In marine satellite navigation systems, optimizing ionospheric correction algorithms is crucial for reducing positioning errors. The ionosphere, a region in Earth's atmosphere filled with free electrons and ions, significantly impacts satellite signal propagation, causing signal path curvature and changes in propagation speed, thus leading to pseudorange measurement errors. This error is particularly pronounced in open marine environments, where the lack of ground reference points makes ionospheric disturbances a major source of error. Therefore, optimizing ionospheric correction algorithms can effectively improve the positioning accuracy of marine satellite navigation systems.
Dual-frequency observation technology is one of the core methods of ionospheric correction. Marine satellite navigation systems simultaneously receive satellite signals at two different frequencies, utilizing the difference in refraction of different frequencies by the ionosphere to construct a combined observation without ionospheric delay. This technology is based on the principle that the ionospheric refractive index is inversely proportional to the square of the signal frequency. Through the linear combination of the two frequency signals, it directly eliminates the first-order ionospheric delay, significantly reducing positioning errors. For high-precision marine positioning requirements, dual-frequency receivers have become standard equipment, and their correction effect is particularly outstanding in open sea areas.
The ionospheric grid correction method further improves correction accuracy by incorporating spatial electron content distribution data. This method first obtains ionospheric electron content grid data from the space center, then combines user location and satellite orbit information to calculate the geographical coordinates of the signal propagation path and the ionospheric penetration point. Using the inverse square distance method, it estimates the vertical delay value of the penetration point using ionospheric zenith delay data at the grid points, and finally calculates the additional ionospheric delay by combining the projection function and signal frequency. This method performs excellently in long-distance maritime positioning, effectively mitigating the impact of ionospheric spatial inhomogeneities on positioning.
The application of real-time ionospheric models provides marine satellite navigation systems with dynamic correction capabilities. Traditional ionospheric models are built based on historical observation data and struggle to reflect real-time changes in ionospheric state. Real-time models, however, dynamically update ionospheric delay parameters by receiving dual-frequency observation data from ground reference stations and broadcast the correction information to user receivers via satellite navigation messages. This technology enables marine users to obtain near-real-time ionospheric corrections, significantly improving dynamic positioning accuracy, especially during periods of ionospheric activity or peak solar activity.
Multi-frequency combination technology further taps into the potential of ionospheric correction by expanding the signal frequency range. In addition to traditional dual-frequency combinations, combinations of three or more frequencies can eliminate higher-order ionospheric delays, such as second- and third-order terms. This technology is particularly important for high-precision oceanographic surveys, such as marine geological exploration and seabed topography mapping, where positioning accuracy can be improved to the centimeter level. Simultaneously, multi-frequency combination can also enhance signal anti-interference capabilities and strengthen the system's stability in complex electromagnetic environments.
Regional ionospheric correction strategies are optimized for specific ocean regions. Ionospheric characteristics differ across sea areas; for example, the equatorial region has an anomalously active ionosphere, while high-latitude regions are significantly affected by the aurora. By constructing regional ionospheric models and combining them with local observational data, more accurate corrections can be achieved. For instance, in equatorial waters, the model can focus on correcting ionospheric scintillation effects; in polar waters, it can optimize the handling of aurora-related ionospheric disturbances. This regionalization strategy significantly improves the adaptability of marine satellite navigation systems across global ocean areas.
The development of comprehensive correction algorithms is driving the evolution of ionospheric correction towards intelligence. Modern marine satellite navigation systems construct a comprehensive correction framework by fusing multi-source data, such as dual-frequency observations, real-time models, and regional grids. The algorithm can dynamically adjust the correction strategy based on environmental conditions, such as prioritizing real-time model data or switching to dual-frequency combined correction when the model fails. This intelligent processing significantly improves system robustness, ensuring high-precision positioning capabilities are maintained even in complex marine environments.
