Figure.2 Management Diagram of “MultiGateStation” 
5. GPS POSITIONING TECHNOLOGY 
There are two methods of high-accuracy GPS-based 
positioning: Differential GPS (DGPS) and Real-Time 
Kinematic GPS (RTK-GPS). As shown in Figure 3, DGPS 
is a GPS-based positioning technique in which a GPS 
receiver is set up at a known location and at a meas 
urement location, and range errors at the measurement 
location are corrected by transmitting range errors meas 
ured at the known location (ARn) to the meas-urement 
location as correction data. The positioning accuracy of 
DGPS is several meters (about 1-2 m in the horizontal 
direction and about 3-5 m in the vertical direc-tion), which 
are suitable for rough positioning but are not accurate 
enough for precise location management. GPS receivers 
used for DGPS, however, are inexpensive. In addition, in 
Japan, DGPS service that uses FM multiplex broadcasts 
for vehicle-mounted navigation systems is a-vailable in 
almost all parts of the country. Consequently, in areas 
within range of the FM multiplex broadcasts, DGPS 
positioning can be carried out relatively inexpensively. 
Another advantage of DGPS is that extreme deterioration 
of accuracy does not occur even when satellite trans 
missions or correction data transmissions are interrupted 
under the influence of the surrounding topography: sys 
tem operation returns to normal immediately after com 
munications are reestablished. 
The set-up for RTK-GPS is similar to DGPS, as shown in 
Figure 4: a GPS receiver is set up at a known location and 
at a measurement location. In the case of RTK-GPS, 
however, information transmitted to the measurement 
location is not correction information but GPS satellites' 
Figure.3 Differential GPS (DGPS) 
observation information obtained from measurement at 
the known location. At the measurement location, this 
information is used to calculate the measurement location 
through baseline analysis between the known location 
and the measurement location. The positioning accuracy 
of RTK-GPS is several centimeters (about 2 cm in the 
horizontal direction and about 5 cm in the vertical 
direction), which is high enough for precise location 
management. Since, however, RTK-GPS receivers are 
more expensive than DGPS receivers, an RTK-GPS- 
based rolling management system is more expensive 
than a DGPS-based system. Another disadvantage of