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Faraday cages and GNSS reception

By Spirent On May 11, 2012
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Although the concept of the Faraday cage is generally well understood, it is all too easy to fail to appreciate that many of today's structures can easily incorporate Faraday cages—or at least partial electromagnetic screens. The net result can be that a perfectly designed and constructed GNSS receiver can fail to operate, or (potentially worse) will show greatly degraded performance.

The classical Faraday cage is a structure that comprises a contiguous enclosure made from any conducting material or from a mesh of conducting material. Its effect on electromagnetic radiation is remarkable: all but the strongest electromagnetic fields cannot penetrate the cage, or at best are severely attenuated. What's more, the “cage” can have openings, such as those in the “mesh”, and so the effect of the Faraday cage can crop up in some unexpected structures.

The problem, of course, is that GNSS satellite signals are extremely weak, and so even a partial Faraday cage can have a catastrophic effect on the performance of any receiver located inside it. Indeed, this is precisely the effect that makes in-building navigation virtually impossible using GNSS signals alone.

Almost any man-made structure can create the effect, whether it be the metal frame of a building, the superstructure of a ship or the body of a car. Worse still, many modern materials and components can conceal conductive meshes that exacerbate the situation. Take, for example, the modern automotive windscreen with an inbuilt defrosting element. In this case, the mesh is microscopically fine so as to allow full visibility through the screen. However, the conductive mesh is there, and it only serves to complete the Faraday cage formed by the rest of the car body.

The problem, of course, is not an insurmountable one: Hybrid navigation systems are used to improve in-building performance, and maritime and automotive navigation systems will use external antennas. However, there will be occasions where users will attempt to use a portable receiver within a Faraday cage and will be disappointed with the performance. So manufacturers would be well advised to include an explanation of the effect within their product documentation.

 
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