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Concept descriptions
Basic sonar theory
Layers and deflections
Daily operation of the ST90 system may be disturbed by environmental situations that you are unable to control. Typical conditions are temperature and salinity layers, as well as deflections caused by increasing or decreasing water temperature.
Sonar beam deflections
On warm days – especially during summer – the sea surface will be heated by the sun. The water temperature will gradually decrease with increasing depth. On cold days, you may experience the opposite. The cold water will be close to the surface, and the water temperature increases with increasing depth.

About this manual
The purpose of this publication is to provide the descriptions, procedures and detailed parameter explanations required to allow for safe and efficient use of the ST90 system. The publication also provides you with a thorough understanding of the parameters and adjustments provided by the ST90 system.
Simrad ST90
Operating procedures
Maintenance procedures
User interface
Menu system
Functions and dialog boxes
Concept descriptions
- Important settings
- Basic sonar theory
  - Vessel coordinate system
    The vessel coordinate system is established to define the relative physical locations of system units and sensors.
  - Layers and deflections
    Daily operation of the ST90 system may be disturbed by environmental situations that you are unable to control. Typical conditions are temperature and salinity layers, as well as deflections caused by increasing or decreasing water temperature.
    - Temperature and salinity layers
      You may experience that temperature and/or salinity layers exist in the water column. Since the speed of sound in water depends on its temperature and salinity, such layers will cause the sound beam to be reflected.
    - Sonar beam deflections
      On warm days – especially during summer – the sea surface will be heated by the sun. The water temperature will gradually decrease with increasing depth. On cold days, you may experience the opposite. The cold water will be close to the surface, and the water temperature increases with increasing depth.
  - What is sampling?
    The information from the transducer on the ST90 system is a continuous flow of analogue data. In signal processing, sampling is the reduction of this continuous signal to a discrete signal.
- Acoustic noise
Technical specifications
The technical specifications summarize the main functional and operational characteristics of the ST90 system. They also provide information related to power requirements, physical properties and environmental conditions.
Datagram formats
The ST90 system can send and receive information to and from several different peripherals. All transmissions take place as datagrams with data sentences. Each datagram has a defined format and length.
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Sonar beam deflections

On warm days – especially during summer – the sea surface will be heated by the sun. The water temperature will gradually decrease with increasing depth. On cold days, you may experience the opposite. The cold water will be close to the surface, and the water temperature increases with increasing depth.

Water temperatures on a warm day

When the temperature decreases (A) closer to the bottom, the sonar beam is deflected down as indicated by the illustration. The school of fish (B) you thought you would see will not be visible at all.

Water temperatures on a cold day

When the temperature decreases (A) closer to the surface, the sonar beam is deflected up as indicated by the illustration. The school of fish (B) you thought you would see will not be visible at all. Instead you will probably see a ring of surface echoes.