The echogram settings control how data is collected and processed by the Echosounder. Firstly, to enable Echosounder configurations, the user must ensure the Echosounder firmware is installed and select one of the following options in Signature Deployment:

• For Signature 500 and Signature 1000:
  • “Burst with 4 beams”, “Burst with 5 beams” or “Burst using vertical beam” plans (single or concurrent) ticking the “Echosounder” box; or
  • “Burst with echosounder beams” plan (single or concurrent); or
  • “Wave height and direction” or “Waves and burst using 5 beams” plans (single or concurrent) ticking the “Echosounder” box.
• For Signature 100:
  • “Echo sounder” plan (single or concurrent).

Once the Echosounder measurements are enabled, that will activate the "Echosounder Step", visible on the left panel. Within the "Echosounder Step", the user will see the range configuration along with 3 Echograms options. It is up to the user to select the Echosounder configuration as desired according to the objective of the study. Further reading is available here.

Echogram 1 has the option to enable Pulse compression, which is a Broadband signal with a wider frequency range. This FAQ focuses on the Signature 100, where this range is between 67.5 and 112.5 kHz. For the Signature 500 and Signature 1000 configuration please refer to this FAQ. The signal is sent out in the form of a frequency modulated signal, also known as a chirp or a matched filter (Figure 1). The advantage of the Pulse compression is the noise reduction, higher accuracy and longer range due to the combination of long- and high energy pulses. However, Pulse compression is not ideal for shallow water as it can yield side lobe effects near the boundary and may come at a cost of higher power usage.

Figure 1: shape of a chirp signal

The user also has the choice to select the data to be processed into single beam (centered at 90 kHz) or five frequency bins, centered at 73.8, 82.3, 90.9 and 99.4 kHz. With five frequency bins, the user gets a broader range of data points, which can be useful for more detailed analysis. If only a single frequency is needed (e.g., 90 kHz), the best option it to choose the 1-bin option and still use pulse compression for better quality.

If Pulse compression is disabled, Echogram 1 will be programmed in Narrowband Mode. Echograms 2 and 3 are always collected in narrowband, regardless of the settings. In Narrowband mode, the data is collected at three user-selectable frequencies - 70 kHz, 90 kHz or 120 kHz with a reduced bandwidth of ± 6.25%. For the Signature 500 and Signature 1000, each device is centered around its respective specific frequency (Figure 2).

Figure 2: shape of a narrowband signal

Now, to an example of how this configuration plays out with 3 different configurations:

• Configuration 1: Echogram 1 (90 kHz) with pulse compression disabled, Echogram 2 (70 kHz) and Echogram 3 (120 kHz) enabled: this gives 3 processed data sets, one for each frequency. The raw data for each will be narrowband with 6.25% bandwidth.
• Configuration 2: Echogram 1 (90 kHz) with pulse compression enabled with 1 bin, Echogram 2 (70 kHz) and Echogram 3 (120 kHz) enabled: Echograms 2 and 3 will be narrowband with 6.25% bandwidth, Echogram 1 will be “broadband” ranging from 67.5 and 112.5 kHz.
• Configuration 3: Echogram 1 (90 kHz) with pulse compression enabled with 5 bins, Echogram 2 (70 kHz) and Echogram 3 (120 kHz) enabled: Echograms 2 and 3 will be narrowband with 6.25% bandwidth, Echogram 1 will be 'broadband' ranging from 67.5 and 112.5 kHz. This results in 7 processed datasets centred at 73.8, 82.3, 90.9 and 99.4 kHz

To the question, is it better to use broadband or narrowband mode? The choice between broadband and narrowband mode depends on your specific measurement needs. If more frequency data points and better noise suppression are needed, Broadband Mode with Pulse Compression is the better option – always considering the caveats detailed here. However, if specific frequencies are needed, Narrowband Mode may be sufficient. If the deployment is an initial trial, both modes can be selected and analysed during the processing stage. If sufficient power and memory are available, both modes can be stored either as raw data (I/Q) or just the processed data for each of these frequencies in the following tab by selecting “Store raw echo data”. The raw broadband data will be the same whether the data is saved for one or five bins, with different output.