China’s successful global positioning system (GPS) carries a vital filter produced from research at City University of Hong Kong.
Unlike its American rival GPS which merely tells users their location, China’s system is bi-directional which means it also reports the user’s location to the operator.

From top: The filter device design that Prof Chan based his work on; how the filter design was transformed and simplified; the filter as a component in a GPS terminal; mesh filter screen of a microwave oven; Prof Chan (centre) and some of his research team

Senior scientific officer at CityU, Dr Xue Quan, said: “The Chinese system is like a mobile phone rather than a pager.” The CityU filter used in the Chinese system isolates microwave signals, so that outward signals from the base station do not interfere with incoming signals from the satellite at a different frequency. Without the filter, the two signals may interfere with each other.
The filter was invented from research left on the shelf at University of California (UCLA), said Principal Investigator Prof Chi-hou Chan
Prof Chan’s starting point was the UCLA filter concept based on a transmission line over a flat plane consisting of a dielectric or insulating surface, such as plastic or ceramics, on a perforated conductor plate (see graphic).
It was found that the electrical performance in the transmission line depended on the perforated structure in the conductor plate. It also depended on the angle of the transmission line going over the perforations.
“In simple terms,” said Prof Chan, “depending on the perforation pattern, the signal in the transmission line will travel at different speeds.”
As a next step, Prof Chan and his researchers discovered that the same result could be obtained by perforating the transmission line. This meant perforations on the baseboard were unnecessary and helped compact and simplify the filter design.
Another step in the research was to miniaturise the entire filter so that it could be incorporated in a printed circuit (see graphic).
For applications using a printed circuit, the filter could therefore be included at virtually no extra cost.
An additional benefit of the CityU filter, termed a Compact Microstrip Resonant Cell (CMRC), is that it eliminates a fault in the original design where the perforations led to energy leakage at certain frequencies.
The filter works like the perforated screen door on a microwave oven, explained Prof Chan. “In the microwave oven, the perforated screen allows light to go through the door but blocks the microwave frequency so is called a high pass filter.
“Our filter is slightly different as it blocks high frequencies but allows low frequencies to pass, so is a low pass filter. The distance between the perforations is critical.” A development of the filter the researchers are working on will allow a band of frequencies to pass, blocking frequencies both above and below the band.
Prof Chan sees huge potential for China’s global positioning system. “For example, if all cargo containers are fitted with them, an operator would know all their locations at any one time. The system could also be fitted to tourist buses,” he said. “Ultimately, terminals could be standard on most cars using China’s roads.”

Principal Investigator
Prof Chi-hou Chan : eechic@cityu.edu.hk