How to do GPS time sync for your automation system for $55

[spinjector]

Hi all,

After a conversation I had with RCS tech support a couple months ago, I set off on a quest for cheap GPS time sync, and I figured out how to do it for only $55. I’ve been running our Nexgen on the following setup for a couple weeks now, and the clock drift hasn’t been much more than 100-200 milliseconds per daily update. It seems to me it should/could be more accurate than that, but I can’t tell if the problem is the NEMA/ASCII data running at 4800 baud, a processing delay in Windows, or because the RTC chip on the motherboard in the Nexgen Utility PC is really drifty.

USB GPS dongle, provides NEMA data via USB virtual COM port ~$30
USB-over-CAT5, to get the dongle outside or near a window ~$15
GPS time software, only uses NEMA data over COM port ~$10
…or, GPS software that also listens for 1PPS pulse on CTS ~$15
…and a CAT5 cable of appropriate length.

See this pic of what it looks like together. The other end is plugged into the Nexgen Utility PC, with the GPS software. GlobalSat also has GPS “pucks” with magnets on them for a few dollars more, which could conceivably be used to stick it to an HVAC unit, STL tower, or some other chunk of steel outside the building. The dongle version could be put in a window with a good view of the satellites, or mounted in a waterproof box outside, which is what I’ll be doing with ours.

Note that the second of the two time utilities will respond to GPS devices that provide the 1PPS signal on the RS232 CTS line. The 1PPS signal comes right from the Cesium-decay counter on the satellite, and provides accuracy measured in nanoseconds. But that’s overkill for automation, and I have yet to find a cheap USB GPS dongle that advertises the 1PPS signal. I could build one for about $100 in parts from Adafruit.com, but I haven’t tried yet. The ideal device would be one with 1PPS, 10 updates per second, and 9600 baud for the data.

 

[Bill_981]

Assuming you're running Win XP or later, and the Nexgen has Internet connectivity, why not use Windows' built-in time set utility? I've used it for years, resettng the clock every 4 hours, and the drift is usually under 50 milliseconds. If you're running Windows Server, you can use NetTime 3.14 http://timesynctool.com/. I have it on my Win 2003 Server as a service and update every 4 hours. I just checked and the last adjustment was 3 hours 50 minutes ago, and that was for 309 ms. Certainly accurate enough for automation. Hell, the networks aren't that close. If you need greater accuracy, just decrease the query time. Works fine, and if $55 cheaper.

I'd be interested to know what RCS Tech Support had to say concerning the need for such accuracy.

 

[bradgoehl]

Interesting way to do it, Ive been considering moving to GPS time sync for our building

 

[Goat Rodeo Cowboy]

Where many of us live and work, the Internet can make this a simple thing to solve with a free or close-to-free piece of software.

I think I know where Spinjector operates, and full-time reliable Internet connection may not be as practical as it is for some of us.

And radio people have ALWAYS loved being innovators. I remember walking into an AM station in southeastern Arkansas years ago and meeting both of the owners, who both had droopy eyelids because they had been at the station all hours of the night trying some things. They were beside themselves with happiness. One of them was downright GIDDY! Through experimentation they found that by removing the power transformer in the bottom of the transmitter, putting BREAD PAN upside down where it sat, and the setting the transformer on the bread pan and some kind of isolation material, they had reduced the signal-to-noise ration by 2 dB. You would have thought that had just invented sliced bread.

Part of the "fun" of radio for generations has been to be able to craft some of your own unique technical ideas and unique programming ideas. Opportunities to do that are getting harder and harder.

Knock yourself out, Spinjector!

 

[Grounded Grid]

My ideal setup would be a GPS reference, then the ability to offset that time by so many MS. Some of my network audio comes in over the internet (Barix boxes), and the latency conflicts with uber-accurate clock accuracy.

There have been a couple of software applications that would handle the adjustable offset, but I haven't seen anything lately.