# The Use of Underwater Gliders as Acoustic Sensing Platforms
## 3 Self-Noise of the Petrel II Glider  
Before studying the underwater ambient noise and signal detection capability, the self-noise of the Petrel II glider needs to be analyzed. Although the glider has no propeller system, the mechanical structures of the glider for changing the buoyancy and the barycenter still lead to some self-noise. The self-noise mainly comes from two motors in the glider: the pump motor and the roll motor. The function of the pump motor is to change the buoyancy of the glider on the bottom of each dive.  
Figure 1: The “Petrel II” glider in the August 2018 experiment. Two SCHs are indicated by red circles, one at the front of the glider and the other at the tail.  
Figure 2: The location of the experiment in April 2018 is marked with a red circle on the map. Map data: © OpenStreetMap contributors, CC BY-SA [21].  
The roll motor will work when the glider deviates from the preset direction. It can control the heading by changing the glider's roll angle. A change of buoyancy also occurs on the surface so the glider can start diving, which is changed by a valve instead of a pump motor.  
Figure 3 shows the time and depth of different kinds of self-noise in several dives. The pump motor noise only appeared at the bottom of each dive, and the roll motor noise appeared randomly during each dive.  
To illustrate the characteristics of the self-noise in time and frequency, spectrograms of the ambient noise, roll motor noise, and the pump motor noise are displayed in Figure 4. These characteristics are measured during an ordinary dive. As shown in the figure, the ambient noise is relatively flat, the roll motor noise is constructed by broadband pulse signals, and the pump motor noise is constructed by multitone signals. The frequency spectrum of the roll motor noise and the pump motor noise are compared with the noise of no-motor time in Figure 5. The pump motor has the highest noise level and the main energy of the noise is concentrated at 3000-6500 Hz. The roll motor noise level is 10-20 dB higher than the self-noise of no-motor time. As it shows, when no motor works, the noise is quite close to the ambient noise at sea state 3 [22], which means the self-noise of the glider at no-motor time is negligible. However, both motor noises will interfere with the acoustic environment acquisition and signal detection ability of the system. To determine the influence of the motor noise on data acquisition, the proportion of motor occurrence time is estimated using all profile data from two sea trials. The results show that the no-motor time is more than 50% of the total dive time. Furthermore, through trade-offs between course-keeping ability and self-noise reduction, the proportion of no-motor time can be further improved. According to the above analyses, the glider has low self-noise, especially when no motor is working.  
In the following, the data when the motor noise occurred will be deleted, so that only motor noise-free data are used.  
Figure 3: The occurrences of different self-noise as a function of time and depth. The blue line indicates the dive profiles of the glider. The yellow dots and red asterisks correspond to the operation time and depth of the roll motor and pump motor, respectively. The roll motor works intermittently, while the pump motor only works in the bottom region of each dive. Measured by statistical data, the no-motor time is more than 50% of the total dive time.