Kongsberg Simrad SN90 Reference Manual page 471

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 SN90 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 SN90 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
used by the transmitted pulse. Use the
range during the transmission.
Each frequency modulated transmission ("ping") from the SN90 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.
381294/E
function to define the centre frequency
Frequency
function to define the frequency
Bandwidth
Functions and dialog boxes
469