Pulse Type function

The Pulse Type function allows you to select the "shape" of the transmitted pulses ("pings"). Different shapes have different qualities. Which pulse type to use depends on the current environmental conditions, your range to the target(s) and the type and speed of the target(s) you are observing.

Specifications

  • CW (Continuous Wave)
  • LFM (Linear Frequency Modulation) (This pulse type is also known as "chirp".)
  • Auto

How to open

Open this function from the Active menu.

Description

For each pulse type, you can select Short, Medium or Long. These options define the pulse duration. This is the current duration ("length") of the transmitted pulse. Use the Frequency function to define the centre frequency used by the transmitted pulse. Use the Bandwidth function to define the frequency range during the transmission.

Note:

The choice you make is by default only applied to the currently selected (active) view. Before you can change the settings related to a view, you must click inside the view to activate it.

There are no specific procedures that tell you exactly when to use which pulse type. The use of this function depends largely on the range you use to study your target(s), how the target(s) behave, and the current environmental conditions.

Tip:
  • Frequency modulated transmissions may provide a more stable echo presentation. You may see an increase in target detection because noise and reverberation are reduced.
  • With CW transmissions, increasing the duration of the transmitted pulse increases its energy and improves target detection capability. However, this will also reduce the range resolution. Small targets may then be harder to detect. CW pulses with long duration are also sensitive to Doppler.
  • Decreasing the duration of the transmitted CW pulse increases the range resolution.
  • When you operate with frequency modulated pulses, the range resolution is independent of the pulse duration. You can increase the pulse duration without decreasing the resolution.
  • If you wish to investigate targets close to the bottom, a short pulse duration is always recommended.
  • We recommend that you use Auto or a CW setting during normal operation.

Details

Auto

Select Auto to make the ST90 system choose pulse type and duration for you. The resulting setting is based on the current range. By default, the Auto setting will use LFM Medium on shorter ranges, and then automatically switch to LFM Long when the range is increased.

CW (Continuous Wave)

In CW transmissions the frequency is fixed during the entire duration of the pulse.

Tip:

If you select different centre frequency for each view, you will reduce the risk of interference between them. If you operate in the close vicinity of other vessels that are also using a ST90 system - or another sonar system operating within the same frequency range – set each system to use different centre frequency.

LFM (Linear Frequency Modulation)

In LFM transmissions the frequency changes during the duration of the pulse. The change is linear. Use the Frequency function to define the centre frequency used by the transmitted pulse. Use the Bandwidth function to define the frequency range during the transmission.

Each frequency modulated transmission ("ping") from the ST90 system consists of a frequency sweep around the centre frequency. The difference between the lowest and the highest frequency in each transmission is referred to as bandwidth. The selected bandwidth will be distributed equally on both sides of the centre frequency.

Related functionality

Pulse Duration

The pulse duration can be selected according to the current range and what kind of targets you are looking at. The longer range you wish to use, the longer pulse duration should be selected. However, when using CW transmissions a long pulse duration will reduce the resolution. It will also cause the ST90 system to transmit less frequent.

Further, increased pulse duration will cause reduced source level when using consecutive transmissions. For example, if the ST90 system is set up to use one horizontal ping and two vertical pings these are transmitted in series. The first ping will reduce the voltage level on the output capacitors, and this reduces the source level on the following transmissions.

Longer pulses will also increase the blind zone close to the transducer and increase the reverberation level.

For these reasons, the use of long pulse durations may not be the best choice.

For CW (Continuous Wave) transmissions:

  • Long pulses provides longer detection range. They make the ST90 system less sensitive for noise, but offer lower range resolution.
  • Short pulses provides shorter detection range. They make the ST90 system more sensitive for noise, but offer higher range resolution.

For FM (Frequency Modulated) transmissions:

  • Long pulses provide longer detection range, and the range resolution is independent of the pulse duration..
  • Short pulses provide shorter detection range, and they make the ST90 system more sensitive for noise.
Bandwidth function

The bandwidth used by the frequency modulated transmissions is controlled by the Bandwidth function. Each frequency modulated transmission ("ping") from the ST90 system consists of a frequency sweep around the centre frequency. The difference between the lowest and the highest frequency in each transmission is referred to as bandwidth. The chosen bandwidth will spread equally to both sides of the centre frequency.

The bandwidth function does not apply to CW (Continuous Wave) transmissions.

Open this function from the Active menu.

Frequency function

Use the Frequency function to specify the centre frequency for the frequency modulated (FM) transmissions (pings) and the operating frequency for the continuous wave (CW) pings. You can change the frequency in predefined steps from a "start frequency" to an "end frequency".

Open this function from the Active menu.