The dual axis Doppler speed log system (DL2) works by the Doppler principle. This principle is that
a sound bouncing off a moving object will change in frequency. This principle can be utilized by
making a narrow beam of sound and analyzing the frequency of the returning sound. This frequency
change is proportional to the relative speed of the reecting object.
On a vessel, a transducer is positioned pointing slightly in one direction and the reected sound
comes from particles in the water (STW) and the bottom (SOG). The frequency of these echoes
is translated to speed. By having 2 transducers pointing in 90deg directions, the speed can be
measured in two axis.
The DL2 system sends from both beams at the same time. The transducers are moulded into the
same sensor head together with the ampliers and detection circuitry (transceiver).
The sensor also contains temperature sensors and tilt sensors to allow compensation of the data.
The transceiver contains a small computer which processes the data and signals and converts them
to speeds. This data is sent to the electronic unit (JB70D2-XX) where it is formatted and presented
on screen and web page, and as data in formats to be integrated into the vessels navigation and
presentation systems such as repeaters and conning.
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