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SatRef – What makes positioning infrastructure work

Simon Kwok
Chief Land Surveyor,
Lands Department
Hong Kong Special Administrative Region
China
Email: [email protected]

A satellite positioning infrastructure establishes the reference frame to integrate the spatial information of a region. It provides a link between the regional reference system to the global reference system. The infrastructure also helps users to improve accuracy of positioning, enhance efficiency, increase productivity and reduce the cost of operation. To make a satellite positioning infrastructure work and deliver the aforesaid outcomes, the organisation running the infrastructure should address the issues of quality assurance, risk mitigation, business continuity, legal requirements, training and growing an environment for innovation. The article discusses aspects based on the experiences gained from developing the Hong Kong Satellite Positioning Reference Station Network (SatRef) during the past decade.

System Overview
The SatRef has twelve stations, one date centre and one control centre. Fig. 1 shows the network. The average station spacing is about 15 km. It is designed to provide positioning service for metropolitan areas. It ensures continuous achievement of cm level accuracy for real time positioning and static positioning with short observation time (15 mins). Such station distribution also provides redundancy to ensure business continuity, as from time to time some of the sites may be out of service due to maintenance or equipment failure. Fig. 2 shows the photo of a typical station and its design model.

The services provided by the system are:

  • RINEX data service for post processing by the user
  • Network RTK real time positioning service
  • DGPS real time positioning service
  • Automatic position computation service using RINEX data

Data Centre and Control Centre relationship
The Data Centre and Control Centre are situated at different locations. The purpose is to ensure high level of security and provide flexibility for the operator to control the system from their normal place of work. The hosting service of the Data Centre is provided by a telecommunication company. The Data Centre has strict IT and physical security control. It also provides fire protection, backup power supply and optimum environment for the operation of the computer system. The Control Centre is located at the Lands Department.

Quality Checking Mechanism

Hourly checking on RINEX data quality, equipment error and communication problem

  • RINEX files collected from the reference stations are processed every hour
  • Hourly solution is compared with the known position (dN,dE,dH)
  • Poor results or no solution indicate problem in data quality, equipment error and communication problem.

Network-RTK Data Quality Checking
Three independent monitoring stations are set up in Hong Kong to monitor the quality of network RTK data. Every epoch (1 second) of RTK measurement is checked against the known position of the monitoring station.

Station Stability Check
Daily REINX files of each station are processed using Bernese software

  • Preliminary solution: use quick orbit
  • Final solution: use precise orbit

Detect trend in station movement with time series of years of observation

DGPS data quality checking
One reference station receives DGPS correction continuously from the SatRef system. Its measured position is compared with the known position. Alarm will be sent to operators by email if error outside tolerance is detected.

Risk Mitigation
IT security measures are essential to mitigate risks to business continuity. IT security audits are conducted regularly to review management control and operation control. The aspects to be audited include: risk management, review of security controls, system security plan, personal security, physical and environmental security, production, input/output control, contingency planning, hardware and system software maintenance, data integrity, change control management, document, security awareness and incident response capability.

Legal Requirements – Accuracy Standards and Code of Practice
The Code of Practice issued under the Land Survey Ordinance was revised to include the “Specifications and practice guides for establishing GPS control stations for land boundary surveys”. It regulates the quality of land boundary survey conducted using GPS.

Human resource development
Building up the ability of the people to do the task well is essential for making the positioning infrastructure work. All stakeholders have to participate in building the capacity. It involves the government, professional institutions, academics, industry, equipment manufacturer and the users. The Satellite Positioning Reference Station Data Services is designed for multi-discipline applications. Other than land surveying and mapping, the system has been used for:

  • Structure and slope monitoring
  • Land use control
  • Weather forecast
  • GIS data collection
  • Asia and the Pacific Regional Geodetic Network
  • Location base services

The consumer market has not yet full explored the potential of DGPS, RTK, wireless internet, consumer information, light weight and low cost devices. It is necessary to stimulate an environment for innovation and facilitate development of value added product and services.The Lands Department is now providing Satellite Positioning Reference Station data service to government departments and contractors of government projects. In 2010, the Lands Department will open the Satellite Positioning Reference Station data service to the public. There will be no charge for the services provided (Network RTK, DGPS and REINX data).

It will facilitate market participation in the provision of value added products and services for the economic benefit of Hong Kong.

The Hong Kong’s Satellite Positioning Infrastructure brings innovation to business, speeding up work efficiency, reducing operation cost, enabling creation of value added services and improving quality of life of the people.