# The Use of Underwater Gliders as Acoustic Sensing Platforms
## 4 Application of Gliders to Underwater Acoustics
### Underwater Passive Surveillance
#### 4.2.3 Performance of Target Azimuth Estimation  
After the target has been detected, the target azimuth is another key parameter in underwater passive surveillance. The azimuth of the target tells the glider which direction the target is in, and location algorithms can be further implemented to locate and track the target [28; 29; 30]. To test the target azimuth estimation of the Petrel II glider, a source was installed on a ship to transmit various broadband signals every 320 s at 13 m depth. During the transmission, the Petrel II glider was set to travel northward, but, affected by ocean currents, it travelled westward at an average horizontal speed of 0.243 m/s. The maximum depth of the dive was about 500 m. Meanwhile, the ship drifted north at an average speed of 0.566 m/s. The M-sequence signal is chosen for estimating the azimuth in the paper. It has a frequency band of 4000-7000 Hz with a duration of 12 s. Figure 12 shows the spectrogram and the cross section of the received M-sequence signal. The received signal sound level is about 57 dB in the frequency band and the center frequency is 5.5 kHz. The signal DOAs are estimated through the time difference of arrival (TDOA) method [10; 12]. The TDOA \(\Delta T\) is estimated by calculating the cross-correlation between two SCHs. Due to the good autocorrelation of the broadband signals, the output of the cross-correlation has a sharp peak [31]. The DOA can be calculated based on the following equation:  
\[\alpha=\text{acos}(\Delta Tc_{w}/d), \tag{7}\]  
where \(d\) is the distance between two SCHs, \(c_{w}\) is the sound speed in the water, and \(\alpha\) is the DOA. Note that \(\alpha\) here is in the glider's \"body coordinates.\" The altitude of the glider is indispensable when converting the \(\alpha\) into global coordinates [12]. Every time a signal is received, the altitude of the Petrel II glider is read from the onboard sensor to calculate the azimuth of the received signal in the global coordinate. If not specifically mentioned, the DOAs in the paper have been converted to global coordinates. However, when the received SNR is low, the energy of the ambient noise in the low frequencies has a great impact on the estimated results. Thus, only the frequencies in the range of 4000-7000 Hz are utilized to calculate the final results. The original dead-reckoned locations have large deviations from the actual positions. To get more accurate results, the estimated positions of the glider are calibrated by the depth-averaged current,  
Figure 12: The spectrogram (**a**) and one example cross section (**b**) of the received broadband signal. The signal frequency band is 4-7 kHz. which can be estimated from the relationship between the dead-reckoned and the actual surface GPS position fix [2]. These positions can be more accurate if a computational model of the ambient currents is used such as the regional ocean modeling system (ROMS) [32], the finite volume coast and ocean model (FVCOM) [33], or the Copernicus marine environment monitoring service (CMEMs) [34]. The depth-averaged current does not reflect the specific information of currents at different depths. However, a computational model can provide vertical and horizontal currents at different depths. Since only two SCHs are mounted on the glider, the estimated azimuths suffer from port-starboard ambiguity. Figure 13 shows the diagram of ambiguity, where DOAs are symmetrical to each other with respect to the glider's heading. In this case, we cannot identify which DOA is formed by the target. In order to distinguish the true DOA, a simple scheme is used. If the glider changes its heading, the image DOA will also change accordingly, but the DOA formed by the true target will remain unchanged. Based on this feature, the true DOA can be easily distinguished. Figure 14 shows the estimated azimuth results of both sides during the experiment as a function of time. It is obvious that the portside DOAs are more stable. However, the starboard side DOAs shows a strong consistency with glider's heading.