DOPPLER SPEED LOGS
Doppler speed logs work on the principle of the Doppler effect, which is a shift in frequency between a transmitted signal and a received signal caused by the motion of a vessel over the sea bottom. A transducer broadcasts a continuous beam of sound vibrations at about a 60-degree angle from the keel. A second transducer receives the diffusely reflected signal returning from the seabed. Unlike the fathometer, which times the returning signal, the Doppler speed log registers the change in frequency between the transmitted signal and the received signal and then calculates the velocity of the vessel based on the amount of the frequency shift.
There are several differences between Doppler beams and fathometer beams. Doppler beams are continuous, narrower (about 3 degrees in width), and higher in frequency. In addition to the transducer set facing forward, there is a second transducer set facing aft. This is called a Janus configuration (named for the two-faced Greek god) and allows the system to calculate frequency shift in two directions thus insuring a more accurate speed measurement. The placing of the Janus configuration in a fore and aft direction is known as a single axis system and is used to calculate speed over ground in the forward and after direction. A dual axis system places a second grouping of Janus configured transducers in an athwartships direction allowing for the calculation of a vessel's speed when moving sideways through the water, as in docking. The beam width of the athwartship installation is about 8 degrees to account for the possibility of a vessel's rolling.
The Doppler system calculates speed to within an accuracy of about 0.5 percent of the distance traveled. It functions well for all speeds that modern vessels can attain and works from a minimum depth of about 1.5 feet to a maximum depth of about 600 feet. Frequencies employed are between 100 kHz and 600 kHz. There are primarily four errors to be aware of when using the Doppler system:
1. Transducer orientation error caused when the pitching or rolling of the vessel becomes excessive
2. Vessel motion error caused by excessive vibration of the vessel as it moves through the water
3. Velocity of sound errors due to changes in water temperature or density due to salinity and particle content
4. Signal loss errors caused by attenuation ofthe vibrations during transit through the water or upon reflection from the bottom
The Doppler system normally measures speed over ground to about 600 feet. Mter this depth signals may be returned by a dense, colder layer of water located throughout the oceans called the deep scattering layer (DSL). Signals received off the DSL are not as accurate as signals received from bottom reflections but can still be used to provide an indication of speed through the water instead of speed over ground when bottom tracking. Your unit may have a manual or automatic system which will switch from bottom tracking to water tracking at increased depth.
The Doppler system can be connected with other electronic navigation systems providing generally accurate speed input. The navigator should be cautioned, that precise speed should be determined not only by using the Doppler but also from careful calculations of distances between accurate navigational fixes.
