Another example of interest is the detection of the acoustic pinger (e.g., Teledyne Benthos ELP-362D) from the flight data recorder on an aircraft that is lost over the ocean. The pinger transmits a series of 9 ms long pulses of sound at a frequency of 37.5 kHz with source strength of 160.5 dB at 1 m. At this frequency, the absorption is per km, which must be added to the geometric propagation loss. This would take a much larger lidar system, perhaps similar to that in Table 1, and a larger void fraction. The requirement for larger void fraction means that the system would need to scan the ocean surface for the large returns associated with bubble plumes. When it found one, it would need to track that position on the surface as long as possible to capture the ping. That time could be as long as a second. The result, assuming a plume with a void fraction of , is an SNR of 154 dB and detection to a range of 4 km. Background noise level at this frequency is generally determined by wind speed through breaking waves and bubbles. Typical recent values in a 100 Hz bandwidth are 53 dB45 to 63 dB42 for sea state six (wind 11 to ), well below the pinger signal level of 140.5 dB at 4 km. Aliasing of spatial variability at this frequency can also be neglected, since the spatial wavelength would be .