The use of “Self Propelled Semi-Submersibles” (SPSS), semi-submersibles that run with only a snorkel above water, and true submersibles by drug smuggling organizations represents a serious challenge to efforts to interdict drug trafficking. SPSS and snorkels are difficult radar and visual targets. They can be detected, but sweep widths are dramatically reduced. True submersibles are invisible to radar and only rarely visible from the air, when the water is clear and calm, and even then, only at relatively short distances.

Source: “Introduction to Naval Weapons Engineering”

There is a way to detect these vessels at ranges greater than typical radar ranges against even normal surface contacts. Passive Acoustic Towed Arrays routinely make detections beyond the radar horizon. Direct path detection ranges vary a lot. They are typically inside the radar horizon, but there are common phenomenon that make detection at extended ranges possible. These include the deep sound channel, bottom bounce, surface ducting and convergence zone (all explained here. You can skip the math and go down to “Propagation Paths” that starts about a third of the way down). These conditions frequently allow detection at well over 60 miles. These conditions are also recognizable and predictable. Passive acoustic detection is not as unambiguous as radar, in that passive sonar does not provide range, but it does give other information radar does not, that can aid in classification. In the past, success with passive systems has been highly dependent on the skill of the operator, but reportedly newer systems like the Multi-Function Towed Array (MFTA), planned for the Littoral Combat Ship, provide computer assistance that makes them much more intuitive. Range can sometimes be inferred from knowledge about the water conditions, and it can also be deduced by target motion analysis. Whatever information can be gleaned from the passive contact can be used to cue a helicopter to search a specific area.

Because this sensor is passive, it permits searches without betraying own ship’s position. It is not too hard to believe that drug runners who can build submarines are sophisticated enough to use Electronic Support Measures (ESM) to detect and identify Law Enforcement units  based on their radar emissions. A towed array gives the commander the option of searching a large area while going passive electronically.

It would be reasonable to expect that the Navy would pay for these systems, just as they did sonar systems in the past, so that they could be available for military operations. The Offshore Patrol Cutters should incorporate provision for this system in their design. (Previous related posts here, here, and here.)