Accurate Time Keeping on your PC

An important part of any satellite reception operations is accurate time keeping. This is particularly important for receiving High Resolution Picture Transmission (HRPT) data where an antenna must be kept pointing at a satellite that is tracking rapidly across the sky, but it is also important for automated operations where the accuracy of a geographic overlay on the image may rely on knowing exactly when that image was received.

Running on many Internet servers is the Network Time Protocol (NTP) which allows hierarchical synchronisation of a network of machines to one common reference. Accuracies down to milliseconds are easily achievable. Which software package you use depends on whether you have a dial-up or permanent Internet connection (e.g. ADSL, Cable Modem etc.), and whether you run Windows NT/2000/XP or Windows 9X. With some of the software you can also use an alternative reference source such as a GPS or Radio Clock receiver.

I have a permanent connection

I use Windows NT/2000/XP

Personally, I now recommend and use the Terje Mathisen NTP Windows port on all my systems. This runs as a service on Windows, meaning that it starts with Windows and requires no user intervention. When the time error is small (less than 128 milliseconds) the software works in a fine frequency change mode, rather than crudely stepping the clock to a new value. NTP software is available for a wide number of machine types (not just Windows). You can download a Windows NT/2000/XP version from:

http://www.ntp.org/links.html

Please note that you may need to set the W32Time service (Windows Time) to disabled to stop it interfering with the operation of NTP. If you want to see the accuracies I get one my system, please see my timekeeping accuracy data, which is updated every few hours. Be aware that because of the plotting software I use, 100msec is added to the values (the software can't plot negative values) so that values plotted as 100msec actually mean zero offset.

I use Windows XP

Windows XP does have limited NTP support, although the default polling of once a week is not adequate for good time keeping on most consumer PCs. You can alter the polling interval of the XP client, and Microsoft Knowledge Base Article 223184 describes the registry entries you would need to edit. I recommend using the NTP software mentioned above in preference to the built-in Windows XP NTP support.

I have a dial-up connection

When I used dial-up, I used Tardis, and was very happy with it:

http://www.kaska.demon.co.uk/tardis.htm

This software will work on all versions of Windows. Please note that this is shareware. The unregistered version is fully functional, although you should support the author by registering if you continue using this software.

If you simple want to sync your computer to its own CMOS Time Of Year (TOY) clock, Geoff Whitbourn recommends ClockMon (http://www.softdevlabs.com/ClockMon/ClockMon.html). This program addresses a slightly different issue, that of lost interrupts causing the OS time to become out-of-step with real time.

How accurate is NTP?

Please see my NTP Monitoring data (updated every 90 minutes).