# The Use of Underwater Gliders as Acoustic Sensing Platforms
## 5 Conclusions  
This paper illustrates the application of the Petrel II glider to underwater acoustics by sea trials. First, the self-noise of the glider was studied to show the ability of the glider to serve as an underwater acoustic sensing platform. Both the spectrogram and spectrum of different kinds of self-noise were displayed and compared with the ambient noise. The results showed the quietness of the Petrel II glider. Then, the applications of gliders in underwater acoustics were analyzed from two aspects: ocean ambient noise acquisition and underwater passive surveillance. Since the self-noise of the glider appeared intermittently, the ambient noise could be recorded during the no-motor noise time. By estimating the transmission loss using transmitted signals, the signal receiving ability of the Petrel II glider was discussed. Meanwhile, a simulation of target detection probability was carried out, and the results indicated that the glider had a great advantage over other underwater vehicles with propeller systems. Moreover, in order to reduce the influence of self-noise, an adaptive line enhancement algorithm was introduced and tested using experimental data.  
Because the glider had two SCHs at both ends, the target azimuth estimation ability was analyzed using the broadband signals transmitted by a drifting ship, and a simple scheme to distinguish the true DOA from the port-starboard ambiguity was illustrated. Two SCHs formed a simple linear  
Figure 15: The sea trial scenario with the estimate DOAs (**a**) and the local enlargement of the black dotted bordered rectangle (**b**). The black line is the path of the glider, and the blue circles are the positions of each signal reception. The yellow line with dots is the path of the ship, and the drift direction of the ship is also marked. The glider started diving from the square and ended at the red asterisk. The red and blue arrows represent the estimated DOAs in the port and starboard sides of the glider, respectively. The short green arrows represent the headings of the glider at each signal reception time. The glider was set to travel southward, but due to the influence of ocean currents, the heading of the glider is not consistent with the travel direction.  
array with a large aperture, through which the glider could locate an unknown target. The array also had lower energy consumption compared with other linear arrays with more elements. Moreover, this installation reduced the impact of the self-noise because the two SCHs were as far away from the glider as possible. Furthermore, the glider can improve the signal receiving ability through array signal processing technology such as beamforming. To make the scheme more reliable, the law of heading changes should be designed carefully, and the balance of energy consumption and azimuth estimation ability needs to be considered.  
Conceptualization, C.J., J.L. and W.X.; data curation, C.J. and J.L.; methodology, C.J., J.L. and W.X.; software, C.J.; validation, C.J., J.L. and W.X.; investigation, C.J., J.L. and W.X.; writing--original draft preparation, C.J.; writing--review and editing, J.L. and W.X.; funding acquisition, J.L.  
This research was funded by the National Key Research and Development Program of China (Grant No. 2017 YFC0305905).  
The authors would especially like to thank [PERSON], [PERSON], and other members of Petrel II team from Tianjin University.  
The authors declare no conflict of interest.