Page 45 - Electronic Navigation Cyber Book

For measuring the aftwart ship speed, a similar Janus configuration is
mounted at an angle of 90 deg. with the along ships transducers;

The distance from the bridge of a large tanker to the bows may be 250
meters, so special information about the athwart ships speed both fore
and aft is required when mooring.
In that case athwart ships transmitting and receiving transducers are
mounted both fore and aft.

Janus configuration. A term describing orientations of the beams of
acoustic or electromagnetic energy employed with Doppler navigation
systems.
The Janus configuration normally used with Doppler sonar speed logs,
and docking aids employs four beams of ultrasonic energy, displaced
laterally 90° from each other and each directed obliquely (30° from the
vertical) from the ship’s bottom. This is to obtain true ground speed in the
fore and aft and athwart ship directions.

These speeds are measured as Doppler frequency shifts in the reflected
beams. Certain errors in data extracted from one beam tend to cancel
the errors associated with the opposite directed beam.

Pitching and rolling

From the figure we see that the speed for the dotted position of the ship,
and for the forward-directed beam increases to V1’; for the backward-
directed beam V1decreases to V1’’.
© 2018 Digital Galaxy Index 45