GPS Data and Correction Services GPS người dùng sử dụng các kỹ thuật khác biệt giữa các để đạt được độ chính xác cao nhất có thể vị trí. Một cách đơn giản để làm điều này là sử dụng hai máy thu GPS, cơ sở và một rover, như được thảo luận trong chương 5. Tuy nhiên, điều này có thể không được hiệu quả chi phí trong nhiều trường hợp. Một thay thế, mà có thể làm giảm đáng kể chi phí khảo sát xuống cấp mà không cần độ chính xác vị trí, là sử dụng một trong. | 7 GPS Data and Correction Services GPS users employ differential techniques to achieve the highest possible positional accuracy. A straightforward manner of doing this is to use two GPS receivers a base and a rover as discussed in Chapter 5. However this may not be cost effective in many instances. An alternative which could significantly reduce the survey cost without degrading the positional accuracy is to use one of the available GPS data and correction services. If such a service is available within the project area a GPS user would only need one receiver to be used as a rover no base receiver is required. A number of various GPS services are currently available with various levels of accuracy and cost. Some services are even provided with no user fees required. For high-accuracy static positioning primarily for establishing new control points GPS users may use one of the highly precise permanent GPS reference station networks established by several organizations around the world. These services are currently available free of charge at the global level . the International GPS Service for Geodynamics IGS network as well as at the regional level . the Continuously Operating Reference Station CORS network in the United States 1-3 . The Canadian Active Control System CACS is another regional GPS service which is available to users at nominal fees 4 . The reference stations within these systems are 91 92 Introduction to GPS operating on a continuous basis and provide access to the modern reference frames such as the ITRF and the improved NAD 83. Some countries around the world have established networks of reference stations around their coastal areas which continuously broadcast real-time DGPS corrections in a special format known as the RTCM format see Chapter 8 . This service is primarily designed to enhance the safety of marine navigation but is available at no cost to all users within the coverage area. Although this service is available primarily free of
