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Large structures, e.g. long bridges, towers and tall buildings, may be exposed to
vibration and displacement during the earthquake, temperature change, variable or impact
loading and strong wind, like typhoon. Climate changes are also important. To measure
the structural vibration and displacement some classical measurement methods, such as
accelerometer, strain gauges, linear variable displacement transducers (LVDT) and total
stations are common tools for experts involved in monitoring. Laser interferometer and
electronic distance measurement instruments are also adopted for this purpose. Bridge
monitoring is now very important to transportation authorities in all countries.
Monitoring of structure is evidently important, for both safety and planning of
maintenance and repair. The structures and equipment are today exposed to high loading
in order to regain cost competitiveness: trains are designed for higher speeds, requiring
that the tracks conform to more severe standards for rail alignment, mechanical loads and
working pressures in internal combustion engines are increasing and the requirements for
pressure equipment are very strict. According to the Pressure equipment directive
(97/23/EC) the products should not endanger working personnel, environment and
population. A crack in a welded joint can lead to a critical failure with a tremendous
destroying capacity for a large vessel, and heavy consequences to humans, property and
natural environment. Pressure equipment must be designed, manufactured and checked,
applicable equipped and installed in such a way as to ensure its safety when put into
service in accordance with the manufacturer's instructions, or in reasonably foreseeable
conditions. On the other hand, mechanical loads and working pressures in pressure
vessels are increasing for higher energy efficiency and cost competitiveness.
2. GLOBAL POSITIONING SYSTEM (GPS) TECHNIQUE
Global positioning system (GPS) is a satellite-based navigation system made up of a
network of 24 satellites placed into orbit by the U.S. Department of Defence (NAVSTAR
is the official U.S. Department of Defence name for GPS). It was originally intended for
military applications, but in the 1980s, the government made the system available for
civilian use. System works in any weather conditions, anywhere in the world, 24 hours a
day. There are no subscription fees or setup charges to use GPS.
Sensor systems which monitor the geometry and deformations of large civil structures
are not new. Most conventional bridge monitoring systems rely on a fixed network of
sensors that transmit their data back to a central site for processing and analysis. Instead,
the emerging GPS application of structural deformation monitoring is used. Recent
advances in GPS technology have made it a cost-effective tool for safety monitoring.
The receiver of GPS tracks the satellite signals and computes the range and phase
measurements. It uses triangulation to calculate the user's exact location. Essentially, the
GPS receiver compares the time a signal was transmitted by a satellite with the time it
was received. The time difference tells the GPS receiver how far away the satellite is.
Now, with distance measurements from a few more satellites, the receiver can determine
the user's position and display it on the unit's monitor /1/.
The use of classical methods is connected with some inconvenience and difficulties.
For example, accelerometer cannot measure the swing of total vibration of structure
because acceleration cannot be obtained properly when structure moves slowly. It
requires direct contact with the structure as well wiring to link it to a central recording
unit. The wiring can be easily damaged. Use of laser interferometer and electronic
distance measurement instrument is limited by climate condition, i.e. clear line of sight is