## 1 Introduction  
Determining the position coordinates with high accuracy during the process of conducting marine underwater navigation is becoming a significant problem. This is mainly due to the fact that the Global Navigational Satellite System (GNSS) cannot be used. Sometimes using satellite navigation during emersion could be also difficult, especially in port areas, where infrastructure facilities and other vessels can block and reflect navigation signals [1, 2]. Due to these difficulties, it becomes necessary to use dedicated navigation systems (NS) in such cases [3, 4, 5, 6, 7, 8, 9, 10]. These dedicated navigation systems generally:  
* use devices to carry out navigational parameter measurements in water, such as log, hydrostatic pressure sensor, or echo-sounder,
* estimate the position coordinates as a result of the fusion of various navigational parameters, e.g., speed with course or submergence [11, 12, 13, 14, 15, 16, 17, 18, 19, 20].  
In the case of BAUV, a selection of devices and methods for determining the position can be made dependent on the parameters of its operation, shape, and size of the hull (construction), and type of propulsion used.  
A BAUV, imitating fish in terms of the design and movement, is currently being prepared at the Polish Naval Academy (PNA) (Figure 1).  
Starting from the bow to the tail fin, the BAUV consists of the following modules [9]:  
* module with camera and \"looking\" forward echo sounder (\"wet\" compartment),
* module of the sensors (in the upper part: USBL, hydro modem and \"looking\" up echo sounder; in the bottom part: sonar and \"looking\" down echo sounder--\"wet\" compartment),
* module of the lateral fins (\"dry\" compartment),
* module of electronics and batteries (the batteries located in the bottom part, which can move along the longitudinal axis of the vehicle giving the possibility of trimming; two computers PC-104 and power management system mounted in the upper part--the whole compartment is \"dry\"),
* module of the caudal fin consisting of two segments (the tail fin driven by an electric motor with nominal power 250 W, rotary motion converted into an oscillating motion).  
The range of swimming is designed for about 2 nautical miles and assumed optimum cruise speed is around 1 m/s (2 knots). The swimming range is an important operating parameter and it is a determining factor in the construction of the designed NS.  
Several NS variants have been designed for the conceptual stage of the BAUV. They differ from each other, mainly in applied navigation devices and in algorithms of the position coordinates estimation. The conceptual NS described in this publication consists of components installed on the BAUV:  
* GPS Aided Inertial Navigation System (GPS/INS) \"VN-200\" [21]--due to the operating environment, it is not possible to use GPS, therefore this subsystem is referred to as INS \"VN-200\" in subsequent parts of this article,
* \"ALIZE\" electro-magnetic log [22],
* \"WIKA S-20\" hydrostatic pressure sensor [23],
* USBL \"MicronNav System\" transponder system [24],
* \"MICRON DATA MODEM\" slave hydromodem [25],  
and components located on the seabed:  
* USBL \"MicronNav System\" transceiver system,
* \"MICRON DATA MODEM\" master.  
The USBL system will be dedicated to measure the approximate position coordinates (sending to the BAUV by hydromodem). The INS \"VN-200\" will measure the course, while the log will measure  
Figure 1: Side view of 3D model of the BAUV built in Polish Naval Academy (3D design by [PERSON]) [9].  
speed, and the pressure sensor will be used to submergence determination. These parameters will then be used to estimate submerged BAUV position by the extended Kalman filter. The BAUV moves using wave motion.  
The NS constructed in this way will be subject to a comparative evaluation of the dead reckoning NS (that uses only the course and speed measurement), and the NS based only on the USBL system. The basic criterion for this assessment is the accuracy of the estimated position coordinates.