Design of a magnetometer using multiple COTS sensors for attitude control

Abstract

The Earth magnetic field is used in many applications such as navigation. Magnetic field sensors are mainly used on satellites for attitude control and in limited missions as scientific instruments. These magnetic field instruments are: specifically designed for the mission, expensive and/or require knowledge of the magnetic noise sources present in the complete satellite. Therefore a proposal was made of a magnetic field instrument consisting of multiple COTS sensors that doesn't require knowledge of the satellite and could be placed inside the satellite. Two measurement processing methods were used to test the effectiveness of such a system. Both methods were tested simultaneously testing if an ideal sensor placement exists and the effect of the number of sensors was considered.

The requirements for a sensor system to be used for attitude control and/or scientific purposes are as follows. Firstly for both kind of systems (attitude or scientific) a measuring frequency of at least 10 Hz is preferred. Secondly the sensitivity for an attitude control instrument needs to be 50 nT or less and for a scientific instrument 0.5 nT or less. Lastly the accuracy for an attitude control instrument needs to be 50 nT and/or 3 degrees or less and for a scientific instrument 0.5 nT or less.

The two measurement processing methods used in this research are the averaging method which averages the measurements of multiple sensors and the SOE method which uses a system of equations to determine the location, direction and magnetic strength of the noise sources. It was found that the SOE method can maintain an accuracy of less than 50 nT if a determined or overdetermined system of equation could be realized. This can only be the case if the number of sensors is $3rac{1}{3}$ times as much as the noise sources which is not feasible for a satellite. Another limitation of the SOE method is the computational time which is at least 7 order of magnitude bigger than the computational time of the averaging method. The averaging method can produce better accuracies than 50 nT and/or 3 degrees but is very dependent on the sensor placing and less dependent on the number of sensors. An increase in accuracy can only be gained if all sensors are placed far from the biggest noise sources and there should be a minimum distance from small noise sources. If one of the sensors doesn't adhere to this then there is a decrease in accuracy and a single sensor could perform better. The sensitivity of a sensor system improves when multiple sensors are used.