[GPS_Standard] preferred GPS

[Dave Platt]

Bob Bownes wrote:
> So, do folks have a preferred GPS module to discipline clocks? Clearly
> some are better than others. I'd like one that will output in NMEA so
> I can use it to drive some other things as well, but other than that,
> it has become clear that the Rockwell MicroTrack TU00 just isn't going
> to cut it as it only locks to 4 satellites, has quite a bit of jitter,
> doesn't hold a very good 3D lock (+/-100m just isn't good enough for
> me...)

On the used / new-old-stock market, I'd suggest looking for one
of the newer Motorola Oncore VP modules (the 8-satellite ones).
They seem to have reasonably low (although not ideal) jitter,
and support both NMEA and Motorola binary protocol.

Motorola made some Oncore models optimized for timing purposes
(I think the UT and UT+ were of this sort) but some of these
don't speak NMEA.

The one I'm using myself is an older VP, purchased as
new old stock from Synergy Systems a few years ago... 6
satellites, no NMEA.  I was concerned about the lack of
NMEA support, but the "gpsd" daemon has recently been
upgraded with a driver that understands Motorola binary
packet protocol.  I had to hack the driver a bit to get
it to work - as released it understands only the
8-satellite reports, and can't either request or parse
the 6-satellite reports from the older VP receivers - but
that wasn't hard to do.

The newer VP/UT-family modules support a "site survey
and automatic position hold" mode - they'll average
out the positions they calculate for a day or so, then
store the average in NVRAM and switch to position-hold
mode.  This can improve the precision of the PPS
timing pulses somewhat.  The older 6-channel VP modules
do support position hold but don't have the automatic
survey-and-average feature - the average or median
position must be determined by external software and
then programmed back into the NVRAM before position-hold
mode can be used.

Used/working Oncore modules are pretty easily available
on eBay for a reasonable price, mostly from a number of
well-stocked sellers in China.  My impression is that a
bunch of older cellphone sites (which use GPS for
timing synchronization) have been decommissioned or
upgraded, and the GPS modules pulled for sale.  There
are a bunch of Motorola UT+ modules available, for
as low as $24 delivered.

Newer modules... well, the more recent Oncore line
has the M12+T, which seems to be optimized for timing
purposes... I believe it's based on one of the SiRF
chipsets.  Most newer modules are *not* 5-volt - they
typically require 3 or 3.3 volts for power, and use
this voltage for their I/O as well.  Dropping one of
these into a design intended for a 5-volt module might
not be the simplest approach.

SparkFun has a whole bunch of modules, about which I
know little... mostly 3- or 3.3 volt, most with their
own internal antennas (which is probably not a good
thing for this particular application).