Review on the Processing Methods of Sound Speed Errors in GNSS‐Acoustic Seafloor Positioning

Global navigation satellite system(GNSS)‐acoustic seafloor positioning is a technology proposed and developed for deformation monitoring of plates in the submarine subduction zone. As an important technology for the construction of marine space‐time reference network, it has broad application prospects. Although the current research results of GNSS‐acoustic seafloor positioning cannot meet the construction requirement of marine space‐time reference network, its data processing method, especially the fine process‍ing method of sound speed error, has important reference significance for the construction of seafloor part (seafloor geodetic ref‍erence station) of marine space‐time reference network.This paper firstly introduc‍es the origin of GNSS‐acoustic seafloor positioning and summarizes it into two categories: Static measurements and dynamic measurements. At the same time, the sound speed error processing method is used as the development line to refine three development stages of the technology: Only assuming the vertical stratification of ocean sound speed, considering the temporal variation of sound speed, and considering the horizontal gradi‍ent of sound speed.For the stage of only assuming vertical stratification of ocean sound speed, foreign scholars used geometric structure symmetry to weaken the influence of sound speed error; domestic scholars mainly studied the error sources besides the sound speed (lever arm offset, time tag deviation, attitude angle error, etc.) and used the stochastic model to weaken the influence of system error on the positioning results. For the stage of considering the time‐domain variation of sound speed, foreign scholars used fitting method (polynomial fitting or cubic spline fitting) combined with parameter smoothing constraint to solve the time‐domain variation of sound speed, and improve the stability of positioning results. Based on this, domestic scholars have refined the parameter fitting method (considering the variation characteristics of the long period term of parameters). In addition, domestic scholars have proposed the underwater differential positioning algorithm to innovate the seafloor positioning technology. For the stage of consid‍ering the sound speed horizontal gradient, scholars have solved the sound speed horizontal gradient parameters in the GNSS‐acoustic seafloor positioning processing, which improves the stability of the horizontal positioning results, and verified the reliability of the results by using the marine numerical model.This paper presents the prospect of high‐precision data processing methods of GNSS‐acoustic seafloor position‍ing for the seafloor reference station construction, and introduces the concept of small spatial‐temporal scale sound speed tomography (sound speed error processing method based on ocean positioning reference network) in order to solve the problem that numerical prediction model cannot provide small spatial‐temporal scale products, so as to provide high‐precision sound speed error correction services for underwater vehicles.