3-3-2 
1. INTRODUCTION 
The successful application of GPS is 
dependent on understanding its coordinate 
system, achievable accuracy and 
limitations. All GPS modes of operation 
are not the same. Accuracy ranging from 
hundred meters to a few millimeters can be 
achieved depending upon the data 
collection and processing techniques, 
hardware, software, satellite geometry and 
atmospheric condition. 
In order to optimise the survey time, it is 
necessary to know the accuracy achievable 
in different modes of operation. It is also 
necessary to know the procedures in order 
to achieve higher accuracy. A GPS can be 
used for level survey provided that the 
accuracy achievable is good enough for the 
desired application. In this research a 
procedural was developed to achieve 
centimeter level accuracy in height 
component, which could be useful for level 
survey. 
2. OBJECTIVE 
The main objective of this research is to 
improve the height accuracy observed in 
real time kinematic (RTK) mode. 
3. RESEARCH STRUCTURE 
The structure of this research is shown in 
Figure 1. The structure mainly consists of 
four components. The first one is to 
develop the system itself, since this is the 
first time a base station was installed in 
Thailand (and hence at AIT) using mobile 
phone for real time differential correction. 
The next component includes the accuracy 
Research Structure 
Figure 1 
analysis in both the static and kinematic 
modes. In static mode, the accuracy was 
analyzed for planimetrie and height 
components where as for the kinematic 
mode, only the height accuracy was 
anlaysed. The height accuracy achieved in 
kinematic mode was compared with the 
level survey. An overhead road crossing at 
a baselength of three kilometer was chosen 
for real time kinematic survey. The road is 
about 750 meters long, elevated up to eight 
meters (approximately) at the center of the 
road and has 31 piers. The error at each 
pier location was calculated and analysed 
to see whether there was any effect of 
antenna velocity (vehicle speed) on 
accuracy. Next, the analysis was done to 
see whether the errors could be minimised. 
The RMS error, the standard deviation of 
the error itself and the scatter plot between 
the errors at the calibration points were 
analysed. Based on these analysis and the 
two calibration points, an error correction