CHANGES IN POSITIONING AND NAVIGATION DUE TO GPS In the past, surveyors require sophisticated survey equipment in fixing the position of newpoints by making observations and measurements from well established geodetic control points. A large number of Continuously Operating Reference Stations (CORS) have been established in different places of the world to provide round-the-clock GPS data services in support of all kinds of positioning activities for mapping, geodetic surveying, fleet management, logistics, etc. SatRef is the first CORS system in Hong Kong to provide full coverage of the territory for different GPS positioning activities. The collected GPS data and correctional information of the CORS improve accuracy and efficiency of those positioning operations. SYSTEM REALISATION OF SatRef The SatRef Phase I was implemented in 2001-02 with six GPS Permanent ReferenceStations constructed in the northwest and northern part of the Hong Kong territory. To extend the network coverage, SatRef Phase II for the establishment of another six GPS Permanent Reference Stations in the Lantau Island, Hong Kong Island and eastern part of the territory was commenced in early 2004. At present, there are a total number of twelve continuously operating GPS reference stations in the SatRef System. These stations are scattered in different places of the territory with a separation of about 10 km to 15 km between one and other (Fig 1). A system enhancement of SatRef was implemented in June 2007 to provide the real-time GPS correction data service on internet via NTRIP (Network Transport of RTCM via Internet Protocol). In principle, the SatRef can deliver the correctional data to almost unlimited number of users by means of NTRIP to meet the demand of users in positioning and navigation.  Figure 1 Layout Plan of Hong Kong SatRef Stations SYSTEM DESCRIPTION OF SatRef The SatRef System consists of three major parts, namely, GPS Permanent ReferenceStations (GPSRSs), Control Centre (CC) and Data Centre (DC). The GPSRSs are connected to the DC with 64K leased line whilst the DC is connected with CC with a 512K leased line. GPS Permanent Reference Stations As mentioned above, 9 of the GPS reference stations are hilltop stations and 3 stations are rooftop stations. These 9 hilltop stations are built at elevation ranging from about 30 meter to 360 meter above mean sea level. The hilltop GPS reference station is constructed with a 5m high concrete antenna pillar erected on a concrete platform. The area is fenced to prevent trespassing of people and animals. Each stationis installed with a dual frequencies geodetic grade GPS receiver, choke ring antenna, meteorological sensor, tilt sensor, GSM modem, power supply, UPS, 64K leased line, lightning and power surge protection device and etc. The station collects GPS data, meteorological data and tilt data at every 5 seconds and then sends the data back to Data Centre for processing at every 10 minutes interval. The rooftop station has the same functions and equipment (except tilt sensor) as the hilltopstation and it is constructed with either a 0.5m high concrete pillar [Stonecutters Island and Peng Chau or a 1.7m high stainless steel antenna stand (Fanling). |