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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 
$ 
t ©) Airborne LIDAR 
o ©) Aerial Photo 
Qo 
2 
® Oo Wireless Sensor Networks 
RJ 
e (motes) 
o 
© Web Cams 
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. 
  
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Sensor Web Sensor Layer 
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Pu Information Layer A Sensor Networks 
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b b or See ov he pit Td, i 
t I 1 
: b Low-cost, off-the-shelf \ 
b (9 sensors (e.g. web cams) 
£5) 
  
  
  
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