Application requirements: 
Long-term underwater operations require high-precision positioning (such as ocean surveying and mapping, resource exploration).

Plan composition: 

Core sensor: Fiber Optic Gyroscope (FOG) or Laser Gyroscope (RLG) + High Accuracy Accelerometer.

Combined navigation: Inertial navigation system (INS) is tightly coupled with Doppler meter (DVL), supplemented by periodic GPS correction (when surfaced).

algorithm:
Kalman filtering (such as EKF, UKF) combines multi-sensor data to suppress the accumulation of inertial navigation errors.