PC clock : Computer Hardware Buyers’ Glossary

Built-in Clock

Every PC comes with a time of day clock chip on the motherboard. Unfortunately, it is not very accurate. You have correct it every hour or so by syncing it with an accurate atomic-synced clock on the Internet using either SMTP or ever better NTP protocol.

Radio-Synced Clock

You can also put a clock card inside your PC or use a USB (Universal Serial Bus) fob containing a clock that is synced to radio time signals, such as the one from Fort Collins. Meinberg makes such devices. I asked them how you ensure good reception of the time signal. They said you attach an antenna to the device.

I asked do you adjust to account for your distance from the Fort Collins transmitter? For example, I am 2,229.0 km away. The speed of light is 299,792,458 metres per second. It would take 7.4 ms for the time signal to get to me. They don’t bother with this because there are other inaccuracies caused by atmospheric disturbance, ionic reflections and the USB connection.

Because of the vagaries of the atmosphere, you cannot count on continuous reception. This is the big weakness of radio-syncing.

Another big problem with radio syncing is the card has to be designed to receive the type of time signal used near you. It is not one size fits all. For Fort Collins (North America), you need a WWVB device.

GPS (Global Positioning System) Clock

GPS antenna

GPS antenna

For greater accuracy you can use a card with GPS. You need to use an external antenna to get line of sight to the satellites. GPS inherently accounts for the speed of light and relativistic effects. You can even adjust for the antenna length. The accuracy is better than 100 nanoseconds. The drawback of GPS is the US military can turn it off any time when it is worrying its enemies are benefitting from it.

Atomic Clock

The fanatic could get an atomic/cesium clock.

Rant

It is high time that Internet Access providers delivered accurate time to all their subscribers. Then you could have accurate wall clocks that could get good time from WiFi (Wireless Fidelity) or an Ethernet cable in a uniform way.