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Basic System Components > Radio Link

General Issues

Earthquake resistance

As already mentioned, the radio link for the RED System is a direct terrestrial link. What would happen to the equipment in a major earthquake? Let´s look at the basic system components separately:

• Front-end unit
• Receiver
• Stability of the mast
• UPS facility

Front-end unit

It would be incorrect to claim that every RED Cube would stay intact during a major urban earthquake. In a dramatic scenario, where a whole building collapses, the hardware could simply be crushed––this unit wouldn‘t have a chance of staying operational. Other sensor units could be lost in cases of explosions or large fires. Protecting all of the system‘s sensors during a really destructive earthquake would be financially impossible.

However, there is a considerable chance that a RED Cube would stay operational even in a location where heavy destruction has occurred. The RED Cube is a compact and self-contained unit. And its design has been carefully optimized in order to achieve an outstanding level of robustness--mechanically as well as electrically.

First of all, the system´s chassis is made of a durable plastic compound with 0.33“ wall thickness--much more stability than standard plastic cabinets would offer. Since the antenna is placed inside the protective cap, there is no vulnerable external antenna construction involved. In other words, this cap also protects the antenna, thus serving as a radome.

Furthermore, the built-in antenna system is based on an omnidirectional (low-gain) construction. Hence, there are no strict rules for the orientation of the RED Cube‘s integrated antenna. This could be a highly valuable feature in a chaotic scenario where the RED Cube unit is tossed about and finally lands on the ground in a tilted position. Normally, the radio link will have enough power reserve that even signals from this wrecked unit still have a good chance of being received properly. See Precise Time Stamps.

The protective cap is extremely resistant to heavy impact--on the one hand, because of its pyramid-shaped structure and on the other hand, because it is made of polycarbonate, a preferred material for bulletproof constructions. There is also plenty of clearance between the protective cap and the electronic modules--a crash-zone. Thus, a RED Cube could potentially tolerate severe deformation of the cap without negative consequences for the system´s basic functions.

Loss of line power supply is likely to occur in a severe earthquake. To cope with this threat, the RED Cube has been equipped with sufficient backup capacity for several days of autonomous operation.

Compared to today´s networks, the seismic sensors of the RED System exhibit a unique robustness. But there is another important feature that enhances the system´s ability to provide useful output under dramatic conditions: the large number of units in the field.

In a dense grid of stations (e.g. 1 x 1 km), there would normally be four neighboring stations within a short radius (typically 1 km) to fill in for a unit that might be rendered out of service. This degree of redundancy is so enormous that the quality of the output in general would not suffer even if one-third of the units were lost in an extremely severe disaster.

Let us support this statement with a mathematical example. Imagine a big urban earthquake with severe destruction spread over a region of 300 square kilometers. The sensors would be located in a more or less regular grid of 1 x 1 km.

There would be no substantial difference in the conclusions related to instant emergency response activity if we had

(a) exactly 295 sensor units, all reporting a peak acceleration of more than 30% g , or if

(b) we were restricted to receiving results from only 196 units, reporting the same peak acceleration for essentially the same area as in (a)

Based on statistics, one could say: there is no reason to assume a significant difference in the damage to this urban quarter whether the observation is based on sample (a) or (b), both taken randomly.

Receiver

Design

During network planning we have to be careful to connect every seismic sensor in the region to at least three different receivers (base stations) at the same time.

Note that the typical distance between each of the six base stations is about 30 km. The mountain-tops selected belong to different fault structures with distances like these, it is very unlikely that more than one or two stations will be hit by a shaking in excess of ±1 g.

Therefore, our design goal for a base station is that the complete receiver installation be able to resist a shaking of up to ±1 g without malfunction. Further, whenever a specific receiver location is involved in stronger shaking, there will be enough neighboring sites still in operation to substitute for its loss.

Verification

A receiver chain consists of seven different components

(1) Antenna
(2) Downconverter
(3) Mast
(4) Cabling
(5) IF processor
(6) Digital signal correlator
(7) UPS facility

All these components are engineered to withstand an acceleration of ±1 g in the frequency range of 0.5 to 10 Hz for ten minutes on a shake table without any malfunction or damage. For most of these components this condition is easily fulfilled because mechnically moving parts have strictly been avoided (especially HD drives). However, two aspects are worth discussing:

Stability of the mast

The antenna of a SkyLINK base station needs to be supported by a mast with a height of approximately 10 m (33 feet). Thanks to the small, lightweight design of the antenna (20 kg), a self-supporting construction can be employed. Identifying the appropriate dimensions of standard steel tubing for this application should not pose a major engineering problem. A bit of over-engineering would be a prudent decision. A professional, high-quality flagmast might be employed--with or without colors hoisted. Compared to commercial communication towers, this is a really smart solution for an antenna carrier.

However, there is still a problematic aspect in this construction: the (invisible) foundation. Experience has shown that the best resistance to strong shaking is achieved by thoroughly anchoring the mast in a solid formation of rock. Therefore, geological expertise must be combined with earthquake engineering know-how to find the best solution for each of the receiver locations.

In those locations where solid rock is out of reach, another approach might be taken--more or less the opposite of solid anchoring. The basic idea is shown in the picture. This construction would partially decouple the mast support from the ground motion in a severe earthquake. The whole framework would be tossed back and forth, sliding on the ground. But it would not be damaged because of the enormous stiffness inherent in its tetrahedral structure. And, due to the very low position of the centroid of the skeleton, it could never fall over.

UPS facility

Another concern refers to the battery pack in the UPS unit. The lead acid battery cells would not be problematic, as their electrolyte is completely immobilized in gel. However, the whole battery block is extremely heavy: 50 kg (110 pounds) in one 19“ plug-in.

To better accommodate this weight, the heavy lead acid cells are fixed with large metal brackets. See UPS.