ISPRS, Vol.34, Part 2W2, “Dynamic and Multi-Dimensional GIS”, Bangkok, May 23-25, 2001 
DIFFERENTIAL SATELLITE POSITIONING OVER INTERNET 
Ying. GAO and Zhi. LIU 
Department of Geomatics Engineering 
The University of Calgary 
2500 University Drive N.W. 
Calgary, Alberta, Canada T2N 1N4 
Tel: 403-220-6174 Fax: 403-284-1980 
Email: gao@geomatics.ucalgary.ca 
KEYWORDS: GPS, Wireless, Internet, Differential Positioning 
ABSTRACT 
A differential GPS positioning system is able to provide more precise position solutions than a stand-alone system through the 
application of corrections calculated at a reference station or a network of reference stations with known surveyed coordinates. 
Differential corrections are usually transmitted to the users via radio, beacon or communication satellite. In this paper, the concept of 
differential GPS positioning based on wireless Internet has been be described. To assess the feasibility of the proposed method, a 
prototype system has been developed and tested in the field using CDPD-based wireless Internet access. The field results have shown 
satisfactory positioning accuracy and differential data latency over the Internet. In addition to Internet advantages for wireless 
communication, the use of the Internet to develop new positioning methods has also been discussed. 
1. INTRODUCTION 
Measurements made from Global Positioning System (GPS) are 
affected by a number of error sources including satellite orbit 
error, satellite clock error and atmospheric effects. Autonomous 
GPS positioning is therefore subject to the effects of all the 
above error sources and can provide positioning accuracy only 
in the neighborhood of about 10 meters. Therefore, in order to 
achieve higher positioning accuracy such as in the order of 
meter to centimeter level, differential GPS (DGPS) techniques 
must be employed. 
The objective for DGPS is to reduce the error sources within the 
GPS satellite clock and orbit data, atmosphere effects as well as 
other errors due to GPS receivers. Using DGPS method, at 
least two GPS receivers must be used with one serving as a 
reference receiver station with precise known coordinates, and 
the other as the rover station for which positioning is required. 
The reference station is used to generate differential corrections 
be applied by the rover station to reduce the above mentioned 
error sources and subsequently to derive an improved position 
solution. Due to the use of a reference station, the method is 
effective only for short reference-rover separations because the 
spatial correlation of the error sources between the reference 
and rover stations reduces as the increase of the reference- 
rover separation. DGPS positioning using a single reference 
station is often referred as Local Area DGPS (LADGPS). 
To increase the effective area of the generated differential 
corrections, multiple reference receiver stations are often 
employed to form a reference network. Dependent on the size of 
the network, there are two different types of reference networks, 
namely, 
a) Wide Area DGPS (WADGPS) network and 
b) Regional Area DGPS (RADGPS) network. 
A WADGPS network focus on providing differential correction 
service continental-wide even worldwide while a RADGPS 
network focus on a region of hundred kilometers in dimension 
(Gao et al., 1997). A number of different WADGPS and 
RADGPS networks have been implemented to date and many 
others are currently under development. The US Wide Area 
Augmentation System (WAAS) is a typical example of 
WADGPS networks whose reference station separations are 
typically a few thousand kilometers apart (Loh, 1995). WAAS 
differential corrections are currently available with obtainable 
position accuracy at the meter-level although the network is not 
yet fully operational. On the other hand, a RADGPS network 
consist of multiple reference stations separated in the range of 
several hundred kilometers and the Swedish SWEPOS network 
is a typical example of such networks (Hedling, et al., 1996). 
For real-time applications, no matter what type of DGPS 
systems you may implement, a continuous data link must be 
established between the reference network and the remote 
users in order for the DGPS users to receive the network 
generated differential corrections. For local and regional area 
differential positioning, radios and local communication systems 
are typically used while for wide area differential positioning 
satellite communication is appropriate although it is much more 
expensive to use. As the advance of Internet technology and its 
fast expansion of the coverage and mobile accessibility, it has 
been widely demonstrated that the Internet could become a 
cost-effective and efficient alternative for a wide range of 
applications including differential positioning that we will discuss 
in this paper. 
This paper describes recent research results in the use of 
Internet as the communication link for differential positioning 
and navigation applications. The paper will show that Internet- 
based differential positioning systems are advantageous when 
compared to current DGPS systems. The technology will not 
only improve the efficiency of implementing DGPS technologies 
but also potentially expand significantly the application spectrum 
of DGPS technology with new differential positioning methods. 
The paper is organized as follows. The paper will first provide a 
brief description of the Internet and its characteristics as a 
communication tool. Differential satellite positioning using 
Internet and a prototype system are then described followed 
with the introduction of a new Internet-based mobile-to-mobile 
solution to multiple moving platform applications. Field test 
results are finally provided to assess the obtainable differential 
positioning accuracy of an Internet-based system and its 
feasibility to be used in operational environments. 
2. WIRELESS INTERNET ADVANTAGES 
Internet is characterized by its low cost, easy accessibility, 
availability, flexibility, and expandability. Currently the expense