Just one further question.
When the pps input triggers, so my linux box knows a second has just
ticked; is the time of that second the one the NMEA sentence has just
sent, or will send next?
Or to put it another way, when I receive an NMEA sentence is this the
current time (as was when the sentence was constructed) or the time at
the next PPS 'tick'?
Thanks in advance.
KenD
On 19/08/12 11:23, Bill Dailey wrote:
I will jump in a bit. I, and many have been right where you are. You are correct...USB
is a no go for accurate time. Same on windows. So you need a Linux box with serial
port. Anything from a Beaglebone, pandabox...or pc will work. You certainly need a gps
with a pulse per second output (most have) and you can wire that up to the appropriate
line on the serial cable or send to the target computer via gpio pin. The pps thing is
fairly simple really. If you are receiving gps data via serial connection it takes a
variable amount of time to get each status report from the gps...list time etc etc in
text format and sends it repeatedly. This gets ntp to within a second or some fraction
thereof...the pps part refines that..no text or anything... Just a one pulse per second
separate from the gps info that acts as a exclamation point to tell ntp "right here
is the actual start of the second!" alone the pps wouldn't be useful for time but
with the time info ntp already has from the nmea sentences it is priceless for really
precise time. That's about it. Once you have gps and pps configured on Linux you
should be in the sub 5 microsecond range. It gets tricky getting better than that and
you have to Ntpns and really worry about hardware issues that affect precision (system
clock stability etc) but it can be done.
Doc
KX0O
Sent from my iPad
On Aug 18, 2012, at 11:25 PM, Sarah White <kuze...@gmail.com> wrote:
Hi, this is my first post.
First off. Windows 7 USB connection to the GPS (no serial ports / modern
computer) and I'm pretty sure that is my main problem.
Past few months, I've been trying to figure out my timing issues. Lots
of reading & trying to figure out how to best configure everything. I'm
typically still off (randomly) by 20-100 miliseconds. I'd like to at
least get to within 50 microseconds (nanoseconds would be wonderful)
1) I need a computer with a serial port. The curent GPS module I'm using
is INTERNALLY RS232 --> USB converter, and recognized by my windows 7
computer as: "Prolific USB-to-Serial Comm Port (COM3)" ... the latency
and jitter is horrible, and both are seemingly random.
2) I need to run my stratum 1 clock (connected to the stratum 0 time
source via old-school RS232 serial) on linux or a form of BSD with
support for kernel timestamps, and a version of NTP with a driver to
supports my reference clock... points 1 and 2 seem fine.
3) I'm clueless about mounting an antenna, running cable, grounding /
lightning protection, etc... Really want an easy to install one.
For software, I've used 4.2.6 (stable / production) as well as 4.2.7
(dev version) NTP and haven't been able to tell any difference.
Just using the generic NMEA driver / this is a no-name cheapo SIRF module.
Also, trying to wrap my head around these:
http://linuxpps.org/wiki/index.php/LinuxPPS_installation
http://linuxpps.org/wiki/index.php/LinuxPPS_NTPD_support
And here is where I give up. As the subject line suggests:
HELP!!! I'd like to convert L1 GPS timing signal(s) into local time on
computer(s)
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