There are two main types of techniques used by Nortek's profiling ADCPs. These two techniques are generally referred to as Narrowband and Broadband.
Overview
Narrowband gets its name from the bandwidth of the acoustic signal normally used in these systems. Narrowband systems operate by sending a single pulse of acoustic energy, which is relatively long (up to a few meters), and then listening to the echo these pulses generate as they bounce off particles in the water column. The frequency of the pulse is known when it leaves the transducer and its echo’s frequency is measured upon return. The difference between the transmit frequency and the return frequency is the Doppler shift, and it is proportional to the along-beam velocity of the water.
Broadband systems operate by sending two pulses of sound and listening to their echoes. A complex function is then used to calculate the phase difference between these pulses. This phase difference is proportional to the speed of the water. As they are often in the water at the same time, they are coded in order for the electronics to be able to differentiate between the two.
The different processing techniques have implications for how a system operates as they pertain to profiling range, maximum measurable velocity, short-term uncertainty (precision), power consumption, ease of operation and data output rate. These different factors will be discussed next.
Profiling range
Although profiling range is primarily a consequence of the instrument’s transducer frequency, transducer size (diameter) and amount of power used to transmit the acoustic pulse, it is also related the measurement technique used. For any given frequency and similar transducer size, Narrowband will generate a maximum profiling range about 20–30% greater than a Broadband system.
Maximum measurable velocity
Similarly to profiling range, the maximum measurable velocity that a Narrowband system can detect is greater than for either a Broadband system. This is due primarily to the fact that the Narrowband technique relies on a direct Doppler shift measurement, and is therefore not limited to restrictions between transmit and receive timing that are applicable to techniques measuring phase differences between pulse pairs.
Precision
For acoustic Doppler systems, precision refers to the short-term (typically 1 second) uncertainty in a measurement. Of the two techniques, Broadband offers the better precision. Narrowband systems have generally low precision and as a consequence must average data longer than either a Broadband system.
Power consumption
Power consumption is directly related to the amount of power used to transmit acoustic pulses. Therefore, it’s clear to see that Narrowband systems will generally use more power than Broadband systems as they must transmit more pulses into the water to achieve the same precision that a Broadband system would. Generally, a Narrowband system will use about 30–40% more power than an equivalent Broadband system to achieve the same precision over the same amount of time.
Ease of operation
As far as ease of operation is concerned, Narrowband systems are by far the simplest instruments to program, operate and data-analyze. This is because the Narrowband technique is very robust and indifferent to the maximum desirable velocity and/or profiling range, provided they are within the system’s specifications. For a Broadband system, the maximum profiling range, velocity and precision desired all need to be carefully considered when programming the instrument and analyzing the data, as wrong configurations may lead to completely unusable data.
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