The document describes Sichuan Network GPS services including: 1. A network of 13 reference stations spaced 42-132km apart connected to a central control center 2. The network provides RTK accuracy of ±2.5cm within a 4,500 sq km service area and RTD accuracy of ±33cm within a 36,000 sq km area 3. Evaluations found the network achieves better than specified accuracies with rapid attenuation beyond 50km from the network boundaries

1. Sichuan Network GPS Service Introduction Sichuan Seismological Bureau 2006.8

2. Review of Positioning Single Point Using only one receiver to receive signal (C/A code). Position information is calculated independently. Advantage: Distance: Unlimited Communication: No needed Complexity: LOW Disadvantage: Accuracy: 10M, LOW

3. Single Base Differentiation One receiver as base station and another as rover station. Rover receives the position correction data to improve its accuracy. DGPS AND RTK The accuracy of DGPS is 3m(C/A code) or 50cm (C/A code + L1). RTK(L1,L2) can reach 3cm accuracy.

4. Note :Need to set up a base station. Need a data link for communication. Single base differentiation is base on the high correlation of GPS signal on high correlation of GPS signals in nearby area. Taking the data link in account, the distance between rover station and base station is less than 15km. Problem: Uneven Accuracy—degrade as distance increase Low reliability— Single reference; No integrity monitoring; May import error when setting reference station Frequent resetting base stations.

5. Network GPS As the technology of satellite positioning, wireless communication and computer networks process, in the 1990s, Network RTK was proposed. In early 2000s, as commercial software of Network RTK being developed, Realtime differentiation technology is moving rapidly into multi-base Network RTK. The most interested is virtual reference station (VRS)

6. Formation: 1 A base station network made by more than 2 running GPS base stations. Number of stations:≥3 Distance:50km—70km

7. Formation 2 A control center running Network RTK system software

8. Formation 3 Reliable data link

10. Advantage 1 Increase working range, improve efficiency 2 Significantly lower system error and initiation time, improve accuracy 3 All users are in the same coordinate system 4 Personal surveying system, no local reference station needed

11. Introduction to Sichuan Province Network GPS Service System

12. Reference Station Network 13 Stations Distance: Min 42km Max 132km Avg. 86km

14. Control Center

15. Control center and users communicate thought Internet and wireless networks. For wireless networks, system support GPRS/CDMA/GSM 。 . Terminals require wireless support hardware such as GPRS/CDMA modules and GSM Modem Sichuan Province Network GPS Service System

16. The system RTK service area 4.5 万 km 2 ， covering major developed area. RTK accuracy±2.5cm(better than±3.0cm) RTD service area36 万 km 2 RTD accuracy ±33cm(better than±50cm)

17. G eodesy Determination To reach the accuracy 0f ±3cm in Chendu, 44 GPS/Geodesy sites are set in 12km distance grid. 8 GPS/Geodesy sites are set randomly in the area for counter check To reach the accuracy of ±5cm in Ya’an, 20 GPS/Geodesy sites are set in 10km distance grid. All sites perform a 4-hour GPS static observation and level 2 geodesy observation

18. 成都地区外检验不符值的 Max 5.7cm 平均差值为 2.1cm 中误差为 ±2.5cm 。 雅安地区外外检验不符值的 最大值为 6.2cm 平均差值为 2.5cm 中误差为 ±3.2cm 。

20. System Accuracy Evaluation Two accuracy evaluations First evaluation after 4 GPS reference sites and control center are constructed Second evaluation done after 12 reference stations are networked

21. First evaluation uses RTK accuracy from GPSNet system Evaluation content RTK/RTD accuracy inside the network(20 km from border ) Analysis of uniformity of accuracy inside the network Analysis of distance correlation of accuracy outside the network

23. Attenuation with distance in N axis (m) Attenuation with distance in E axis (m) Attenuation with distance in height (m)

24. Result of the two evaluations By using Sichuan Network GPS Positioning Service The RTK accuracy Better than ±2.5cm in horizontal direction and even inside network Attenuate rapidly 50 km outside the network

25. Features and Applications

26. 1.Combination of fix stations and portable devices. The system not only monitors active rift in Plain Chendu and southern area of Sichuan Province but also helps in scientific researches and commercial uses.

27. With 86km average-distance between reference stations, the network GPS system still satisfy the design requirement, which makes our system the largest area per station among known GPS networks. In this way investment efficiency is achieved.

28. The network GPS positioning service software using VRS technology, VRSNET, has achieve comparative performance through long term parallel test with Trimble GPSNet and Leica Spider. During the software development, a lot of researches are done. Such as “Modified model of distance correlation”, “Optimized network RTK arithmetic ”, “Modeling arithmetic for correcting network error” and “Implementing internet real-time service”. First VRS service software ever developed in China.

29. Bring forward solutions for evaluating network GPS positioning service using distribution curve of uniformity of network accuracy and attenuation along the distance from the network.

30. In Chengdu and Ya’an, geometry is determined, making “Using GPS Technology to obtain real 3D Terrestrial Information” a reality.

31. Network GPS user management system is effective and secure. Feasible for commercial usage.

32. Multiple communication method for terminals. Users can select one from GSM/CDMA/GPRS depending their requirement and the quality of service in the area.

33. The problem with different data transmit standards is solved. Easy and reliable for users with different system.

34. Balanced Power System is a low cost, high reliability power supply for man-free GPS observation station. It solves the problem of lighting strike and ripple in the power line.

35. Using the observation record, the atmosphere water vapor level can be determined for short-time weather forecast.