A Role of the Celestial Navigation in the Future

This paper discusses the current and future role of celestial navigation and its application on board ships and in maritime educational institutions. Accordingly, the question is whether the governing bodies of the maritime industry should maintain their position unchanged or whether they should increase or decrease the importance of celestial navigation as part of the competencies of future officers. Celestial navigation, its evolution and the various methods of application, the possibilities of implementing future technologies, have all been the subject of many papers, but still no clear answer in which direction to go and what we can expect in years to come. The paper will systematically review and analyze existing legislation and recommendations, especially STCW (Standards of Training Certification and Watchkeeping and IMO (International Maritime Organization) acts. Also, the paper will provide an overview of various papers on this topic and the existing most used methods. An overview of what is considered a modern solution is also provided. The focus is on the advantages and disadvantages of celestial navigation in the context of the future environment in the maritime industry.

Navigation technology is indispensable for both animal and human. Navigation sensor plays an important role in emergency rescue, precision-guided weapons, ship exploration, aircraft navigation, and satellites positioning system. To reach nests or favorable grounds, many insects have evolved navigational ability that can obtain the solar azimuth by detecting the polarized skylight. This polarized pattern in the sky arises, due to the sunlight scattering in the atmosphere. In the single-scattering Rayleigh atmosphere, the direction of polarization is perpendicular to the scattering plane determined by the observer, the observed celestial point and the sun. Some insects can finish long-distance migration to survive in the seasonal changing environments. Inspired by the amazing capacity, kinds of sensors have been developed depending on the structure of the insects’compound eyes and the insects’ visual mechanism. Integrated polarization dependent sensors have the features of compact structure, high precision, strong robustness, and simple fabrication process. This kind of sensor is composed of a complementary-metal-oxide-semiconductor sensor integrating with a multiorientation nanowire grid polarizer, and also the working principle of the sensor is described. The astronomy navigation technology based on polarized skylight has already become the innovation and research focus, even though the navigational mechanism of insects is still not very clear. In the meantime, the miniaturization and integration are new directions for the future research of navigation sensors. Additionally, to prevent navigational errors, it is necessary to calibrate orientation compass. Modern navigation technologies such as GPS have made themselves straightforward for mankind to locate global positions. These techniques are classified as satellite navigation, radio navigation, inertia navigation or celestial navigation. Unfortunately each of these techniques has their own weakness. For example, satellite navigation has the possibility of jamming and satellite signal loss. Radio navigation cannot work without base stations. Inertia navigation needs to correct its peculiar accumulated errors. And celestial navigation is complex and very expensive. The existing researches show that the astronomy navigation technology is real-time and autonomous and can work without accumulation of errors. Polarized skylight is one of the multitude of sensory information used by animals, thus there is much future work waiting for researchers to develop.

Spacecraft's attitude information plays an important role in celestial navigation. The attitude is mainly determined by matching the star's centroid in the obtained image with its corresponding information in star catalog. Generally, the star image can be regarded as a spot with a diameter <5 pixels. Therefore, it is very difficult to extract the star centroid with sub-pixel accuracy, especially in the hardware system, such as FPGAs. The existing spot centroid extraction methods with high accuracy require plenty of pixels to realize the complex computations. Limited to the star's diameter and hardware requirements, such methods are not suitable for star centroid extraction in hardware system. To solve the problem, a two-step extraction method for star centroid with sub-pixel accuracy is proposed. The maximum pixel-level center can be located through zero crossing of the first derivative in a small region. Taking the pixel-level center as the middle of the window with fixed size, the sub-pixel offsets to the sub-pixel center can be calculated using fixed window weighted centroid method. The sub-pixel center of the star is then obtained by adding the offsets to the pixel-level center. This method can be implemented in hardware to increase processing speed, using Verilog hardware description languages. A simulation is performed on computer and FPGA. Experimental results show the excellent performance in accuracy and processing speed of two-step method. In addition, two-step method has strong ability of resisting noise and good robustness compared to other methods.

Star pattern recognition plays an important role in celestial navigation. Aiming at the shortcomings of the traditional star map recognition algorithm such as large storage space and high recognition error rate, an improved star pattern recognition navigation algorithm is proposed. Firstly, the navigation star database is constructed based on the idea of star angular distance. Then a new companion is introduced to eliminate the mismatched results and reduce the mismatched rate. Finally, the position information of the spacecraft is solved by the star position information in the field of view of the star sensor. Simulation results demonstrate the effectiveness and high accuracy of the proposed navigation method.

The star’s centroid plays a vital role in celestial navigation, star images which be gotten during daytime, due to the strong sky background, have a low SNR, and the star objectives are nearly submerged in the background, takes a great trouble to the centroid localization. Traditional methods, such as a moment method, weighted centroid calculation method is simple but has a big error, especially in the condition of a low SNR. Gaussian method has a high positioning accuracy, but the computational complexity. Analysis of the energy distribution in star image, a location method for star target centroids based on multi-step minimum energy difference is proposed. This method uses the linear superposition to narrow the centroid area, in the certain narrow area uses a certain number of interpolation to pixels for the pixels’ segmentation, and then using the symmetry of the stellar energy distribution, tentatively to get the centroid position: assume that the current pixel is the star centroid position, and then calculates and gets the difference of the sum of the energy which in the symmetric direction(in this paper we take the two directions of transverse and longitudinal) and the equal step length(which can be decided through different conditions, the paper takes 9 as the step length) of the current pixel, and obtain the centroid position in this direction when the minimum difference appears, and so do the other directions, then the validation comparison of simulated star images, and compare with several traditional methods, experiments shows that the positioning accuracy of the method up to 0.001 pixel, has good effect to calculate the centroid of low SNR conditions; at the same time, uses this method on a star map which got at the fixed observation site during daytime in near-infrared band, compare the results of the paper’s method with the position messages which were known of the star, it shows that :the multi-step minimum energy difference method achieves a better effect.