Research On Key Technology For Underwater Gravimetry Based On Inertial Navigation System

The gravity field of the earth is one of the basic physical fields as the geomagnetic field,geothermal field and electric field,which is able to reflect the movement,variation and distribution of the earth interior.Constructing a high-precision and high-resolution gravity field model by accurate measurement will not only benefit the development of geophysics,but also contribute to resource exploration,navigation and earthquake prediction,etc.However,conventional gravity measurement methods cannot generate dynamic accurate data of the underwater gravity field.In order to solve this problem,underwater continuous dynamic gravimetry is proposed and considered as one of the most important research interests to build precise global gravity field.This article is conducting studies on the related core technologies of underwater dynamic gravity measurement based on single-axis rotation inertial navigation system.Gravity measuring error model and multi-layer precise temperature control structure have been construced while SINS/LDV integrated navigation technology have also been studied in order to meet the accuracy requirement of gravity measurement.Based on this system,vehicle and marine gravity experiments have been carefully designed and carriered out to simulate and testify underwater dynamic gravity measurements.The main research results are summarized as follows:1.Based on the single-axis rotation inertial navigation system,the principles and errors analysis of dynamic gravity measurement have been pointed out.Besides,error characteristics of gravity sensor are discussed while accuracy requirments for inertial navigation system and aided position system have been proposed.Parameters of FIR lowpass filter are given while the gravity accuracy assessment method is presented.2.Multi-Layer precise temperature control method has been studied.Principles of inertial sensors,theoretical simulations on precise control system have been analyzed and temperature control accuracy have also been proposed.Based on the structural characteristcs of the single-axis rotation modulation inertial navigation system,temperature control schemes and physical designs are determined.At last,BP neural network simulation,static and dynamic experiments are carried out to validate the performance of control algorithm and the multi-layer temperature control system.Operation temperature variation of up-accelerometer can be limited to 0.01℃ while dynamic gravity internal RSME is limited to 1.4mGal.3.Hardward construction,data post-processing and error compensation are studied.Single-axis inertial navigation system is proposed to replace the strapdown inertial system as the core part of dynamic gravimeter to provide higher attitude and position results.Besides,Error resources of underwater dynamic gravimeter are discussed and horizontaldamp navigation method is adopted to solve the periodic error of the inertial system.In order to verify this dynamic gravimeter,vehicle experiments are conducted and mean Internal/External RSME accuracy are limited to 1.6mGal and 1.8mGal separately.Besides,marine experiments are also conducted and Internal RSME is 0.4986 mGal。At last,attitude information of the vessel is used as an extra method to assess the gravity quality.4.Considering of the feasures of underwater environment,marine and vehicle experiments and gravity measuring data are adopted to conduct Semi-physical simulations.With proper errors aided to the gravity data,performance of dynamic gravimeter is verified in condition of the simulated underwater environment.Besides,Laser Doppler velocimeter is adopted and tested in order to figure out extra positioning system which can operate in the underwater in addition to the GNSS system.According to experiments,the mean Internal/External RSME accuracy are limited to 1.8mGal and1.9mGal separatelyn.The experimental results reflected that the Laser Doppler velocimeter can achieve the approximately accuracy of gravity measurement.