The Numerical And Analytical Investigation Of Multiple Magnetometers Measurement Technique

The magnetic field generated by the spacecraft platform and instruments onboard it can be a strong artificial disturbance on in-situ measurements of the magnetic field.The wide-used approaches to reduce the effect are mainly including:application of a very long boom,magnetic cleanliness,circuit layout optimization and multiple magnetometers measurement technique.However,the long boom does not benefit for the attitude stability of spacecraft;magnetic cleanliness is a costly and complicated engineering task;the circuit layout optimization is too complex for spacecraft design.Multiple magnetometers measurement technique which is based on the gradients of magnetic field obtained by multiple magnetometers,can effectively remove the disturbances generated by the spacecraft from the magnetic field measurements,with low cost and relatively high accuracy,and thus are widely employed in missions for high precision detection of space magnetic field.At present,there is no systematic research or scheme optimization for this technology domestically.In this thesis,we will analyze the optimal method to mount the magnetometers radially along the boom,and verify the mounted scheme with this method for magnetometers onboard the Cluster spacecraft.Using two magnetometers to measure the in-situ magnetic field was proposed by Ness in 1971.It is one of the most widely used methods for in-situ magnetic field measurements.The method assumes an eccentric magnetic dipole model of the spacecraft field,and the gradient of the residual spacecraft magnetic field,can be obtained by the multiple magnetometers mounted in the radial direction along the boom,thus the dipole moments of spacecraft magnetic field is deduced,and then the ambient space magnetic field can be obtained by excluding this residual spacecraft field from the observed total field.According to the applicability constraint of the magnetic dipole model,the field generated by the spacecraft platform can be equivalent to an eccentric magnetic dipole,and then we study the optimal number of magnetometers and their corresponding positions on the boom of the spacecraft.Three cases are discussed:one magnetometer radially mounted on the boom,two magnetometers radially mounted on the boom,and three magnetometers radially mounted on the boom.In the second and third cases,according to the different positions of the inner magnetometer,the optimal mounted positions of the outer magnetometer are studied,also,the magnetic measurement errors under different mounted schemes are analyzed.The main results are as follows:for the cases of two magnetometers mounted in the radial direction of the boom,when the mounted position of the inner magnetometer is within the range of 2.2?5.5L,it can achieve high magnetic measurement accuracy on a short boom.As the mounted position of the inner magnetometer is within the range of 2.2?5.5L,each scheme can be obtained a minimum value of the measurement error of the space magnetic field,and the ratio of the relative distance between the inner and outer magnetometer corresponding to these minima and the mounted position of the inner magnetometer are all around 10.The mounted position parameters corresponding to the minimum error of space magnetic field measurement are the optimal mounted position for the two magnetometers in each scheme.In addition,we verify our mounted method by using the scheme and parameters of the multiple magnetometers mounted on the existing Cluster spacecraft boom.The results show that the magnetic measurement error obtained under the same mounted parameters according to our research method is 0.015 n T,however,the actual magnetic measurement accuracy of Cluster is 0.1 n T.In fact,the optimal mounted position parameters calculated with our research method can obtain a much smaller minimum value of the measurement error of the space magnetic field,whose value is about|?B|=3.4 10^-5n T,compared to the magnetic measurement accuracy of the actual Cluster spacecraft.The difference is due to that we have not taken into account the measurement accuracy of the magnetometer itself,the zero offset of the magnetometer and the subsystem's magnetic properties of subsystem in the spacecraft platform.