[email protected] said: > this is a no-name cheapo SIRF module > 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.
The serial data stream basically doesn't work very well for timekeeping. The problem is a software bug. The info has a wander of roughly 100 ms. I say wander rather than jitter because it is very low frequency. You can't reasonably filter it out. The time constant is hours. http://www.megapathdsl.net/~hmurray/ntp/GPSSiRF-off.gif It is easy to correct for a constant offset. You also need a GPS unit that puts out a PPS signal. I don't know of any low cost ready-to-go units. A low cost unit is a demo board from Sure. http://www.sureelectronics.net/goods.php?id=99 http://www.satsignal.eu/ntp/Sure-GPS.htm You need to add a wire or two. You can also use the Garmin 18X LVC. Google will find directions. The other alternative is something like a Thunderbolt. It will keep good time if the GPS signals fades out - holdover. > 3) I'm clueless about mounting an antenna, running cable, grounding / > lightning protection, etc... Really want an easy to install one. Modern GPS receivers are much more sensitive than older gear. They may work well enough without an external antenna. On the other hand, a TBolt only needs 1 satellite after the survey. I'd suggest trying the Sure unit. It's only $35 plus shipping, and that includes antenna and power supply. You do have to add a wire or two. If it works well enough, you are done. If not, you will have learned a lot and can upgrade to a TBolt and/or try an external antenna. > 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 They contain a lot of noise from ages ago when the PPS stuff wasn't included in the Linux kernel. On a modern Linux system, the kernel is ready to go. You may want to install the pps-tools package. I think you need pps-tools-devel if you want to recompile ntpd and have it support PPS. You will have to do something like this before starting ntpd: if [ ! -e /dev/gps10 ]; then # GPS 18 LVC ln -s /dev/ttyS0 /dev/gps10 setserial /dev/ttyS0 low_latency ldattach PPS /dev/gps10 # makes /dev/pps0 ln -s /dev/pps0 /dev/gpspps10 fi You will also have to learn enough about general system administration to put that chunk of code in a reasonable place. The details probably depend upon which distribution you are using. On Fedora, I used to put it in /etc/sysconfig/ntpd but that broke when they switched to systemd. (I haven't worked out a clean solution yet. I just hacked it in to rc.local) For FreeBSD, I think you have to recompile the kernel to include PPS support. -- These are my opinions. I hate spam. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
