How do I arrange for accurate timekeeping on my system?
(information provided by Chris Chapman)
Although SIDS can be monitored without extremely accurate timekeeping,
timekeeping is critical for gamma ray bursts and some other phenomena.
Although most computers provide 'software clocks,' these clocks
can vary by over 10 sec per day.
There are a collection of on-line time services,
although even these seem to be getting overloaded
and errors of a second or more have been reported.
To further complicate matters, the delay in getting the
signal from the server to the SID location over a phone line may be variable.
Analog channels do not tend to experience large delays, but digital ones
may be less reliable.
One of the better services is
This product runs in the background and can be set to update the PC clock from
the internet every few minutes. It is reasonably accurate because it gets the
time from a time server and then 'pings' the server to check the signal
delay, which is adjusted accordingly.
The cheapest precision clocks with typical errors of less than 20
milliseconds are provided by radio corrected quartz clocks using WWVB, MSF,
DCF77, JJY, and RTZ -- all VLF radio time signals. Check out
Radio Time Signals and other internet references.
Compared to other time-signal transmissions in higher bands (WWV, GPS,
etc.) long-wave signals have a number of advantages: They can go around
obstacles such as mountains or buildings. Since no line-of-sight is
necessary between the transmitter and receiver, a single very powerful
station can cover a huge geographic area. Long-wave signals even
penetrate the walls of most buildings quite well. Propagation happens
mostly in the form of a ground wave, such that transmission delay is
less affected by the variability of the ionosphere.
However, they are susceptible to local radio interference, particularly
from CRT-type computer monitors! You need a gap larger than 6 feet.
The transmitters may
temporarily cease operation if a local thunderstorm threatens the very large
aerial system. The clocks may operate OK at night in 'fringe reception'
areas over 2000 km from the transmitter. 3,000 km is about the far limit for
night time ionospheric reception. For the WWVB reception areas and times
in North America, see WWVB.
You can also
buy the 60 KHz aerial and radio modules. Decoder modules should be