February 2020 – TD 266 OPERATING MANUAL – ZPulse® DCS 4420/4830/4520/4930 Page 11
CHAPTER 1 Short description and specifications of the DCS
1.1 Description
The AADI Aanderaa ZPulse
®
Doppler Current Sensor is a single point current sensor based on the Doppler Shift
principle.
To measure current speed and direction the sensor transmits acoustic pulses (pings) into the water and samples the
backscattered (echo) signal at specific distance away from the sensor. Complex acoustic pulses comprising two distinct
frequencies are combined into a single acoustic pulse which is sent out in two orthogonal directions at regular time
intervals. The ZPulse
®
based DCS separates the received signal into different frequency bands, one for each frequency
in the transmitted signal. Further it analyses the frequency shift using a high speed Digital Signal Processor. An ARMA
based parametric model processing algorithm is used to find the Doppler frequency.
It uses four acoustic transducers (beams) but only two are necessary to be able to calculate the 2D-current. The sensor
can be configured to use all four beams or only two or three if necessary. It can also be configured to only use the 2
transducer that is pointing towards the current. This is done to avoid turbulence that might occur behind the sensor.
The sensor includes a solid-state 3-axis magnetometer and inclinometer providing the tilting and compass heading of
the sensor. The tilt is necessary for correct horizontal placement of the cell. The compass also uses the tilt internally to
find the correct horizontal components of the earth magnetic field to be able to calculate the correct compass heading
from the earth magnetic field. The compass heading and tilt (pitch, roll) is measured for each acoustic ping and is used
for calculation of the current vectors (north, east) for each ping. At the end of the recording interval a vector average is
calculated to obtain the correct averaged horizontal speed and direction. The sensor can also output the horizontal north
and east speed components in addition to the horizontal speed and direction. The sensor also calculates and outputs
several quality data which can be used to identify any suspicious data (user enabled). The different quality data from the
sensor are described in details in this manual. To minimize the effect of marine fouling and local turbulence, the ZPulse
®
DCS is measuring the horizontal current at least 0.4 meter away from the instrument, refer .
In ZPulse
®
mode, a measurement interval comprising 150 pings (150 on each axis) will normally provide a good
estimate of the current speed. The number of pings can be reduced to lower the power consumption, but this also
increases the statistical noise (standard deviation).
The ZPulse
®
technology reduces statistical variance with a factor of
. This again reduces the required number of
Pings needed in order to achieve an acceptable statistical error.
The ZPulse
®
DCS 4420/4830 has POM Housing and is depth rated to 300m; All 4830 sensors and 4420 sensors has a
POM housing but titanium base plate. ZPulse
®
DCS 4520/4930 has a Titanium Housing and base plate and is depth
rated to 3000m or 6000m. The output formats of the 4420/4830/4520/4930 are AiCaP and RS-232, while the output
format of the 4420R/4830R/4520R/4930R are RS-422. The sensor can be used in polled mode when set to Smart
Sensor Terminal mode.
The ZPulse
®
DCS 4420/4830/4520/4930 fits directly onto the Top-end plate of the Aanderaa SeaGuard or SeaGuardII
data logger; the sensor is then used in AiCaP mode. When used with SmartGuard both AiCaP and RS-232 mode can
be used. The sensor may also be used as a stand-alone RS-232 sensor for other applications. The R-version cannot be
used on a SeaGuard platform. This sensor is for use with SmartGuard or as stand-alone RS-422 sensor on longer
cables. RS-422 is based on differential data transmission, which makes it more insensitive to noise than RS-232. RS-
422 was designed for greater distances and higher Baud rates than RS-232.
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