Low Frequency Analyzer and Recorder

Originally the analysis was electromechanical and the display was produced on electrostatic recording paper, a Lofargram, with stronger frequencies presented as lines against background noise.

[1][2] As a result, the Navy formed a study group designated Project Hartwell under Massachusetts Institute of Technology (MIT) leadership.

[3][4] One recommendation was a system to monitor low-frequency sound in the SOFAR channel using multiple listening sites equipped with hydrophones and a processing facility that could calculate submarine positions over hundreds of miles.

When representing frequencies generated by propeller blades or machinery those could form a submarine or surface ship signature that could be recognized and used to locate and identify the source.

With regard to vessels that could be speed or other changes, including a Doppler shift indicating direction changes, having an effect of frequencies received.

The shore stations where the operational array and cable, composing a surveillance sonar set, terminated, were given the generic and non revealing term Naval Facilitity (NAVFAC).

"[11] Jezebel and LOFAR branched into the localization of submarines with the AN/SSQ-28 passive omnidirectional Jezebel-LOFAR sonobuoy introduced in 1956 for use by the air antisubmarine forces.

This, and later specialized, sonobuoys equipped with a small explosive charge could be used in an active mode to detect the echo off the target.