For safety of navigation, SOLAS in chapter V regulation 19 made it a requirement that all ships of 300 gross tonnage and upwards and all passenger ships irrespective of size shall have a radar on board.

Two types of radars Are required by SOLAS.  Therefore, when you go onboard big ships, you would find two or three Radar display units. One is the X band and the other is  the S band.

The X band and the S band may come in identical display units But they Are functionally independent and different in more ways than one…and most people do not know the difference.

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A Furuno Radar display unit


The X band uses a higher frequency and a shorter wave length. Its Frequency ranges from 8 – 12GHz (SOLAS requirement is 9Ghz). Its Wavelength ranges from 3.75 – 2.5 cm.
On the other hand, the S band uses a lower frequency and a longer wavelength.  Its Frequency ranges from 2 – 4 GHz (SOLAS requirement is 3Ghz). Wavelength ranges from 15 – 7.5 cm.

The S band comes with a large antenna. From what i have seen, a furuno sband radar antenna can be between 10 -12feet long so it Is common in larger vessels with a wide monkey deck or bridge top. On the other hand, the X band antenna is smaller in size and therefore common in smaller vessels. The size of antennas is as a result of the frequencies used. A lower frequency (sband) requires a larger antenna and a higher frequency (xband) requires a smaller antenna.

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a picture showing the x band (above) and s band (below)

The X band has a Better resolution than the s band.  This is because it uses a higher frequency. It can spot smaller targets and this is important for safe navigation But it is easily affected by the weather. The S band on the other hand can easily see through heavy weather or precipitation Which is common at sea. Larger vessels Are at the advantage here because they have both the xband and sband radar onboard so they can have What they want at any weather condition.

The last reason Am given is probably the most important.
X Band radars can detect radar transponders including the SART (search and rescue transponders)
The S Band radars on the other hand cannot detect the SART or any other radar transponders.

To know more about the SART CLICK HERE

That is why every ship 300 gross tonnage and above must carry an X band radar while the S Band Radar is to be fitted as the second radar for ships of 3000 gross tonnage and above.