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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B-YF. Istanbul 2004
With these inexpensive and powerful embedded computing
processors, small sensors are now equipped with "brains"
(processing units); are able to “think” (processing data); and are
able to “talk” (communicate) to each other with the integration
of equipped computing processor, wire/wireless communication
and embedded operation system(Liang et al., 2003).
1.2.3 Scalable
The Sensor Web will accommodate enormous amounts of
distributed and heterogeneous sensing resources. New sensors
can be added into the SW easily at later stages and without
changing the existing design. To date sensor network
community has pursued a variety of pioneering research
activities in this field. One of the most important examples of
this is in the development of 'self-configurable and adaptive
sensors’ which can be randomly ‘seeded’ to the field in large
numbers without a sophisticated deployment process(Sohrabi et
al., 2002).
1.2.4 High Resolution
The collaboration between various types of sensors and the
fusion of sensing information will provide full-scale integrated
sensing that has both high spatial coverage and high temporal
resolution. (Figure 1.) Remote sensing has larger spatial
coverage and in-situ sensing has higher temporal resolution.
The SW connects both remote sensing and in-situ sensors, fuses
their observations and provides integrated sensing(Teillet et al.,
2002b) that has both large spatial coverage and high temporal
resolution.
o SPOT, LandSAT
oO IKONOS, Quickbird
O Airborne SAR
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o ©) Aerial Photo
Qo
2
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RJ
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o
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Oo Traffic Sensors
(Car presence)
Oo Weather Stations
O Marine Sensors
Temporal Resolution
Figure 1. Sensor Web is an integrated sensing which has both
large spatial coverage and high temporal resolution.
1.3 Layers of the Sensor Web
The sensor web is comprised of the following three layers:
e Sensor Layer
e Communication Layer
e Information Layer (Figure 2)
Through close integration of the three layers, the sensor web
concept can be achieved. The following section introduces the
advances, characteristics, and capabilities of each of these three
layers.
Sensor Web
Airborne Remote Sensin
ng Spaceborne Remote Sensing
Sensor Web Sensor Layer
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: b Low-cost, off-the-shelf \
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Figure 2. Conceptual Diagram of Sensor Web
1.3.1 Sensor Layer:
By definition, a sensor is a device that provides a usable output
in response to a specific physical quantity, property, or
condition which is measured (National Research Council, 1995).
In the past ten years, sensor technology has been advanced
significantly. Over 100 physical, chemical and biological
properties can now be measured by sensors. Sensors have
become smaller, cheaper, more reliable, more power efficient,
more widely available, and more intelligent. Trillions of
sophisticated sensors will be embedded into our daily lives,
thereby providing extensive monitoring in the near
future.(Estrin et al., 2001)
Sensors can be classified in several ways. One way is in terms
of the medium or object that the sensors are sensing. Based on
this, sensors can be classified as either in-situ or remote sensors,
both of which have different characteristics and advantages. In-
situ sensors are less expensive per unit, have higher accuracy,
and have better temporal resolution. However, remote sensors
provide much greater spatial extent then do in-situ sensors. The
benefit of the SW is that it integrates both in-situ and remote
sensors and thereby achieves truly integrated sensing.
1.3.2 Communication Layer:
The second of the three SW layers is the Communication layer,
which controls the data / command transmission within and
between the sensor layer and the information layer. It includes
media, protocols, topologies, etc. This layer can be an Internet,
satellite, cellphone or radio-based network. Configuration of
the layer depends on the environment, the requirements and the
constraints of the particular context.
Radio communication has long been used for meteorological
sensors that obtain periodic readings of temperature, light,
humidity, wind directions, etc. In urban areas, cell-phone based
wireless network communication has been widely used for
mobile sensors, such as GPS sensors for vehicle tracking.
European Commission’s Mobihealth project is an example of
using 2.5G and 3G cell-phone-based networks to transmit bio-
sensors’ physical measurements. In many areas, satellite
communication is still the only option for linking sensors. For
example, CCRS’s proWISE project deployed weather sensors
in Saskatchewan, CANADA and used satellite communication
for transmitting observations back to the workstation in Ottawa,
CANADA (Teillet et al., 2002a)
In recent years, advances in miniaturization; low-power circuit
design; simple, low power, yet reasonably efficient wireless