Ground Traffic Detection on Airports
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Fiber-optic axle sensors manufactured by SENSOR LINE have now been applied with great success in road traffic for more than a decade. This is mainly due to their precision and durability; the fact that they are working electrically completely passive is fine but far from being their key advantage.
These features are not obtained by magic but by a special construction. Advantageousproperties result from certain characteristics given to the sensor which can not simply be omitted afterwards if they appear less desirable. One of these characteristics is some offset load applied to the optical sensing fiber which finally enables load detection while preserving a completely even road surface. However, this offset load has an offset effect on the fiber: its optical loss becomes significantly higher than that of an unaffected fiber, plus, environmental effects acting upon that loss grow to the same extent.
Since optical loss of any kind increases exponentially along the optical path the dynamic range of any instrument to measure that loss must increase the same way, and as a consequence fiber-optic axle sensors with an offset load can only be mastered up to some maximum length. It is difficult to tell an exact value for this. As a rule of thumb sensors up to four or five meters can be considered completely safe whereas such longer than six meters already deserve some suspicion: one may be lucky or not.
Shorter sensors have still more advantages over longer ones. It is a well-proven fact that the quality of sensor installation affects the performance more than everything else. Quality of installation means dimensional accuracy of the slots cut to accept the sensor as well as care and precision applied when mixing and pouring the embedding material. This has led to the approach of machining the slots into special “treadle frames” and pre-embedding the sensors under controlled working conditions to obtain so-called “treadle strips”, where the frames are installed in the road and the strips then inserted. This procedure is no longer mandatory as it was in former times because of the short life of mechanical treadle strips but it is still convenient with respect to installation quality and the possibility of quick sensor replacement persists. However, pre-embedded sensors can no longer be handled like a cable, they are quite stiff and the effort for handling and transport is very much affected by their sheer length.
While single sensors with four or five meters of length are far sufficient for road traffic purposes they are much too short for the width of taxiways or even runways on airports. Aside from the fact that it might never be possible to get sensors with 30 or even 60 meters of length working these would also be extremely inconvenient to handle and to install. The only viable solution are therefore sensor arrays.
Sensor arrays have one big disadvantage: each sensor element requires a feeder cable to be installed as well, this resulting in a multitude of optical fiber cables emerging from each array. Attempting to run all these cables along the large distances on an airport is hardly a viable approach. It is, however, possible to do the optical signal processing on-site and transmit only the result along one single fiber. Fibers which specially allow for long-distance data transmissions can not be used as sensor feeders but they are even cheaper and there is only one needed. Thus, at the single expense of having to provide electrical power at the measuring site, it is possible to provide a fiber-optic sensor system for airports which is not only reliable and durable but also economic and easy to maintain. Detecting aircraft on the ground is not as easy as detecting cars on a road. Runways and taxiways are much wider than roads, the distances between sensors and the signal processing equipment are much larger, and an aircraft moves much higher above the ground than a car (even when not flying). Also, an airport is a wide open plain, so any metal is much more likely to attract lightning there than elsewhere. Traditional methods of vehicle detection encounter problems under these circumstances. There are microwave detectors used instead, but they are often disturbed or even destroyed by aircraft radar.
Airports are therefore ideal places to benefit from the unique features of fiber optic sensors, especially load sensors. For airport applications SENSOR LINE provides the special FPL series fiber optic load sensors in conjunction with high power versions of the MD-220 opto-electronic interface.
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