ISPRS, Vol.34, Part 2W2, “Dynamic and Multi-Dimensional GIS”, Bangkok, May 23-25, 2001 
problem without adding new permanent reference station, a 
mobile reference station can be used as a bridge to tie the rover 
users to the permanent reference stations. The concept is 
depicted in Figure 2. 
From Figure 2, we see that the proposed mobile-to-mobile 
solution employs a mobile reference station whose separation to 
either the permanent station or the rover user is less than the 
separation between the rover user and the permanent reference 
station. Because the shorter baseline length between the 
permanent reference station and the mobile reference station, 
the latter’s position can be precisely determined using RTK 
technology where integer ambiguity resolution becomes 
feasible. The difference between the mobile reference station 
and the permanent reference station is that the mobile reference 
station is usually mounted in a vehicle so the mobile reference 
station must wirelessly access the Internet. Once the precise 
position of the mobile reference station is known in real-time, it 
can be applied to serve as a reference station for the rover 
users. Since the baseline length between the mobile reference 
station and the rover user is shorter than before when the rover 
user is directly differencing with respect to the permanent 
reference station, it allows the rover user to conduct high 
precision RTK positioning because integer ambiguity resolution 
becomes feasible now. 
The above concept can be extended to more general multiple 
moving platform situations where precise RTK positioning is 
conducted with respect to multiple moving or mobile reference 
stations (Luo, 2001). Since each mobile user need to access 
differential corrections from all mobile reference stations, 
communication bandwidth is a concern if using conventional 
radios. In this regard, the Internet provides an excellent 
alternative with no bandwidth barriers. The concept is described 
in Figure 3. 
Figure 3: Concept of Mobile-to-Mobile RTK System 
5. FIELD TESTS AND DATA ANALYSIS 
The Internet-based prototype system described in Section 3 has 
been tested in the field to assess its positioning performance 
and feasibility under operational environments. The performance 
analysis is conducted in a kinematic mode using phase-based 
Real-Time-Kinematic (RTK) technology. 
The wireless equipment used in the test includes an Ethernet 
NIC linking the reference station to the Internet and a Merlin 
Wireless PC Card from NovAtel Wireless Inc to allow the rover 
user to wirelessly access Internet via a Cellular Digital Packet 
Data (CDPD) network. In the city of Calgary, the CDPD service 
is provided by Telus™ Corporation with an operational speed of 
19.2 Kbps which is sufficiently high for the RTCM message 
transmission in RTK positioning. 
A kinematic field test was conducted in Calgary on February 11, 
2001. Two Ashtech GPS+GLONASS single frequency receivers 
were used as the reference and rover receivers, respectively. 
The antenna for the reference receiver was setup on the roof of 
the Engineering Building at the University of Calgary with 
precisely known coordinates (Figure 4). During the test period, 
the reference receiver was connected to the server PC from 
which the differential data was made on-line over the Internet. 
The rover receiver was installed in a vehicle and the receiver 
antenna was set up on the roof of the vehicle (Figure 5). 
Connected to a laptop computer, the rover receiver retrieves the 
reference differential data from the CDPD modem installed on 
the laptop. 
Figure 4: Reference Receiver Station 
Figure 5: Rover Vehicle Station 
During the field test, the vehicle was driven away from the 
campus and the maximum baseline length between the 
reference receiver and the rover receiver was up to about 12