Holux M-241 (GPS Logger) as GPS time source for ntpd

slick · Bodhisattva Joined: 20 Apr 2003 Posts: 3495

This is a short documentation howto use the Holux M-241 GPS Logger als GPS Time Source for ntpd.

This device has a battery, but you dont need a battery inside. (Remove it!) Just plugin a usb cable connected to your computer.

You need USB Support (*HCI) depend on your computer and the following Kernel options:

steveL · Posted: Sun Jun 01, 2014 5:41 pm Post subject:

Just in case no-one else says it: This is a great post, slick.

I don't have one of those loggers, but the post makes me want to buy one (I find ntpd setup a real pita, and would love to just have a GPS variant that doesn't require network and always works.)

slick · Bodhisattva Joined: 20 Apr 2003 Posts: 3495

The Holux M-241 ist not cheap, currently ~80 €. But I bought it some years ago, but dont use it and forgot it. I give it a second life now. I put it on a tiny photo tripod and place it on my desk. (Picture) Looks very cool, my own satellite downlink

But I guess there are a lot of other gps gevices that may usable. (All that are supportet by gpsd?)

steveL · Posted: Mon Jun 02, 2014 6:03 pm Post subject:

slick · Bodhisattva Joined: 20 Apr 2003 Posts: 3495

Would be nice if anybody knows how to setup the initial time from gpsd before ntpd start. Like the following, but that does NOT work.

Aiken · Posted: Wed Jun 04, 2014 1:15 am Post subject:

127.127.28.0 is not an ip address. So can not use ntpdate to get the time from it. A while since I used gpsd and can not think of a way. Could try lowering minpoll which will have ntp syncing faster but if you have too much jitter that may not work. With how much the time can jump around when using nmea as a time source I have preferred longer poll periods.

If all you are doing is using nmea and/or pps then you don't need to use gpsd. Ntp can use a gps directly. Where I have found gpsd handy is viewing the data and sending it around the network. Gps on 1 machine, viewing gps data on a 2nd machine, logging all nmea on a 3rd machine. Which is how I did use gpsd last.

Odds on you will find a time stamp generated from reading the nmea sentences jumps around. It is not sent at a set time in the second like a pps is. A few gps where I have looked at the timing of the rmc and gga sentences there seems to be a correlation between the number of satellites in use and the when in the second the sentences are sent. My view is someone who just wants their clock with in 1 second is not that relevant but I found it interesting.

aevertett · n00b Joined: 23 Jun 2014 Posts: 4

I realize this is resurrecting an old post, but I've been conducting some tests with a NMEA GPS receiver in conjunction with NTP. Using the ZDA time sentence for timing. Not only do I find that the ZDA sentence is generated at variable intervals after the 1PPS output, but on occasions, it can actually be generated either side of the respective 1PPS output. I suspect that not all NMEA based GPS timing receivers are like this, but these sort of issues make using NMEA GPS receivers inadequate for timing purposes. With the lower cost of GPS-referenced NTP servers, I suspect that these may be a better and less time consuming option.

Eve

Aiken · Posted: Fri Jul 04, 2014 8:26 pm Post subject:

Nmea can wonder around bit. With the iblue mentioned above last time I used it as a time source the nmea was around +/- 100ms. Had an old sirf II gps that was a bit worse. The gps I use on my time server is an older garmin gps25 and I found the nmea sometimes would wonder enough to lose it's prefer peer status. The nmea at least stays in the correct second. At the moment I use the isp's ntp server as the perfer peer so I can use the pps. That has it's own problems as when the net drops out I lose my prefer peer so the pps stop being accepted by ntp.

With a well behaved gps I would expect to be able to use the nmea to get the course time then make use of the pps. The nmea wondering into the next second is not good. Can you adjust the serial baud rate? Can you turn sentences on or off? I have the baud rate on my gps25 at 19200 instead of the 2400 or 4800 it started with. That helped. Unless I specifically want them I have all sentences turned off and when playing I'll turn GGA back on. Another thought is see what gpsd does to the nmea or write something that reads & filters the nmea and feeds the time to ntp via the shm driver. Are GGA or RMC better behaved?
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Beware the grue.

steveL · Posted: Sun Jul 06, 2014 5:32 am Post subject:

@Aiken and slick: would be good to get your input wrt leap seconds and GPS sync - aevertett has mentioned there that they can interact badly with GPS and NTP.