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The Differential Global Positioning System (DGPS) of the Marine Department was upgraded to Differential Global Navigation Satellite System (DGNSS) in March 2016. The broadcasting service is free and the correction signals can be received in all waters of the Hong Kong Special Administrative Region (HKSAR). Any user equipped with a standard DGPS/DGNSS receiver can make use of the correction signals to improve the accuracy of their positions. Although the reference station system is DGNSS ready, it only broadcasts DGPS correction signals of the Radio Technical Commission for Maritime Services (RTCM) SC-104 version 2.0 format before the official launch of the 2.4 format.

System Configuration

The reference station system has two major components, namely the Central Reference Station on Kau Yi Chau and the Control Station on Stonecutters Island. By using a 3G mobile network, the two major components are connected to form a complete system.

Technical Details of the Broadcasting Service

Broadcasting Period:	24 hours continuously
Station ID:	852 & 853
Frequency:	289.0 kHz
Bit Rate:	200 bps
Modulation Mode:	Minimum Shift Keying
Differential Data Format:	Radio Technical Commission for Maritime Services (RTCM) SC-104 Version 2.0 format
Coverage:	All waters of the HKSAR
RTCM Message Type:	1,3,5,7 and 16
Signal Warning:	Type 16 'screen pop-up' message

The Central Reference Station on Kau Yi Chau

The Central Reference Station on Kau Yi Chau consists of two GNSS reference stations, a Near Field Integrity Monitor (NFIM) and a Radio Transmission System. All system instruments, including two NetR9 GNSS reference station receivers, two NetM3 MSK modulators, a NetR9 IM receiver, a set of dual 200W radio transmitters, a 3G digital router with firewall, and a Reference Station Integrity Monitor (RSIM) computer, are collectively installed in a RSIM rack. The RSIM computer is connected to the computer system of the Control Station on Stonecutters Island through a 3G mobile network. The two reference stations, each consisting of a 440-channel dual frequency GNSS reference station receiver and a geodetic grade Choke Ring Antenna, are set up at two known survey points on Kau Yi Chau. One is active while the other serves as a redundant unit. The receiver receives and records the ranges from satellites within its observation extent. By making comparison between the observed ranges to individual satellites and their respective values, range corrections to satellites can be determined. The correction signals are broadcast continuously by the Radio Transmission System.

To monitor the quality of broadcast correction signals, an NFIM is set up near the broadcasting site. Similar to the GNSS reference stations, the NFIM is DGNSS ready and set up at a known survey point. It receives the broadcast correction signals and applies to its pseudo-range observations to generate corrected coordinates. The corrected coordinates are then compared with known survey values to work out the positional discrepancies. These discrepancies are continuously monitored in real time to safeguard the quality of broadcast correction signals.

The Control Station on Stonecutters Island

The Control Station on Stonecutters Island is equipped with Trimble Pivot Platform (TPP) and Trimble Coastal Control (TCC) software to monitor and control the quality and performance of broadcast correction signals. Full remote control and monitoring of the Central Reference Station components by the system operator at the Central Control Station is allowed through a 3G mobile network. The operation status of various components of the reference station system is displayed in real time on the panels at the Control Station. If any equipment/component is out of order, the system operator at the Control Station can remotely inspect the malfunction unit and manually turn it off and switch to the redundant unit. Raw GPS/GNSS data can also be remotely downloaded for storage.

A Far Field Integrity Monitor (FFIM) is also set up near the Control Station on Stonecutters Island to monitor in real time the quality of broadcast correction signals.

The major monitoring parameters of both NFIM and FFIM are:

• Strength of the broadcasting signal
• Signal-to-noise ratio of the broadcasting signal
• Age of correction data
• Number of health satellite tracked
• Positional accuracy

Achievable accuracy in Hong Kong waters

The achievable accuracy of correction signal broadcast by the DGNSS Reference Station is highly depended on the type of receiver used. For system integrity checking, the accuracy of the broadcast correction signal is continuously monitored by the NFIM and FFIM located on Kau Yi Chau and Stonecutters Island. Further, the achievable accuracy at different locations over the territory had been evaluated by using dual frequency DGNSS receiver. The surveyed accuracies at different locations are listed as follows:

Location	Achievable accuracy (1δ)
Kau Yi Chau	±0.3m
Stonecutters Island	±0.3m
Tai O Pier	±0.4m
Tai Miu Wan	±0.4m
Wong Shek Pier	±0.7m
Sha Tau Kok	±0.4m
Ping Chau	±0.6m

SYSTEM MAINTENANCE

The maintenance contractor can perform data checking and system updating by remotely logging in the reference station system. Preventive system maintenance is scheduled to be conducted quarterly.

System Maintenance Schedule

Date	Period	Remarks
21-11-2018	09:15~15:00	Due to maintenance. The DGPS signal is suspended.
27-11-2018	09:15~15:30	Due to maintenance. The DGPS signal is suspended.