| The technology of inertial navigation is a technology of system synthesis, which is applied to fix the attitude parameters and position parameters of a moving object, and is deemed as the optimal method for a moving object to control and measure independently. While an accurate positioning of a standard of reference is the primary requirement for an inertial navigation system, north seeker is exactly an instrument in common use for reference positioning.Gyroscope is the key part in a north seeker system as a rotational speed sensor, whose performance directly and decisively orders the precision of north seeker. Compared with those traditional mechanical gyroscopes, fiber optic gyroscope (FOG), as a new generation of rotational speed sensor, possesses significant advantages such as solid-state structure, satisfying reliability, high start rapidity, wide dynamic range, etc., hence is particularly adaptive in north seeker system in small volume but with higher performance and more functions. In the respects of precision and anti-jamming the digital closed loop FOG makes its advancement over analog open loop FOG and this accounts for the application of a digital closed loop FOG, which is called FOG-2B, in our experiment.A set of fine and reasonable way of signal processing also is of key importance to improve the performance of north seeker. The north seeking arithmetic which is based on Vector Modulating Method (VMM) is mainly composed of two parts, the modulation of earth rotational speed and the calculation of the initial phase of modulated signal, namely the north angle, with arithmetic of correlation filtering.The advantage of this north seeker realized by such arithmetic is that noises are well separated from useful the earth's rotational speed and faster north seeking. The accuracy and rapidity of north seeker are mainly rest with the accuracy and dynamic range of FOG, the accuracy of revolving table and its levelness, the accuracy of sampling time and the precision of positioning start point. Based on this thread we conduct this theory as follows:Firstly, considering that Windows operation system is not a real-time system, we resort to DOS, which is with a higher real-time performance, as the system backup and compile sampling and calculating program with Turbo C for the FOG-2B based north seeker model system. This model system can work out north angle in less than 20 seconds with the precision of 1 degree, and the replicability is improved as well.Secondly, after discussing different factors that may well generate error in the system and making a reverse simulation to further prove their possible undesired influences, we apply a revolving table with a higher accuracy of rotational speed ( degree per second), a quartz crystal at the accuracy of microsecond as timer, and a new FOG with weaker noise in low frequency and with wider dynamic range, to improve this model system. After all these improvements, this model north seeker can work out north angle within 10 seconds at the precision of 0.15 degree.At last, considering the strict requirement for hardware by the time-domain analysis based earth rotational speed modulation, we get rid of the restraint from accuracy requirement for rotating speed and sampling time and introduce a new modulation method which only requires equiangular sampling. After adding a 2-second smoothing process in each sampling angle position, we can improve the FOG-2B based north seeker's precision to 0.22 degree in 288 seconds. From the results of this improvement we are ascertained with the fact that equiangular sampling can be adopted in earth's rotational speed modulation, too. Assuming if the gyroscope can sense the earth's rotational speed without a long-time smoothing, then we can use a position sensor, a photoelectric coded disk for example, to realize a successive equiangular sampling. In this way the symmetry of signal and rapidity of north seeking can be preserved without the necessity of highly accurate hardware and this should be a more feasible choice. |
