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General Issues > Telecommunications

Direct Terrestrial Radio

Returning to Earth again, we apply the theoretical limits of radio propagation to the situation in the SFBA. Is there a realistic chance that direct terrestrial radio links could be employed to collect all the seismic alert messages popping up between San Rafael and San Jose?

In fact, the SFBA exhibts topography ideally suited for direct radio messaging. It can be demonstrated that from suitable mountaintops, like Mt. Tamalpais in the north, Mt. Umunhum in the south, and Mt. Diablo in the east, the maximum link distance to cover the entire Bay Area would be about 120 km. Using a UHF channel, the (theoretical) line-of-sight propagation loss for this distance would be roughly 125 dB. A real-world propagation loss would be considerably higher—hence, some reserve in the link budget is compulsory to provide reliable messaging.

Unfortunately, a link attenuation of more than 120 dB is beyond the performance limits of FCC-compliant off-the-shelf radio modems—at least if we want to avoid high-gain directional antennas. During an earthquake, their beams could be misaligned—we would like to avoid such critical risks in the transmission chain.

However, the radio infrastructure we propose for the SFBA uses a very sophisticated fully-digital receiver technology that can definitely handle link attenuations of up to 185 dB. (This while still using simple omnidirectional antennas at the transmitters!) Even at maximum distance, there would be a power excess of 60 dB (energy-wise a factor of 1,000,000) to compensate for obstacles of all kinds. This is a fairly good reserve.

Due to the enormous power reserve even the urban canyons of downtown SF would not be a problem for an advanced digital radio link.

We realize that such a level of performance may be hard to believe, even for experienced RF engineers and HAM radio operators. However, this top-flight technology has been commercially available under the name of SkyLINK since 1997. And it has already been field-proven to cover a radio horizon of much more than 100 km in similar mission-critical applications.

The SkyLINK radio network has been tested often enough in similar tasks for us to predict that this system is a number-one candidate for a seismic network in the SFBA. There is a good chance that only one prominent base station site could cover most of the densely-populated area, thus contributing to reduced seismic hazards for more than five million people.

We all would agree, however, that it is absolutely necessary to install several redundant base stations at other prominent locations—the eggs should not all be in one basket. Each of the seismic sensors should be able to establish a connection to three or even more base stations. This high degree of redundancy is necessary to guarantee smooth transfer of emergency messages, even after partial destruction of the radio infrastructure during a dramatic event in the SFBA.

A possible placement of the receivers around the SFBA would be as follows: With just six base stations on six different mountain-tops—typically separated by a distance of 25 to 50 km—there should be enough redundancy in the network to guarantee a sound 100% availability, even under the unusual scenarios that a really severe earthquake could cause.