Those interested in the Battle of the Atlantic fought during WWII against German submarines may have conjectured how the US Sound Surveillance System (SOSUS) would have performed against snorkel-equipped Type XXI submarines had SOSUS been operational during the war and had Germany been able to deploy Type XXI submarines in significant numbers. Acoustic data collected by Project BRIDGE in 1963 answers that question.

As extensively discussed by Olav Riste in Chapter 8 of THE NORWEGIAN INTELLIGENCE SERVICE, 1945-1970, Project BRIDGE was a Norwegian SOSUS site located on the island of Andoya. It became operational in 1963. Riste's book includes discussions of the performance by BRIDGE against Soviet submarines.

As part of the BRIDGE passive acoustic surveillance capability assessment against submarines operating in the northern Norwegian Sea, the Norwegian Navy provided the services of KNM KYA, a Type VIIC German diesel submarine (ex-U926) that had been snorkel-equipped.

Between 24 and 28 June 1963, the KYA participated in Exercise HUBRO (Owl) which involved snorkel-mode operations at increasing ranges from the BRIDGE hydrophone array out to a maximum distance of 320 nautical miles (nm). Between snorkel periods, the KYA transited on the surface to the next position.

Every snorkel period was detected by Project BRIDGE with an estimated signal to noise ratio of more five dB at maximum range. Assuming a conservative signal loss of five dB per doubling of range (4.1 dB was measured for a 3000 nm target in the Atlantic in 1962: target 27103), that value is consistent with detection of the snorkeling KYA by BRIDGE at ranges as great as 600 nm. Additionally, some of the intervening surface transits at speeds of 10-12 knots were detected by BRIDGE.

The KYA snorkel mode acoustic signature consisted of multiple harmonics of cavitation blade-rate produced by a single three-bladed propeller at about 270 rpm.

These data can be used to estimate how SOSUS would have performed against snorkel mode operations by German Type XXI submarines in the open Atlantic.

When the increased array gain of 6 dB for the early US SOSUS arrays compared with BRIDGE are considered�and consideration also is given to higher ambient noise conditions in the Atlantic versus the northern Norwegain Sea�the KYA could still have been detected at ranges in excess of 1000 nm while snorkeling in the Atlantic.

Since the Type VIIC and the Type XXI had similar acoustic vulnerabilities when on the surface or snorkeling (strong propeller cavitation), SOSUS detection ranges against snorkeling Type XXI units would also have been on the order of 1000 nm while detection ranges against Type XXI units conducting high-speed surface transits to close allied convoys would have been in excess of 500 nm.

Conclusion: had SOSUS been operational during WWII with the same detection capabilities as the first stations in 1954, the system would have achieved notable success in detecting and localizing German Type XXI units operating in the western Atlantic Basin, especially if there had been a SOSUS station on Bermuda.

The writer directed the Exercise HUBRO acoustic data collection effort at the BRIDGE site in June 1963, analyzed the data at the Norwegian Defense Research Establishment (NDRE) Maritime Systems Division Headquarters in Horten on the Oslo Fjord in July, and wrote the Exercise HUBRO final report for NDRE.

POST SCRIPT.

Over the following 36 years, the writer made 20 trips to the Andoya site and elsewhere in Norway for a total time in country of almost two years. Those trips provided the most professionally and personally rewarding experiences of my entire 50 plus year involvement with passive acoustics. As noted by Riste, Project BRIDGE revolutionized our understanding of the threat.

Best to all with whom I worked and especially to Ragnar Schaug-Pettersen and Inga Dahl who came to Louisville, Kentucky - of all improbable places - in April 2013.