Modified Ambient Sea Noise Indicator with ambient noise directionality estimator

The modified Ambient Sea Noise Indicator (ASNI) ID-1872/A MOD which includes an Ambient Noise Directionally Estimator (ANODE) function, provides a comparison of the omnidirectional noise level and the summed noise levels of the two directional channels of a VLAD AN/SSQ-77 sonobuoy when in the ANODE mode. The omnidirectional ASNI absolute noise level can also be read for specific frequencies at the hydrophones of passive omnidirectional sonobuoys without automatic gain control when the modified unit is in the ASNI mode. Passive sonobuoys, located in the sea, will pick up sea noise with pressure-sensitive hydrophones. If the sonobuoy is an AN/SSQ-57, one omnidirectional hydrophone is used. If an AN/SSQ-77 sonobuoy is used, a vertical multielement array of hydrophones provides directional as well as omnidirectional information. The sea noise is transmitted to a receiver in the aircraft. The demodulated signal from the receiver is applied to the indicator. The ASNI and ANODE mode levels are indicated on the front panel meter, which is calibrated in decibels in a 1-Hz band relative to 1 microPascal sq for the omnidirectional noise level and in decibels for the directional channel to omnidirectional channel noise difference. The ambient sea noise levels may be used to plot a graph of ambient sea noise. The ANODE gain may be used to evaluate the effectiveness of the VLAD sonobuoy under various sea states.