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tracking and navigation reduces response times and enables more efficient utilization of
expensive vehicles used by such as police, fire, and search and rescue. GPS can also be
combined with communications to coordinate the actions of multiple ground, sea, and air
emergency vehicles. DGPS systems can also be deployed to emergency locaions to guide
individuals, like firemen in a burning building or expeditions to South Pole.
Because atmospheric and orbital errors increase with the distance between the refe-
rence and the remote station, it is better to set up the reference station at a relatively small
distance from the remote station. When the distance is 1 km or less the atmospheric and
orbital errors are expected to be about a millimeter. The remote station should be located
at the measurement point, the biggest deformation point or the feature point of a structure.
Mobility is a basic characteristic of field tasks. Location-Based Computing (LBC) is
an emerging discipline focused on integrating geoinformatics, telecommunications, and
mobile computing technologies. LBC utilizes geoinformatics technologies, such as Geog-
raphic Information Systems and tracking methods, such as GPS, in a distributed real-time
mobile computing environment. In LBC, elements and events involved in a specific task
are registered according to their locations in a spatial database, and the activities suppor-
ted by the mobile computers are aware of the locations by suitable positioning devices /4/.
2.2. The experience with the application of GPS system to bridges
It turned out that the performances of GPS system are very suitable for bridge
behaviour monitoring. At long-span cable supported bridge, for example, the remote
points are located in the middle of the bridge deck, or on the top of the bridge tower. One
of important applications of GPS is monitoring of cable-supported bridges by vibration
and strain measurement and for real time monitoring of other structural vibrations.
Inertial sensors such as accelerometers and gyros can be added to the GPS antenna
assembly to further increase positioning accuracy.
Using the displacements of measurement points in special directions, vertical, lateral
and longitudinal, the analyses includes the temperature dependence, the wind velocity
dependence, or load dependence of displacement. The data processing includes coordi-
nates transformation, spectral analyses, data extraction and structures health assessment.
The different information about the lifecycle of a bridge (e.g. construction, inspection
and maintenance schedules) integrated to the 3D model of the bridge, result in 4D models
/5/. 4D models will allow for spatial-temporal visualization and analysis that are not
possible with present bridge management systems. Present methods of capturing location
information using paper or digital maps, pictures, drawings and textual description can
lead to ambiguity and errors in interpreting the collected data.
There are two system architectures for structural monitoring with GPS, one based on a
fixed network of sensors, Fig. 4, and the other based on mobile sensors. Fixed network of
sensors are mounted on the structure at sites of interest /6/. Each bridge deformation
monitoring system sensor node consists of a GPS receiver, microcontroller, and data
radio. The GPS receiver tracks the satellite signals and computes necessary data on range
and signal phase, which are transmitted by radio to the central processing unit.
GPS, like any other technology, has limitations for measuring deformations of large
civil structures. However, it is today a powerful and cost-effective tool for monitoring
some types of structural deformation and performance. Foundation settlement, creep, and
different movements generally occur over relatively long periods of time. Therefore,
averaging times of a few hours can be used, producing positions with millimeter-level