Reverse RTK Technical Paper

Development of Reverse Real-Time Kinematic (RTK) in Support Of Global Positioning System (GPS) User-Side

Applications

Ahmad Ikram bin Othman and Prof. Dr Shahrum Bin SesDepartment of Geomatic Engineering,

Faculty of Geoinformation and Real Estate,Universiti Teknologi Malaysia, 81310 Johor Bahru, Johor, Malaysia.

Email: ikramothman@gmail.com, shahrumses@gmail.com

ABSTRACT

Advancement of positioning technique nowadays has increase the requirement to establish continuously operational reference station. This infrastructure is very important for real time positioning techniques which have very high demand in this modern surveying era. Aims of study are to develop and implement new model of real time positioning called reverse real time kinematics (RTK) or also known as server-based RTK and to establish communication between rover and reference station. Research-based network real time positioning system, ISKANDARnet will be used as the test bed of this study. At reverse RTK server, an open source computer programme named RTKLIB is going to be integrated which will act as the processing software for the system. The function of this software is to compute the rover coordinates and sent back the final solution to rover using Networked Transport of RTCM via Internet Protocol (Ntrip). Like standard RTK technique, reverse RTK is likely to be affected by distance dependence error. Findings of this study will help control centre to monitor data quality that is provided by the server as well as help the user to get better positioning solution without need to compute the coordinate at the field.

Key words: Reverse Real-Time Kinematic, Ntrip.

1.0 INTRODUCTION

In modern surveying, demand for real-time measurement by industry is increasing because it provides fast data collection. Many Global Navigation Satellite System techniques for real-time positioning have been developed such as real-time kinematics (RTK) and differential global positioning system (DGPS). Establishment of continuous reference stations (CORS) in support of RTK positioning to provide correction helps improve users coordinate accuracy and reliability. However, accuracy obtained is depends on distance between rover and base station. Instead of establish reference station, the use of reference station network seems to be better. It allows more option for users to choose closest reference station possible from their location. These infrastructures are very treasured in providing high positional accuracy not only for surveying but mapping, navigation and geodesy as well.

RTK positioning needs rover to request correction data by sending its approximate position to base station. Thus, user need to communicate with the reference station in order to obtain correction data from reference stations to calculate the final coordinates. Correction is transmitted in RTCM format which is the standard format in most of rover receiver nowadays. Rover also needs to have a program that contains certain algorithm that able to solve its position after receiving correction from service provider. Because of that, there are some limitations in term of processing ability as rover hardware is not powerful enough to give high accuracy of calculated coordinates.

This study will be about development a new model for RTK positioning called reverse-RTK which will help user to get better accuracy. Instead of getting correction from the base station, the user will get the final coordinate directly without need to compute at the field as it

will be done by the server. Server will be able to control and monitor the data qualities that are transmitted to the user. Besides that, it could reduce burden of the user hardware to compute the final coordinates.

2.0 METHODOLOGY

Development of reverse RTK positioning platform need to undergone various phase and processes. The entire element must take into account involving understanding of existing system, analyse the system, system design and system implementation. After the system successfully implemented, maintenance of the system is very important to ensure it can provide desired solution.

In this study, reverse RTK positioning platform that to be developed only involve single base reference station and ISKANDARnet CORS will be used as study’s test bed. Considering of limited time available, network reverse RTK cannot be done because it need to have many algorithm and complex system architecture. However, by using single base reference station is enough to show reverse RTK can be implemented.

1.1 Planning

Planning has to be carried out at the beginning of work to ensure the workflow is smooth and to avoid any difficulties during overall process. It also helps to achieve objectives of study by using the available time and resources. This phase involve the selection of software and instrument, acquiring permission to use existing system, scheduling as well as operations. Ideal system design must be selected during this phase. Workflow of this study is shown in Figure 1.

Figure 1 Process Workflow

1.2 Software Integration

In reverse RTK, the processing software is very important as it act as the computation module for the user coordinates. RTKLIB version 2.4.1 will be used in this study to be integrated at the reverse RTK server. This software is an open source computer program package for standard and precise GNSS positioning (Takasu, 2011). It supports various modes of positioning for both post-processing and real time. For real time positioning, RTKLIB have a graphical user interface named RTKNAVI which have kinematic mode – See Figure 2. By configuring the rover and the base station data inputs, navigation processing can be executed in real time with on-the-fly (OTF) integer ambiguity resolution – See Figure 3.

Figure 2 RTKNAVI’s user interface (Source: RTKLIB version 2.4.1 Manual)

Figure 3 RTKNAVI’s Data Flow(Source: RTKLIB version 2.4.1 Manual)

1.3 Communication Link

The establishment of communication link between rover and reference station using Ntrip protocol is shown in Figure 4.

1.4 System Test

After the implementation of the reverse RTK positioning platform is finished, several tests will be done to determine whether the system is functional or not. The test will involve communication testing and data collection using reverse RTK technique. Comparison of accuracy between this technique, standard RTK and static observation is planned to be carried out. The effect of distance dependent error also will be carried out by doing measurement with different baseline length of rover and reference station.

3.0 RESULT

Result expected from the reverse RTK positioning testing will be analysed according to these parameters (Feng and Wang, 2008):

i. Base-rover distance.ii. Time latency of the latest RTCM message available at the instance of user sends

their data to server (delays caused by data processing at the base station).iii. Time to first-fix which refers to time required to resolve integer ambiguities and

position estimation. iv. Reverse RTK accuracy compared to ‘true’ value (static observation).

Figure 4 Data Stream for Reverse RTK

Figure 5 Example of results presentation (Source: Feng and Wang, 2008)

4.0 CONCLUSION

Development of reverse RTK in support user application will provide alternative for standard RTK positioning and other positioning technique. By using the ability of server that have more processing power than rover hardware, high precision of user’s coordinate is obtained. At the same time, server can monitor and data quality of end product and eventually can improve the system from time to time. Enhanced correction model able to be applied to compute user coordinates. Findings of this study are hoped can help towards better surveying technique in the future.

AKNOWLEDGEMENT

The authors acknowledge Final Year Project supervisor Prof Dr. Shahrum bin Ses for his supervision, guidance and suggestions for this research. Special thanks to my family members who are giving me support all over the times. My sincere appreciation also extends to all my colleagues and others who have provided assistance at various occasions.

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AUTHOR

Ahmad Ikram bin Othman, was born in 1990. He is a final year student (Geomatic Enginerring) at the Faculty of Geoinformation & Real Estate, Universiti Teknologi Malaysia (UTM). His main interest is in design a real-time kinematics positioning.