【正文】 way above, the results are 177。 ( 12) cm( read the analysis afterwards) , which can meet the control needs of thickness and roughness of filling layers of the paction construction. 5) All weathered 24hours work of many mobile terminals. In the case of using 10 mobile terminals simultaneously, the system has a munication rate no less than 100 Kbps, and can support the application of pointtopoint, pointtomultipoint and work relay station. The system does not suffer from the effect of weather , and meets the need of allweathered 24hours work of dam project . 2 Key problem and solution of the system design 1) RealTime supervision of rolled parameters. Parameters of the roller paction mainly refers to movement track, velocity, rolled times of the roller paction machines for mobile terminals, which need real time supervision, and can be used to control construction quality. Therefore, in order to ensure the realtime feedback of positioning data, the wireless munication data links need to achieve the function of pointtomultipoint and bidirectional high munication rate. The optimal design for solving the problem is to choose the mode of radio or GSM mobile phone. Taking account of the economy and practice ability, the system adopts wireless frequency augmented munication technology , achieves transmitting GPS differential data of the reference station realtimely to each mobile terminal, feeding back the three dimensional spatial location information of each mobile terminal realtimely to supervisory center and supporting position results at interval of one second. T his way,not only the system of each roller paction machine can reflect its own status o f on the display screen, but also the supervisory center and locale subsupervisory station can supervise the working status of each mobile terminal realtimely. 2) Precision of observation data. The precision of GPS RTK is cmlevel in general, considering that the distance between the mobile terminals of Shuibuya project and GPS reference station does not exceed 3 km, its horizontal positioning precision fully meets calculation precision needs of the movement track, velocity and rolled times of the roller paction machines. In order to ensure the elevation precision of thickness and roughness of filling layer, besides choosing high perform ance and antinoise dualfrequency GPS receivers, adopting proper method for acqui ring three elevations data of before filling, after filling and after paction and elev ation fitted model, can make the elevation accuracy increase. By analysis, the elevate ion precision can achieve177。 GPS observation data by means of diferential processing of carrier phases, consequently calculate the spatial location information of the mobile stations, whose positioning precision can increase. T he method above is named DGPS technology. In general, GPS reference station should be set up in the supervisory center for the sake of convenient supporting pow er, management and maintenance. To ensure the supervisory precision, it is stressed that the distance between the GPS reference station and the construction area must be less than 56 km, and the distance controlling requirement can be satisfied in practice. 1. 1. 4 Mobile terminal Mobile terminal includes systemrequiring equipments installed in the project supervisory car and roller paction machines. 1) Mobile terminal installed in the roller paction machine A mobile terminal installed in a roller paction machine is mainly posed of an integrated system unit, a GPS receiver antenna and a wireless munication antenna. The system unit integrates main equipments of industrial platform puter, GPS receiver, wireless munication and so on. Mobile terminal is mobile super visory equipment like GPS moving station, and carries out GPS mobile observation. Its control items mainly include the rolled track, rolled speed, and rolled times of the roller paction machines. Mobile terminals feed back the effective observation result to the supervisory center continuously and realtimely. Meanwhile, the platform puter reflects the work status of the roller paction machine realtimely, by means of figures and tables in the system unit of driver＇ s cab of the roller paction operator of the roller paction machine can check out whether his or her work meets the quality requirement on the display screen. 2) Mobile terminal installed in the project supervisory car Project supervisory car is the working vehicle for construction quality super vising that is equipped exclusively for the project main tasks are carr ying out GPS mobile observation and supervising locale quality . Its main items of observation are the thickness variations of paving layer before and after the pact ion, supporting data for controlling the paving thickness and roughness of filling layer and calculating the pacted rate. T he supervisory car also feeds the observation results back to supervisory center. Besides, according to the system supervisory data, it conducts the locale construction from the point of riew of quality management . Mobile terminal installed in the project supervisory car and mobile terminal installed in the roller paction machine resemble each other. 1. 2 Main characteristics of the system Main characteristics of GPS realtime supervisory system are as follows. 1 ) Continuous realtime supervision. Data munication achieves bidirect ional transmission in a real sense. The movement tracks and velocities of the roller paction machines can be displayed realtimely , and three dimensional location information at the sampling rate of one second can be displayed realtimely on the display screen, no matter whether it is in the supervisory center, on locale subsuper visory station or in mobile terminals. The observation data of the system are cont inuous, and t