# The Use of Underwater Gliders as Acoustic Sensing Platforms
## 4 Application of Gliders to Underwater Acoustics
### Underwater Passive Surveillance
#### 4.2.1 Performance of Signal Receiving  
The broadband and multitone signals are used to estimate the transmission loss to illustrate the advantages of the glider in underwater target detection over other autonomous underwater vehicles. Those signals were transmitted by a source installed on the ship, with a frequency ranging from 4000 to 7000 Hz. During the transmission, the ship was continuously drifting on the surface with a speed of around 1 kt. Figure 7 shows the spectrum of the received multitone signal at a certain time, and the frequency range of the signal is 4500-6500 Hz with an interval of 500 Hz. The sound level of the received signal is around 88 dB, so the transmission loss can be estimated by a sonar equation [22]. As shown in Figure 7, the received signal-to-noise ratio (SNR) is higher than 35 dB, which makes it readily detected by the glider. Figure 6: The raw data (**a**) and the ambient noises (**b**) of individual frequencies from a single profile in April 2018. To obtain the relationship between the transmission loss (TL) and the distance, receptions at different locations were processed. The TL was estimated based on the following sonar equation [22]:  
\[\mathrm{TL}=\mathrm{SL}-\mathrm{NL}+\mathrm{DI}-\mathrm{DT}, \tag{1}\]  
where SL is the source level, which is known. NL is the noise level, which is estimated using the ambient noise. DI is the receiving directivity index. Since the omnidirectional SCH is used to calculate the results, DI is 0. DT is the received SNR, which can be estimated with NL and the received signal level. The left-hand panel in Figure 8 is the estimated TL as a function of the range at different frequencies. The theoretical TLs of the cylindrical spreading and spherical spreading with absorption are also compared with the estimated results [22]. As shown in the figure, the trend of TL increases with the distance: the TL of most frequencies is larger than the TL with cylindrical spreading but lower than TL with spherical spreading, and the largest TL is about 74 dB at 5.5 km. Because of the low noise of the glider, two major factors that affect the ability of target detection are TL and the ambient noise level. However, the detection performance of other AUVs with propeller system will be affected by their self-noise. AUVs are also widely used in underwater surveys [25]. The typical self-noise of AUVs is about 83-116 dB at a frequency range of 4000-7000 Hz [26], which means that the self-noise of the AUV is about 30 dB higher than the ambient noise. However, as for the  
Figure 8: The estimated transmission losses of different frequencies at different ranges (**a**) using both broadband and multitone signals, and the simulated detection probability (**b**). The dashed line represents the theoretical transmission loss of cylindrical spreading and the dotted line represents the spherical spreading. The red plus sign and blue triangle in the right-hand panel represent the detection probability of the glider and the simulated AUV with self-noise of 83 dB, respectively. Figure 7: The spectrum of the received multitone signal. The frequency range of the transmitted signal is 4500–6500 Hz. glider, when there is no motor working, it can receive a signal with the SNR higher than 0 dB as long as the sum of TL and noise level is lower than the source level. Therefore, the SNR of signals received by the AUVs is much lower than that of the glider under the experimental conditions in the paper. To further expound the advantages of the glider in underwater detection, a simulation is carried out based on the estimated TL from the experiment. In the simulation, the self-noise of the AUV is 83 dB, which is the lowest value in [26]. The source level is set to 130 dB, and the ambient noise level is set to 50 dB, which is a typical value in the frequency band of 4000-7000 Hz, as Figure 6 shows. Both the glider and the AUV are mounted with a single hydrophone, and the signal energy [24; 27] is used to detect the source.