Global Navigation Satellite System or GNSS has become an essential tool in various positioning applications in Malaysia. With the full operational constellation and modernisation of the GNSS such as GPS and GLONASS, the accuracy of positioning has definitely improved. However, the integrity of navigation system is more important than the precision and there is always a needs to authenticate and monitor the received signals to ensure the safety used of the space-based positioning in the country.
The National Space Agency (ANGKASA) in collaboration with Universiti Teknologi Malaysia (UTM) has developed the GNSS and DGPS integrity monitoring system for space-based positioning in Malaysia.
The developed system continuously monitor the integrity and quality navigation information on GNSS and DGPS service over the country and provides a warning of any signal anomalies at all time. Several Continuously Operating Reference Station (CORS) are setup as the ground monitoring stations to observe the satellite signals. The observation data are streamed via the Internet to the processing centre and to be enhanced for the purpose of supporting the integrity monitoring system.
All GNSS such as the GPS is consists of three major segments: the space segment, the control segment and the user segment. The space segment consists of the satellites that transmit the signals that give the current satellite position and time. The control segment consists of worldwide monitoring stations that tracks the satellites and maintain the satellites in their proper orbits. The user segment consists of the receiver, which receives the satellite signals and uses the signals information to estimate user’s receiver position and time. These GNSS are being continuously monitored by the system administrator such as the United States Department of Defence for GPS.
In the perspective of GNSS users, there is also a need for an independent monitoring for the GNSS. This independent monitoring system should provide awareness and reliability measure to GNSS users as an important parameter to guarantee users security. In the case of the enhanced satellite signals such as in DGPS service, the users always require good DGPS corrections for a reliable positioning result. Thus, the GNSS and DGPS integrity monitoring system should help the user.
A state-of-the-art of GNSS and DGPS integrity monitoring system should comprises the satellite signals itself, the ground reference stations that enhance the satellite signals and the user who utilize the enhanced signals.
The GNSS and DGPS integrity monitoring system can be staged as follows:
1. The System-level
The conceptual framework of the project is conceived in the model-based design of GNSS and DGPS integrity monitoring as illustrated in figure below.
The system requires a several Continuously Operating Reference Station (CORS) as the ground monitoring stations to observe the satellite signals. The MSK-DGNSS stations by the Jabatan Laut Malaysia (JLM) are considered as part of the CORS for DGPS monitoring. These observation data are streamed via the Internet to the processing centre and to be enhanced for the purpose of supporting the integrity monitoring system.
The integrity of the GNSS signal is of utmost importance where it is the ability of the GNSS navigational system to provide timely warnings to the users when their position fails to meet certain margins of accuracy. This system can also be referred to as the signal reliability measurement system as the system also provides important information to guarantee the users security.
There are five (5) CORS involved in this project, which is one newly established CORS at Jabatan Meteorologi Malaysia Pengkalan Chepa Kelantan and four (4) existing CORS at Bandar Hilir, Melaka; Kuantan, Pahang; Kulai, Johor; and Langkawi, Kedah. The four existing CORS namely JLML (Melaka) and JLKU (Kuantan) – both belongs to SISPELSAT; ISK4 (Kulai) and AGLG (Langkawi) were setup with computer server and internet communication in order to allow for real-time GPS data stream to the UTM processing centre. It is also noted that both SISPELSAT stations, JLML and JLKU, are equipped with DGPS radio beacons. Additional works such as GPS antenna and receiver maintenance for each station have been conducted during the project.