What does the GPSClock 200 do?

The GPSClock 200 is a GPS receiver. It receives signals from the GPS satellites just like a GPS navigation system does. However, instead of being optimized for computing exact position, the GPSClock 200 is optimized for computing the exact time.

A computer connected to a GPSClock 200 can keep time accurate to a few millionths of a second.

You can find the GPSClock 200's specifications here.

What is it for?

The primary application for the GPSClock 200 is computer time synchronization. Because of its low cost, it's useful in other applications where microsecond accuracy is not required, but accurate time is.

For example, a radio base station often needs to record the exact time at which events occur. While the extreme accuracy of the GPSClock 200 is not needed, typically accuracy to a few seconds is sufficient, the GPSClock 200 can certainly do the job.

How does the GPSClock 200 compare to other timing solutions?

Most other timing technologies don't even come close to GPS's accuracy. Affordable WWVB receivers only have 250 millisecond accuracy or so, and when you can't receive WWVB (typically at least four hours a day), you are stuck.

High quality WWV/WWVB receivers have a rated accuracy of about 50 milliseconds. They cost far more than the GPSClock 200.

GOES, ACTS, and other technologies are typically both more expensive and less accurate. ACTS might be cheaper if you live in Colorado, but it's certainly far less accurate.

Using GPS receivers not designed for timing purposes as a clock is not recommended. If no PPS output is available, synchronization must be done off the RS232 output. Many GPS receivers consider their RS232 output a low priority and delay it by random amounts based upon how much navigation processing they have to do.

Even those with PPS outputs often don't have any guaranteed relationship between the PPS output and the NMEA output. Sometimes the PPS is first but sometimes the NMEA output is first. This makes it very hard to match up PPS signals with absolute times.

Check here for more information on using GPS for computer clock synchronization.

What do I have to do to make it work? Can I just plug it in?

You can't just plug it in. While we do plan to offer a more plug and play solution in the future, the GPSClock 200 is aimed more at people who are willing to get their hands a bit dirty.

To make it work, you really want to mount it outside on a piece of PVC pipe. I was able to get all the hardware I needed at Home Depot for less than $5.

Then you need to solder two of the wires at the end of the 30 foot cable to a power supply (almost any wall wart will work, all you need is 9V to 24V DC at 120mA). Then give it about an hour to establish a first fix.

It won't take an hour every time. But the first time you power it up, it has no idea where it is or where the satellites are. So it has to 'search the sky'. Later, if you just unplug it and plug it back in, it will usually get a fix in less than 10 seconds.

What you will now have is a stream of RS232 data coming from the clock on its green wire. It consists of lines of NMEA-0183 data. Some of them will look like this:

$GPRMC,184927,A,3246.3220,N,12453.3241,E,
 000.1,026.9,230699,015.4,E*69

The 184927 is the UTC time. The 230699 is the UTC date (90-99 means 1990 through 1999, 00-40 means 2000 through 2040). When you see this message, the actual wall time is about 400 millseconds before the time encoded.

About 400 milliseconds later, you will see a rising edge on the blue wire. This marks the exact UTC second boundary. So when that pulse starts, you know what time it is.

For timekeeping, that's basically all the clock does. The rest is mostly up to you (or xntpd/ntpd).

What is included with the Clock? What else do I need?

The clock includes a detailed manual, software, and a 30 foot cable. It also includes access to our owner's area, so you can easily download manual and software updates.

We also include a web page builder that builds live views of the GPS constellation. Commented source code is included for this builder, so you can modify it and see how it works.

Our software package currently includes everything you need to make the GPSClock 200 work with xntpd3 or ntpd4 under Linux-2.2 and FreeBSD-3.1. Version 4 is highly recommended.

We currently have experimental software available for Windows NT 4.0. Its accuracy is limited to about 30 milliseconds. This is much less accurate that the Linux/FreeBSD drivers, which are accurate to better than 30 microseconds, but it is adequate for many applications.

We are working on getting a reference clock driver for NTPv4 on NT. This should bring Windows NT accuracy down to about 10 milliseconds.

You will need to do the mounting and wiring. If you want to have an ntp server up and running quickly, you will either need to set up a machine running Linux 2.2.9, 2.2.11, or 2.2.13 with Ulrich Windl's NANO patches or one running FreeBSD 3.1 or later. A processor with an accurate cycle counter (such as the Intel Pentium) is pretty much a must.

If you don't know how to solder, this is not for you. If you don't know your way around Linux or FreeBSD, this is not for you.

How much does the GPSClock 200 cost?

$380 each. Payment may be by check, money order or credit card. Lead time is currently about 2 weeks.

How can I place an order?

You can place an order at the Exit Consulting Store. Our mailing address is at the bottom of this page.

Specifications
Live View
Introduction