1SPRS, Vol.34, Part 2W2, “Dynamic and Multi-Dimensional GIS”, Bangkok, May 23-25, 2001 
109 
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5 technologies 
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ribed followed 
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ns. Field test 
ble differential 
stem and its 
t accessibility, 
y the expense 
of connecting to Internet is very reasonable and will be much 
cheaper in the future. In the past several years, Internet has 
been evolved into one of the most important communication 
methods. Many applications are also increasingly required to be 
Internet-based due to its fast and worldwide accessibility. 
Internet is able to provide many advantages over the 
conventional radio data transmission method if used for 
differential positioning applications. Some of them are described 
in the following. 
First, Internet is not limited by an effective data transmission 
range limitation, which implies the rover station can be away 
from the base station as far as the user needs. In fact, Internet 
theoretically can reach any corner of the world and its 
communication range is not constrained by factors such as the 
horizon requirement. 
Secondly, Internet can be accessed in any place where there is 
an Internet access either in the office or field. This also makes 
it affordable to operate many additional reference sites for large 
WADGPS network beyond those critically needed sites to 
provide improved network redundancy. Subsequently, improved 
redundancy guarantees continuous service even if some 
reference stations go down unexpectedly. Since the Internet 
keeps expansion daily now, its accessibility will be further 
enhanced in the future while with reduced cost. 
In addition to the above, the Internet has also overcome the 
narrow frequency bandwidth associated with the conventional 
radio communication method. As the result, a reference station 
if based on Internet can serve as a virtual reference station and 
the only requirement is that the user should have access to the 
Internet. If using radio communication instead, the rover users 
then have to use the same type of radio and frequency used at 
the reference station in order to receive the differential data from 
the base station for DGPS positioning. 
An Internet-based differential positioning approach is particularly 
advantageous for differential positioning in regions with severe 
signal interference or for applications with large number of rover 
users such as fleet and mobile assets. A convergence of 
technologies, particularly the integration of location, information 
management, wireless communication systems, regulations and 
business opportunities is creating a rapidly emerging market 
known as location-based service, wireless location or location 
commerce, a multi-billion dollar market in the next five years. In 
fact, Internet is now being considered the communication 
standard for the development of future location-sensitive 
information services such as location-aware billing and 
advertising. It is expected that Internet will be increasingly used 
in the future as an efficient communication alternative to 
conventional methods. 
3. AN INTERNET-BASED DIFFERENTIAL POSITIONING 
PROTOTYPE 
An Internet-based WADGPS network implementation has been 
described in Muellerschon et al (2001) where the focus was 
placed on the network development using Internet to transmit 
reference site data to the processing center. In this section, we 
focus on the development of an Internet-based differential 
positioning prototype for the DGPS user to access the network 
differential correction data via Internet. 
The prototype system consists of a reference station and 
multiple rover stations. The system configuration of the 
developed Internet-based system is depicted in Figure 1. The 
reference station consists of a navigation receiver capable of 
generating RTCM differential correction data and a server 
computer. The server computer at the reference station can be 
either directly connected to the Internet via a local area network 
or wirelessly connected to the Internet via a wireless modem. 
On the other hand, each rover station consists of a rover 
navigation receiver, a client PC computer and a wireless 
modem. The client PC computer wirelessly links to the Internet 
using the installed wireless modem via a communication 
network to receive differential data from the reference station. 
The transmission time taken for the user to retrieve the 
differential correction data from the reference receiver station 
determines the differential data latency for the positioning. Less 
than a few seconds of data latency is typically required for most 
DGPS applications. To minimize the data transmission latency 
and its subsequent influence on the DGPS positioning accuracy, 
the Internet protocols should be carefully selected which defines 
how the data are transmitted through the Internet. Internet 
currently uses a Transmission Control Protocol/Internet Protocol 
(TCP/IP) suite to connect all the networks, organizations and 
users across the world. Transmission Control Protocol (TCP) 
and the User Datagram Protocol (UDP) are two important 
transport protocols that have been widely used for Internet 
applications. 
TCP provides a stream delivery and virtual connection service to 
applications through the use of sequenced acknowledgment 
with retransmission of packets when necessary while UDP 
provides a simple message delivery for transaction-oriented 
services. TCP is able to provide highly reliable data 
transmission since it takes extra time to ensure reliability, flow 
control, and connection maintenance. As a price for the superior 
reliability, the TCP protocol may not suitable for high precision 
real-time applications such as Real-Time-Kinematic (RTK) 
positioning because it requires an acknowledgement of data 
arrival and any lost data must be sent again [Hada et al., 1999]. 
Compared to TCP, UDP is able to provide faster data 
transmission although the reliability is not as high as TCP with a 
possibility of data losses. Since fast differential data 
transmission is essential for the success of a real-time 
positioning system to derive accurate positions, UDP protocol 
has been utilized in this research to test the performance of the 
developed Internet-based DGPS system. 
4. INTERNET BEYOND COMMUNICATION 
Although the previous focus has been on the use of the Internet 
as a communication tool for differential positioning applications. 
The Internet advantages, however, could help create new 
methods to the use of satellite navigation systems. It is 
expected that high precision location information will be no 
longer a luxury but basic commodity of daily lives. In the 
following, the concept of mobile-to-mobile differential positioning 
is described as an example. We definitely will see more new 
developments in the near future. 
Although the establishment of a reference network of multiple 
reference stations is able to extend the differential correction 
coverage to large area, the distance between the users to any of 
the reference stations is often still too long for high precision 
positioning. This is particularly true for RTK applications. 
Deployment of highly dense reference stations on a permanent 
basis seems too costly and difficult to implement. To tackle the