International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004
However, all the mentioned limitations are to be solved within
the near future or by a combination with other equipment. New
GPS-satellites will use a stronger signal. The receivers will be
assisted (AGPS) by terrestrial communication signals providing
the satellite almanac data in forehand, which will reduce the
start-up time for a first position fix. Pseudolites at the earth
surface can resolve the limitation of a few visible satellites.
Another promising initiative is the European Navigation System
Galileo. With Galileo the total available satellites will be
doubled and consequently the chance to receive 4 or more of
satellites simultaneously will be very high. A first simulation
study (Verbree et al, 2004) proves this expectation. Figure 2
shows a small part of the city of Delft modelled in 3D. Given
the theoretical GPS constellation and the proposed Galileo
constellation one can determine which satellites are visible from
a certain position. It is known that for 3D positioning are
needed either 4 GPS, or 4 Galileo, or 3 GPS and 2 Galileo, or 2
GPS and 3 Galileo satellites. As the position of the satellites
changes during time, we have to determine the line-of-sight
form the receiver to the satellites for a specific time during the
day. The red lines indicate for a random time the reception of
GPS satellite signal, the purple of the Galileo satellites. Adding
Galileo satellites the availability and thus the usability of GNSS
dramatically improves. Figure 3 shows a combined Galileo GPS
positioning. The green dots indicate for a 90965-10076
availability of GPS and Galileo satellites during a period of 24
hours. This figure indicates that even within the narrow streets
of Delft the combination of GPS and Galileo will work very
well.
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Figure 2: Simulation of GPS (red) and Galileo (purple)
positioning
It is important to take into account the progress in
telecommunications. The GPS/Galileo receivers will be
miniaturized and integrated within devices like PDA's and cell-
phones, creating a situation where the location of these devices
is known prior accessing the disaster management system. This
location awareness. will reduce the input while specifying a
certain location-based query.
The expectations of positioning based on telecom networks are
high. In theory tele-communication networks can trace a mobile
phone by range measurements, comparable to the principle
behind GNNS. But where for example GPS is a global
navigation system, telecom networks are not. While GPS offers
real 3D positions, telecom network cannot due to the
configuration of the transmitters bounded at the earth surface.
While GPS offers a global accuracy of 10 meters or better, the
accuracy of telecom networks corresponds in principle to the
cell-size of the base-station, i.e. it is limited to 100-500 meters.
Any claim to use triangulation between three or more base
stations depends on an adjustment of the current situation and
thus a large investment. The urban areas are again problematic.
A mobile phone can be easily connected to a transmitter (e.g.
on a high building) that is 2-3 km far way from the current
position of the user.
Figure 3: Combined Galileo and GPS: positioning in narrow
streets is possible (only two dark dots remain undetermined)
Telecom based 3D positioning offering the needed decameter
accuracy or better is not foreseen. However, a number of hybrid
systems, e.g. combination of mobile networks, GPS and
additional information (i.e. postal code) are already in use. The
progress in the WLAN offers another alternative to position a
user in close ranges (30-40m). The general research question
here is how to switch between different positioning systems to
be able to provide accurate positioning at any time.
5.2 Communication
Another important aspect is the communication and practically
this includes the interaction between the user and the whole
system. Although third generation of wireless communication
networks (UMTS) will improve the bandwidth and reduce
bottlenecks that currently limit the amount of data that can be
communicated, there will be a demand on reduction of the data,
especially when thinking of communication on different
networks (e.g. UMTS, GPRS). Nonetheless it will be more
important to allow the user not just accessing data that are
already part of the system, but also make sure these data are
handled as effective as possible. Therefore it is necessary to
allow them to update this data and to integrate new data sets.
Here again the system has to support the user in acquiring the
data, complete the data by using the available sensors,
communicate them to the system, check the consistency, detect
semantic dependencies to other existing data sets and integrate
them and the dependencies into the system. These steps are
necessary to enlarge the data pool of the system and make sure
this data are able to be processed and analysed by the system.
Nowadays systems usually do not support online update. All the
described steps are done — often manually -in a post-processing
process after the data is collected that introduces a large delay
between the data collection and the data availability.
The privacy aspect is another topic when dealing with
navigation and positioning devices. In general people do not
like to be tracked and monitored, unless they are in danger
themselves or have to obtain a position fix to help others. The
type of positioning (global or telecom) has different nature.
GNSS are passive devices, meaning the user have to approve
transmitting their coordinates to the geo-mobility server.
Telecom positioning can be initiated by the telecom operator
regardless the wish of the user. This fact will be far more
important in accepting and using GNNS in contradiction to
location devices based on telecommunication systems. Some
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