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July 2019 – TD 304 OPERATING MANUAL – Doppler Current Profiler Sensor
The two methods have their own advantages:
Broadband
gives a lower single ping standard deviation which requires fewer ping (lower energy usage) to get
-
a good measurement result. The disadvantage in
when deployed in a bottom frame, the ambiguity is not a problem (ambiguity lock disabled). If it is moving and
the user knows that the current speeds are always below 1 m/s, enable the ambiguity lock.
If the sensor is moving (as under a buoy for example) and the current speeds are higher than 1m/s, it is
-
recommended to use Narrowband. Though
reached measuring range is slightly longer than in
Enable Ambiguity Lock
value. When the Doppler shift is zero then the phase is also zero. When the Doppler shifts increase, so will the phase. A
Doppler shifts of approximately 1.25 m/s along the beam corresponds to a phase equal to 360 (360 = 0) which is exactly
the same as for zero Doppler shifts. For this reason the cross correlation process is not able to distinguish a Doppler
shift of 1.5 from a Doppler shift of zero. In fact any Doppler shift outside the 1.25m/s range will be wrongly detected to
be within the range 0 – 1.25 m/s. This is called ambiguity and could hamper the correct operation of the instrument if not
corrected for. If ambiguity lock is enabled the result will stay in the first loop also between 0 and 1.25 m/s so if you know
the current will be below 1.25 m/s you can use
In case the ambiguity lock is not selected, several stages of ambiguity solving methods are automatically implemented
in the DCPS in order to achieve a non-ambiguous solution.
Ping Number
is the number of ping transmitted during one recording interval. One ping is a transmitted pulse from all
transducers. These four pulses are sent out simultaneously in four directions.
Enable Burst
Mode. The sensor can run in burst mode or spread mode. When burst mode is enabled the sensor
performs all ping measurements at the beginning or end of the recording interval, dependent on the Burst Period
Placement setting. This can be set to either
are evenly spread out during the recording interval. The instrument activates sleep mode between each measurement,
which reduces the power consumption. Power consumption in spread and burst mode is about the same.
2-25
Figure 2-25: Spread mode and burst mode ping distribution during the recording interval
is only used with Broadband. In
Ambiguity Lock
Start Of Interval
Broadband
is ambiguity. If the sensor is fixed, for example
Narrowband
is more power consuming than Broadband, the
Broadband
mode.
Broadband
the output of the correlation process is a phase
and avoid any error caused by ambiguity,
or
End Of
Interval. If it is disabled the ping measurements
Spread Mode
Burst Mode – End Of Interval
Burst Mode – Start Of Interval
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Refer Figure
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