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GPS-Based Frequency Reference; Part.1

Need a source of very accurate 10MHz and 1MHz signals for calibrating frequency counters, radio receivers and signal generators? Here's just what you need: a frequency reference which is linked to the Global Positioning System (GPS) satellites, to take advantage of their highly accurate on-board caesium-beam "atomic clocks".

By Jim Rowe

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The unit is housed in a plastic case and provides accurate 10MHz and 1MHz reference frequencies via front-panel BNC sockets. A range of data can also be displayed on the LCD, including UTC time and date and the receiving antenna's latitude, longitude and height above mean sea level (see panel).

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.

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