Navigation Solution for Marine Applications using MEMS-based Sensors and GPS

Eskildsen, Bjørn

10. term

Elektronik og IT, Kandidatuddannelsen (Spec. Intelligent Autonomous Systems)

2012

2012-02-28

110 pages

This thesis concerns the estimation of the position, velocity and attitude using inertial sensors, magnetometer and GPS. The purpose is to apply a model based estimation filter to fuse these sensor inputs together to form estimations of the states of interest. This thesis has been made in collaboration with the company: CDL - inertial engineering. The main hardware platform is a MiniSense2 (MS2), produced by CDL. The sensors on the MS2 platform consist of a 3-axis gyroscope, a 3-axis accelerometer and a 3-axis magnetometer. These are all low-price MEMS based sensors. In this thesis, it is chosen to base the design of the navigation solution on a loosely coupling between the sensors of the MiniSense2 and GPS. The estimation filter utilized in this is based on the structure of the extended Kalman filter. Differential equations regarding motion on the rotating earth are derived together with models of the sensors, which includes the assumption of additive biases of the MS2 sensors. The thesis also concerns models of the gravitational field and magnetic field of the earth. As the extended Kalman filter is chosen as basis for the filter design, a linear small state perturbation model is also addressed. Two tests have been conducted for evaluation of the designed navigation solution. The first test was based on a designed simulation environment, as this has the advantage of having the true values of the system states for reference. The simulation environment was based on a point mass which is affected by forces and momentums in such a way that it executes oscillations comparable to the ones of a ship at sea. To simulate the behavior of sensors, noise and biases were added to the state outputs. The second test is based on real world data which originate from a boat trip at sea. Along with the data from a MS2 and a GPS, attitude estimates from a high-grade Attitude and Heading Determination System (AHDS) are available for reference. In both tests, simulated GPS outages have been included to evaluate the handling of this scenario. In the test on the simulated data, the navigation solution was able to correctly estimate all the sensor biases. The performance during GPS coverage and outage were both acceptable. This led to the conclusion that the basic concepts of the designed filter work. In the test on the real world data, the designed navigation solution did not have an acceptable performance even though it was seemingly able to estimate the biases of the gyroscope. The estimates of the magnetometer bias indicated that a non-included effect were present in the magnetometer measurements. It was suggested that a non-included lever between the GPS and MS2 and a ”soft iron” magnetometer effect could be the causes of the bad test results.

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