The ability to detect, locate, fix and attack a submerged target became much easier for an aircraft with the invention of the sonobuoy. Basically, it is a hydrophone attached to a radio transmitter that can be dropped into the water, and will transmit it’s data up to the aircraft for analysis.
Aboard US Navy P-3 Orions, there are two basic sonobuoys. Active and passive. Active sonobuoys are those which emit a sound pulse into the water for echo ranging and location. It’s that pinging sound you hear in movies about submarines. In fact, the slang term for an active sonobuoy is a “pinger”. Initially, the pingers we used were the SSQ 40 series, which had a fixed ping width and active life of about 30 minutes. It maintained a constant ping for it’s life cycle and was good, but not great. This was eventually superceded by the SSQ-50 series CASS buoys. CASS stands for “Command Activated Sonobuoy System”. With the advent of CASS the aircraft could now drop an active buoy barrier along the projected route of the target, and have them power up and wait for commands to activate. Additionally, it allowed the AW’s to vary the pulse width of the ping, in order to remain useful as the submarine closed with the buoy. This was important, because with a standard ping pulse, there came a point where the returning echo was hidden by the outgoing ping, and the submarine could effectively hide in this “eclusion zone”. By shortening the ping width, that zone was almost eliminated, and more accurate fixing data could be generated, which in turn raide the PK (percentage of kill) ratio, always a good thing.
Passive buoys are buoys that activate and listen. They fall into two types: LOFAR, and DIFAR. These stand, respectively, for LOw Frequency Analysis and Ranging, & DIrectional Frequency Analysis and Ranging. These were mainly the SSQ-41 and SSQ-53 series of buoys, though there are other specialist ones within those codes and sub codes. The LOFAR buoys are simply wide-ranging data collectors. they hit the water, activate, and listen. The DIFAR buoys are LOFAR buoys with the addition of a compass that gives you a line of bearing to the sound source. This makes it easier to triangulate the general area of the contact, after which you can beginn to refine the date with smaller patterns, and eventually an active buoy pattern or barrier.
One last type which chould be mentioned is the SSQ-36 BT buoy. this is a Bathy Thermograph buoy. It provides a constant temperature reading from the surface to about 1800 feet depth. This is always the very first buoy a crew drops when it arrives onstation. The temperature data is used by the TACCO and AW’s to refine their speed of sound equations, which is then used to adjust the buoy spacing, depth, etc. It will tell the crew the location of the thermocline, below which subs usually operate. Speed of sound in water is generally figured at 4800fps at the surface, with a 72 degree water temperature. This is then modified by three factors: Temperature, Salinity, and Pressure, and the math equations are the “TeaSPoon formula” in the slang of the users. Accurately determining the speed of sound at the depth of the contact is critical to determining an accurate range from the buoy to the contact, for fixing the target prior to attack. Think of it as determining windage, air temp, etc, for a marksman.
Generally, the sonobuoy is powered by a salt-water activated battery which activates almost immediately upon contact with the water. Both the life of the buoy and the depth of the hydrophone can be pre-set prior to dropping. Buoy life is determined by a series of plugs in the side of the buoy. These corrode in water at a fixed rate, so prior to drop, the ordnanceman selects the number of plugs correlating to the time desired. When they corrode, the buoy fills with water and sinks. The only drawback to the system is that, in the P-3c Orion and later variants which use an external chute system, these must be determined and set prior to take off. Those carried internally may still be set as required prior to deployment.
Anyway, more sonobuoy goodness here