Fig. 1. Schematic of least squares circle fitting principle

Fig. 2. Fitting results of circle fitting algorithm with interference

Fig. 3. Improved algorithm flow chart

Fig. 4. Improved algorithm fitting results

Fig. 5. Underwater light spot collection experimental device

Fig. 6. Underwater laser spot and three-dimensional pseudo-color image. (a) (b) Without background light; (c) (d) with background light

Fig. 7. Image preprocessing process

Fig. 8. Image preprocessing results. (a)‒(e) Without background light; (f)‒(j) with background light

Fig. 9. Center coordinate positioning results of different algorithms. (a) x coordinate; (b) y coordinate

Fig. 10. Underwater laser spot and pseudo-color image with interference at different locations. (a)(b) Left interference; (c)(d) right interference; (e)(f) upper interference; (g)(h) lower interference; (i)(j) far neighbor interference; (k)(l) neighbor interference

Fig. 11. Center coordinates of positioning by three algorithms. (a) x coordinate; (b) y coordinate

Fig. 12. Improved algorithm positioning error. (a) x coordinate error; (b) y coordinate error