or 3.30 release, but one of those made the
NMEA timing even worse, sometimes causing NMEA sentences to continue
past the next top-of-second that could have fit easily in one second
if not so delayed.  The NMEA timing was improved in the 3.70 firmware,
which I recommend all GPS 18x LVC + ntpd users upgrade to,
particularly those using 3.20 or earlier.

The change history included with the 3.70 firmware doesn't completely
align with my recollection.  There's no mention of fixing the bricking
bug I mentioned.  The closest likely mention is of a 3.60 fix:

Version 3.50 to 3.60

1.  Fixed factory firmware flash capabilities.

It does confirm the NMEA timing fix for 3.70, on the other hand.

Cheers,
Dave Hart