What time is it?

Most computers use very imprecise timing circuitry for their clocks. A $10 watch will probably keep better time than your $1,000 computer. Yet, a lot of software assumes that your computer's clock is accurate.

The affects of inaccuracy vary. Embarrassment is the likely result of sending an email with a time stamp off by a few years. Malfunction can result from using software that relies on file timestamps with your clock in the past by even a few minutes.

Numerous Internet protocols send time information between computers. This makes clock accuracy extremely important for computers on the Internet, especially servers and routers.

Many specialized applications require accurate time-stamping. Any computer that produces logs with security information should have its clock accurately synchronized. Also any case where logs made on several computers may need to be cross-checked requires each computer's clock to be accurate.

While programs are available to synchronize your computer's clock from time servers over the Internet, they generally work poorly for computers that aren't permanently connected to the Internet. This is because they have to 'jump' your clock when you connect. If this jump is backwards, the solution can be worse than the problem. In addition, many Internet links have widely varying jitter and delay which makes accurate time synchronization impossible.

A good local clock can provide precise synchronization for all the computers on a LAN. A local clock can also provide a precise 'one pulse per second' signal (PPS signal). The PPS signal for a clock with microsecond accuracy can be used to calibrate any other frequency source to extremely high accuracy. Simply start a counter on one pulse and stop it on the next. You can easily measure frequencies in the 1-50 MHz range with an accuracy of about 1 ppm.

The most interesting and the most accurate time source for a local clock is GPS. GPS is used not only for navigation -- it can provide incredibly accurate timing.

The price of GPS equipment has finally dropped to the point where it is affordable. Receiver/decoder modules are available for under $100. Antennas are available for about $50. Complete hacker-friendly units with antenna, receiver, decoder, and detailed documentation can be had for $400 or less.

A GPS clock can be used to 'discipline' the local clock on one or more computers. These computers can then act as time servers to keep the clocks of nearby computers accurate. However, as mentioned, time synchronization over low-speed, high-latency links is unreliable. The more accurate you want your clock to be, the closer to an accurate clock you have to be. In general, millisecond accuracy can only be achieved over a LAN.

Cheaper solutions using WWVB are available; however, they cannot provide sufficient accuracy to run a stratum one time server. WWVB reception is generally available at most 20 hours a day, so the other 4 hours you are stuck with an uncompensated crystal oscillator.

In addition, most WWVB receivers only synchronize to WWVB intermittently, and do so by jumps. Since they don't report when these jumps take place, there is no good way to process their time stamps. Inexpensive WWVB receivers are typically limited to half-second accuracy.

ACTS provides fairly high accuracy, but the telephone calls can get expensive. In addition, jitter in the telephone system reduces accuracy to about 10 milliseconds at best. Without special hardware, you are back to an uncompensated crystal oscillator between phone calls. We can do much better than this.

Sources

We offer the GPSClock 200, a GPS receiver that is designed for time reference use from the ground up. It weighs about .8 pound and has threads on it to allow it be easily mounted to a PVC pole.

Here are more details.

Further Reading
The Next Level
The Capture Effect