1SPRS, Vol.34, Part 2W2, “Dynamic and Multi-Dimensional GIS”, Bangkok, May 23-25, 2001
109
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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
