The VTMIS is normally used for surface vessel tracking but I see no reason why it can't be extended to sub-surface detection cum tracking particularly in the warning system. Afterall, buoys are also used for static sub detection.
http://www.kongsberg.com/en/KDS/KNCIT/Prod.../VTMIS5060.ashxKongsberg has a wide variety of sub-surface hydroacoustic detection and sonar products
http://www.km.kongsberg.com/ks/web/nokbg03...pdf?OpenElementhttp://www.km.kongsberg.com/ks/web/nokbg02...AA?OpenDocumenthttp://www.km.kongsberg.com/ks/web/nokbg02...2B?OpenDocumentKongsberg is also a leader in diver detection tech. Their white paper on the subject is downloadable from naval tech.
http://www.km.kongsberg.com/ks/web/nokbg02...AA?OpenDocumentThis was an interesting presentation on Singapore port security.
http://www.authorstream.com/Presentation/M...ppt-powerpoint/There is also an MOU between SG and Norway on maritime port R&D (check out MPA.gov.sg website).
There's also quite a lot of firms eg sonardyne, reson selling diver detection systems and setting up offices in SG.
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Separately, Mindef also colloborated with Stratech on its VIPS which the ISAPS module I think included underwater threat detection.
http://www.stratechsystems.com/iv_vips.aspSo it is very likely there is already a sub-surface detection and tracking net in SG.