The Study Of BDS/GPS Network RTK Algorithm Based On Undifferenced Observation Model

Global Navigation Satellite Systems(GNSS)can achieve all-weather,all-day,high-precision navigation and positioning,and it has been widely used in the space technology,homeland security,intelligent transportation and other fields,and it plays an indispensable role.At present,BeiDou Navigation Satellite System(BDS)has officially provided passive positioning,navigation and timing services to the Asia-Pacific region.GNSS standard positioning service can not meet the application requirements of high-precision positioning,Network RTK is an important means to improve the positioning accuracy of GNSS and achieve high-precision real-time dynamic positioning services.The traditional double difference model is generally used in the network RTK,which needs to choose the main reference station from CORS and use the observation data between the main reference station and other reference stations to make the combination of the double difference observations.Although the user can achieve high-precision error correction,the double difference model causes the correlation between the stations and the satellites,the correlation between the double difference observations increases the difficulty of data processing,it limits the flexibility of network RTK.If undifferenced observation model is used in the network RTK algorithm,it does not need to distinguish between the main reference station and the auxiliary reference stations,the model of error correction of network RTK regional scale is established by carrier phase ambiguities,carrier phase observation and the known reference station coordinates from the reference stations.Undifferenced error correction information from the reference stations network is applied to rover,rover is no longer limited by the traditional double difference model.Undifferenced observation model overcomes the correlation between the stations and the satellites for double difference network RTK,it can realize the fusion of different CORS network in the algorithm,the operation mode of network RTK becomes more flexible,it can be used to realize the uniform BDS/GPS real-time dynamic positioning algorithm in the reference stations network coverage area.The positioning principle of BDS/GPS network RTK is explored in this paper.The influence of observation errors and the processing methods of these errors are studied.The resolution algorithm of integer ambiguities of double difference carrier phase between reference stations for BDS/GPS and real-time fast calculation method of undifferenced integer ambiguities are researched based on the above studies.High-precision undifferenced observation model for BDS/GPS in the reference stations network coverage area is established further.The observation error of rover is further processed taking advantage of the method of regional error correction based on the undifferenced observation model between between reference stations for BDS/GPS,then the resolution algorithm of carrier phase integer ambiguities of multi-frequency and un-combined at rover for BDS/GPS is researched.A set of BDS/GPS network RTK data processing software is finally developed according to the algorithm proposed in this paper,the algorithm is analyzed and verified with measured data of BDS/GPS.The main contents and achievements are included in the following parts:1.The basic theory of BDS/GPS network RTK positionining based on undifferenced observation model is systematically researched.It includes time system and the time transformation,the coordinate system and the coordinate transformation,observation and observation equation for BDS/GPS,the differential positioning principle and the linearization of observation equation,the method of least squares parameter estimate and the observation theory of multi-frequency linear combination.2.The error influence and features of high-precision positionining for BDS/GPS are researched,and the method of eliminate and weaken error are further studied.The satellite orbit,ionospheric and tropospheric delay error which has strong correlation with the distance are researched on the basis of the above research.3.Ultra wide-lane ambiguities have the character of long wavelength,and there are linear relationships among triple-frequency ambiguities.The resolution algorithm of triple-frequency integer ambiguities between reference stations for BDS network RTK is proposed based on the above two laws.The observation of B2 frequency and B3 frequency and the strict criterion of ambiguities fixing are used to determine extra wide-lane integer ambiguities.The linear relationships between fixed extra wide-lane integer ambiguities and other wide-lane integer ambiguities are selected as constraint conditions,then the wide-lane integer ambiguity,relative zenith tropospheric delay error and ionospheric delay error are estimated and the wide-lane integer ambiguities can be fixed by searching mode.There are integer linear relationships between fixed wide-lane integer ambiguities and triple-frequency carrier phase integer ambiguities,these linear relationships are added to the observation model of carrier phase ambiguities parameter estimation and the integer ambiguities can be fixed.The resolution algorithms of dual-frequency and multi-frequency integer ambiguities between reference stations for BDS/GPS and GPS are further studied based on the above algorithm.4.An algorithm of ambiguities resolution for low elevation angle satellites between long-range network RTK reference stations for GPS is proposed aiming at the problem of integer ambiguities fixed inaccurate for low elevation angle satellites between long-range reference stations for GPS.The observation of L1 frequency and L2 frequency and the strict criterion of ambiguities fixing are used to determine wide-lane integer ambiguities.The integer linear relationships between fixed wide-lane integer ambiguities and dual-frequency carrier phase integer ambiguities are selected as constraint conditions,then the dual-frequency carrier phase integer ambiguities are determined by using dual-frequency carrier phase ionosphere-free combination observation.Then wide-lane integer ambiguities for low elevation angle satellites can be determined by taking advantage of the atmospheric delay error of fixed ambiguities satellites.The equations of dual-frequency ionosphere-free combination observation for fixed ambiguity satellites and low elevation angle satellites were synthesized,the linear constraints between fixed wide-lane integer ambiguities and dual-frequency carrier phase integer ambiguities for low elevation angle satellites are used simultaneously,and then the dual-frequency carrier phase integer ambiguities for low elevation angle satellites can be determined further.5.A fast calculation method of undifferenced integer ambiguities at single epoch for reference stations network is researched according to the syntagmatic relation between double difference ambiguities and undifferenced integer ambiguities.The method uses double difference ambiguities and the datum ambiguities between reference stations and satellites.High-precision BDS/GPS undifferenced observation model in the reference stations network coverage area is researched based on the above studies,it overcomes the correlation between the stations and the satellites for double difference network RTK.The observation error at rover is further processed to take advantage of the method of regional error correction based on undifferenced observation model between reference stations for BDS/GPS,then the integer features of ambiguities at the rover are recovered.The difference between selected different undifferenced datum ambiguities of undifferenced observation model can be absorbed by ambiguities at rover,the observation error processing at rover is not affected by the selected undifferenced datum ambiguities.6.The linear combination of observation can magnify the noise,and the resolution algorithm of integer ambiguities step by step brings the computation redundancy.A resolution algorithm of integer ambiguities of multi-frequency and un-combined observation for BDS/GPS network RTK at rover is proposed aiming at the above two problems.This method uses observation of the each frequency for each system to parametric estimate at the same time,then it can achieves high-precision positioning for single system,dual system are achieved.7.A set of BDS/GPS network RTK data processing software is developed according to the algorithm proposed in this paper.This algorithm is analyzed and verified with measured data of BDS/GPS.So the usability of the algorithms in this dissertion is testied.