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
