| Gravitational field is the indispensable basic data for the establishment and maintenance of national surveying and mapping datum system,while gravimetry is the only means to achieve gravitational field.Airborne gravity survey has some unique advantages,is the main means and the only way to improve China's gravity blank area and improve the national elevation benchmark level.Aiming at the present situation of airborne gravimetry and gravity blank area in China,based on the large number of airborne gravimetry experiments and measured data,combined with the basic theory and principle of airborne gravimetry,this paper studies and analyses the calibration of airborne gravimetry system and the determination of high precision positioning velocity acceleration in large-scale airborne gravimetry.The key links and technical methods of data quality evaluation and results application,the development of engineering application software,the construction of highprecision airborne gravity measurement and data processing system for engineering production,and the establishment of airborne gravity measurement technology system to meet the needs of Engineering application.The main work of this paper is summarized as follows:(1)Combined with the present situation of airborne gravimetry,this paper investigated,analyzed and collated the existing gravity data,and obtained the existing gravity blank or few-point areas in China.Meanwhile,the external supporting conditions of airborne gravimetry were investigated and analyzed,thus preliminarily grasped the basic conditions of airborne gravimetry.which provided relevant data basis and planning reference for carrying out airborne gravimetry on a large-scale in China.(2)The paper analyzed the development of the existing airborne gravimetry system abroad,and introduced the basic principle of airborne gravimetry.And it was also presented the composition and characteristics of three aerial gravimetry systems.The definition and mutual transformation of common coordinate systems for airborne gravimetry were expounded.Based on the analysis of mathematical models,the impact of position error,velocity error,acceleration error and attitude error on measured results were discussed.Further the low pass filtering,acceleration correction,normal gravity correction,Eotvos correction,eccentricity correction,height reduction and cross point adjustment were also presented in the paper.Then the data preprocessing technique of airborne gravimetry was provided systematically.(3)Systematic calibration of GT-2A airborne gravity measurement system was carried out.Static measurement experiment and elevator experiment were designed.Zero drift rate,resolution and scale factor of gravity signal extraction results of GT-2A airborne gravity measurement system were calculated and analyzed.The results were compared with those of Burris relative gravimeter.The maximum difference of zero drift rate calculated by GT-2A continuous static observations can reach 7.4?Gal/h before and after solid tide correction,while the representative error achieved up to 13.7?Gal/h by applying calibrated data before and after the measurement.The results show that the solid tide had great influence on the determination of zero drift rate.The ability of GT-2A to observe gravitational tide was tested.It was found that the tidal wave with amplitude over 30?Gal/h will affect the amplitudefrequency characteristics of GT-2A measurements.So the resolution of GT-2A is about 30?Gal/h.The vertical gradient of gravity measured by GT-2A is obtained,which was also compared with that measured by relative gravimeter.The value of scale factor was-0.0034±0.0116 by using the observed data within the range of GT-2A.(4)The precision point positioning algorithm solved by fused normal equation based on equivalent parameter elimination was inferred,which perfected the continuity,availability and positioning efficiency of traditional GPS/GLONASS combined precise point positioning method.The innovated method was achieved by superposition of normal equations of GPS and GLONASS system to eliminate parameters.The results demonstrated that the accuracy of GPS/GLONASS integrated dynamic positioning was found to be 5.9mm,indicating that the integrated dynamic positioning system was more stable than single system.(5)In order to verify the accuracy of velocity and acceleration measurements solved by multi-stations differential mode and analyze the precision and reliability of velocity and acceleration measurements under passive mode,the strict relationship between the direct solution method of velocity and acceleration was deduced and established,realizing the solution of velocity and acceleration measurement by GPS/BDS combination.The function model of measuring velocity and acceleration of single and multi-base stations was derived.The static and kinematic tests were designed and method verification and accuracy assessment were also conducted.The results show that the acceleration accuracy of BDS is basically the same as that of GPS.The combination of GPS and BDS by introducing BDS system can effectively improve the accuracy of solution,and the BDS system can meet the needs of airborne gravity survey in China.(6)The scalar airborne gravimetry with single station was proposed.The VADASE method calculating the carrier velocity with high accuracy through the difference of phase observations between epochs was introduced into the application of airborne gravimetry.The dynamic observations of the carrier were obtained by applying single measurement mode without layouting ground base stations.Then the displacement sequences between epochs with high accuracy were first solved,and the carrier acceleration was obtained by applying Taylor's first order central difference.The influence of products of satellite orbit and satellite clock difference on acceleration estimation was emphatically analyzed.Gravity disturbance was solved by employing measured data of GT-2A Gravimeter.Finally,the internal and external precisions were evaluated by adopting discrepancies between measurements on different tracks and measured gravity values.The results showed that different satellite orbital products had little influence on acceleration estimation,while satellite clock difference sampling rate had great impact on acceleration estimation.Based on the GT-2A airborne gravimetry measurements in Shanxi,the gravity disturbance was solved by uniting acceleration calculated by single station,gravity and attitude data,which had good accordance with that computed by DGPS.(7)The airborne gravimetry test areas in three typical geomorphic environments,e.g.Qinling Mountains,Maowusu Desert and Qinghai-Tibet Plateau,were designed and the observed data were obtained.Gravity measurements points were achieved by ground gravity grid.The evaluation methods of internal coincidence accuracy considering the plane position correction and height reduction,and external coincidence accuracy given the observation error and aerial propagation error were put forward in this paper.Then the external coincidence accuracy of GT-2A airborne gravimetry system is systematically evaluated.Statistical results showed that the internal coincidence accuracy of airborne gravimetry was better than 1 m Gal and the external coincidence accuracy was better than1.6m Gal.(8)Experiments on quasi-geoid model based on airborne gravity data were carried out,and the latest airborne gravity measurements were used.A GNSS/leveling route of about 100 km per 1.5 km/node had been laid out in Maowusu Desert by applying the quasi-geoid model established on the measured airborne gravity data.Finally,the elevation anomalies were calculated by utilizing the high-precision GNSS/leveling measurements,and the contribution of airborne gravimetry data to the geoid construction was also evaluated.The experimental results showed that the accuracy of geoid recovered from airborne gravity data was much higher than that of existing models,indicating airborne gravity survey data was very effective,positive and contributed a lot to the recovery of geoid.Simultaneously,to solve the problems existing in engineering application of airborne gravimetry,a software module was developed to satisfy the requirements like data quality control,solution of carrier position velocity and acceleration and construction of geoid model by using airborne gravity data in large-scale airborne gravity survey. |
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