A permanent Global Positioning System (GPS) reference station, located on the top of the island, Kau Yi Chau, was established in 1996. The reference station continuously monitors the status of the GPS and broadcast differential correction signals. The broadcasting service is free; any user equipped with a standard DGPS receiver will be able to make use of the corrections to improve the accuracy of their GPS positions.

The Reference Station at Kau Yi Chau

The GPS Reference Station Receiver Antenna		The Broadcasting Radio Antenna

Technical details for this broadcasting service are as follows:

Broadcasting Period:	24 hours
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.
Range:	50 km
RTCM Message Type:	1,3,5,7 and 16
Signal Warning:	RTCM type 16 'screen pop-up' message

 

EQUIPMENT

The DGPS setup at Kau Yi Chau consists of two GPS Reference Station Receivers, one Integrity Monitor and a Radio Transmission System. All of them are connected to the computer system of the Control Station at Stonecutters Island through a private microwave link and dedicated telephone lines.

The Reference Station Receiver is a 9-channels dual frequency receiver situated on a known survey point. It receives and records the ranges from satellites within its observation extent. By making comparison between the observed ranges to individual satellites and their respective "true" values, range corrections to satellites can be determined. This correction is broadcast continuously by the Radio Transmission System.

The Integrity Monitor is used to monitor the quality of the differential signal. Similar to the Reference Station, the Integrity Monitor itself is a GPS receiver and located at a known survey point. Instead of examining the correction data, it receives the correction from the Reference Station and applies to its GPS fixed coordinates. The corrected coordinates are then compared with the known survey values to get the positional discrepancies. These discrepancies are continuously shown on the screen of the computer system as a Scatter Plot to facilitate real-time monitoring of the broadcasting data.

The GPS Receivers (Two of them as reference station receivers and the third one as Integrity Monitor)		The Radio Transmitters

Control Station (Real-time Monitoring System)

The Control Station is equipped with the Beacon Control System to monitor the performance of the broadcasting system and the quality of the correction data.

The common monitoring parameters are:

• Power Level/Signal to Noise Ratio of the Broadcasting Signal
• Age of the correction data
• Positional Accuracy

SYSTEM MAINTENANCE