NOT TOO MANY decades ago, the only way most people could
generate reasonably accurate frequency signals was by using a quartz crystal
oscillator. Following this, it became possible to achieve slightly better
accuracy by heterodyning a local quartz oscillator with an HF radio signal from
one of the standard frequency and time stations, such as WWV in the USA or VNG
in Australia.
By about 1980, even higher accuracy could be obtained by
locking a local quartz crystal oscillator with the horizontal sync signals from
one of the national TV networks. That’s because the networks used a master
timing clock that was locked to an "atomic clock" based on either a caesium beam
or rubidium vapour oscillator.
The GPS system
The Global Positioning System (GPS) became operational around
1990 and is run by the US Department of Defense. By using this system as a
reference, it’s possible to generate reference frequencies with extremely high
accuracy – even better than using the previously listed methods.
That’s because each of the 22-odd GPS satellites orbiting the
Earth has two caesium beam atomic clocks on board. These are necessary to
generate the very accurate frequency and time signals needed for accurate
positioning. And since there are always at least four GPS satellites "in view"
at any time from any point on the Earth, this means that there’s always access
to an "ensemble" of about eight caesium beam clocks to serve as a frequency
reference – provided you have the right GPS receiving equipment, that is.
