Marine radar is a radar that is mounted and used by ships at sea for collision avoidance and other uses. It is used to detect other ships at sea and any other land obstacles. It is one of the most important safety components at sea and near the shores. But like all radar systems, marine radar has its own limitations and errors. Below are some of the errors and limitations.
Errors
1. Index error: This is the difference between the actual range between two points on a map and the range detected by the radar. This error can be observed when the vessels seat abeam between two points.
2. Beamwidth error: When the radar beam from the vessels moves away from the vessel, the width of the beam tends to widen. This causes distortion of the objects being detected. This distortion error increases as the vessel moves further away from the vessel.
3. Attenuation error: Attenuation is caused by the absorption and subsequent scattering of the beam energy as it is transferred through the atmosphere. This usually leads to a significant reduction in the strength of the echo. Attenuation is more pronounced in instances where there are a high frequency and short wavelengths.
4. Double echoes: These happen when the radar signals bounce off some parts of the ship and back into the receiver.
5. Multiple echoes: Multiple echoes occur as a result of several reverberations of the echoes from a different ship and from own ship multiple times. The display screen may show more than two or three objects being detected.
6. Indirect wave error: When a radar beam is emitted from the vessel, it is supposed to travel in a straight line directly to the contact. However, there are instances where the beam falls into the sea and it is deflected further which makes it travel a longer distance than if it would have traveled in a straight line.
Limitations
1. Clutter: Radar signals are affected by clutter especially from the sea and those caused by rainfall. This is why clutter controls are provided. However, the clutter controls need to be used with great caution because they may end up suppressing weaker objects that are navigating within the clutter zone.
2. Blind/Shadow sectors: The structure of the ship and sometimes the objects on the ship may cause blindness or shadow sectors on the ship. For this reason, it is important to properly mark the shadow or blind sectors in order to let other users understand the limitations of these sectors.
3. Distorted coastline: When a vessel is approaching a straight coastline, the radar may report a curved coastline or vice versa. This is because of the distance the radar takes to reach and return to the receiver from areas that are further away from the centerline of the vessel.
4. Input limitations: There are a number of sources that are used to feed information into modern radars. Some of these sources include GPS and compasses. These inputs may also have their own limitations that may have an effect on the radar itself. Ship navigators should be aware of these input limitations so as to understand their effect on the radar.
5. Heading misalignment: Heading markers are usually manually set on the radar on a vessel. This means that the markers may end up being misaligned for one reason or the other. To ensure the heading marker is not misaligned, you will need to point two vessels directly at each other. Ensure they are at a safe range. Once you have the vessels at this position, take the bearing of your vessel using a compass. The heading marker needs to correspond to this bearing.
6. Bearing discrimination: The radar set on the vessel needs to be able to distinguish between two different targets of the same range but with slightly different bearings. The radar is not able to discriminate and differentiate between these two contacts and may report as the same.