A continuous reading of sound speed at transducer depth from a sensor mounted near the array. Flat-array multibeams need this value to steer each receive beam to its intended angle; if it is wrong, beams point the wrong way no matter how good the profile below is. The live value is also the best alarm for a stale profile, since a growing gap between the surface sensor and the last cast signals changing water.
Surface sound speed's reading 1489 and the profile says 1495 at the head, time for a new cast.
Also heard as
- SSV
- surface sound velocity
- head sound speed
Related terms
SVP
Sound velocity profile: how the speed of sound changes with depth through the water, measured to correct sonar depths.
Cast
One lowering of a sound speed or CTD probe through the water column to measure a profile.
Refraction
The bending of sonar beams as they cross water layers with different sound speeds, which distorts depths if uncorrected.
MBES
Multibeam echo sounder: a sonar that measures a fan of depths across the track on every ping, covering a wide swath.