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.
|Broadcasting Period:||24 hours continuously|
|Station ID:||852 & 853|
|Bit Rate:||200 bps|
|Modulation Mode:||Minimum Shift Keying|
|Differential Data Format:||Radio Technical Commission for Maritime
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 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 Reference Station, Broadcasting Antenna and Anti-lightning Rod
The Broadcasting Antenna
RSIM Rack and Power Supply
Equipment inside RSIM Rack and Power Supply
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
Contol Station Rack
|Location||Achievable accuracy (1δ)|
|Kau Yi Chau||±0.3m|
|Tai O Pier||±0.4m|
|Tai Miu Wan||±0.4m|
|Wong Shek Pier||±0.7m|
|Sha Tau Kok||±0.4m|
|14-12-2021||09:30~16:00||Due to maintenance. The DGPS signal is suspended.|