## 3 Sensor Upgrade
### The UPDAL Underwater Passive Aquatic Listener  
UPAL is an autonomous innovative instrument specifically conceived to collect and process underwater ambient noise data in real-time on any ocean mooring. A real-time embedded operating and signal processing software runs on its microprocessor to detect and to classify sources into geophysical (wind speed, rain rates and type, sea roughness), anthropogenic (ships, sonars, etc.) and biological (marine mammals, etc.). Quantitative estimation is also provided for wind and rain [6].  
The instrument is based on cutting-edge hardware and software engineering technologies consisting of a high-sensitive low-power broadband hydrophone with linear working frequencies down to 5 Hz and up to 90 kHz [30]. The nominal sensitivity of the used hydrophone is \(-\)160 to \(-\)155 dB relative to 1 V/\(\upmu\)Pa and the equivalent oceanic background noise level of the pre-amplifier system is about 28 dB relative to 1 \(\upmu\)Pa\({}^{2}\)Hz\({}^{-1}\). The intrinsic noise of the frequency response is greater than 10 dB below Sea State \"0\" (dB rel. 1 \(\upmu\)Pa/\(\surd\)Hz) and for the used frequencies the values are 21.6, 17.3, 15.5, 13.3, 12.4, 11.3, 10.5 dB rel. 1 \(\upmu\)Pa/\(\surd\)Hz at the 2, 5, 8, 15, 20, 30, 40 kHz, respectively.  
With respect to previous versions, the system has new low-consumption active filters with better cut-off frequencies, high internal data storage components (up to 128 Gb), higher capacitance batteries (>65 Ah) and the versatility to be connected to any external power station. The innovative design is based on new low power, low noise, high bandwidth, wide dynamic range combined microcomputer/microprocessor and analogue-to-digital converter (ADC), based on Field Programmable Gate Array (FPGA) technology capable of accommodating the firmware and the smart processing algorithm.  
One of the most innovative features of UPDAL is its capability to autonomously adapt the duty cycle (sleeping and listening periods) depending on the source of the acoustic signal and mission requirements. When active, UPDAL performs two tasks: the first is devoted to data collection for a specified time interval during which the instrument samples the audio output of the hydrophone at 100 kHz for a time slot of 4.5 s, and performs a Fast Fourier Transform (FFT) using the onboard FPGA to obtain a frequency spectrum of 512 sound pressure levels (SPL) over the entire frequency range of 0.5-50 kHz. The second task averages over a 1 kHz bandwidth eight discrete frequencies (1, 2, 5, 8, 15, 20, 30, and 40 kHz) of the spectrum and applies a tree model approach using these SPLs to classify the acoustic source. Each different detected source is associated to a flag (a specific code for each detected sources of the noise). The basic assumption is that geophysical generated sounds from rain, drizzle or wind are generally stationary even over a short time interval, whereas banging from ships or moorings, or chirps, whistles or clicks are sound signals that usually are non-stationary over a certain time interval.  
The aforementioned evaluation process is used to set the listening (wake up/sleeping) modes of the system saving energy between data collection sequences. This makes the device suitable also to be used as trigger for several monitoring applications such as the groundwater emission from the sea bottom [9].  
Once the noise source is identified as wind or rain, the SPLs at specific frequencies can be used to estimate wind speed or rain rate, respectively [6]. At the end of the duty cycle, the system creates a \"message\" including the values of the SPLs used for the classification, the flags and the quantitative estimates resultant from the classification and quantification algorithms, respectively. The message is made available to any external data logger through RS232 or USB interfaces for near real-time transmission ashorse.