Marine Radar Onboard Ships: An Overview

 

Marine Radar Onboard Ships: An Overview

Your Guide to Seafaring

Marine radar is an indispensable tool in modern maritime navigation, providing critical information that ensures the safety and efficiency of ships at sea. This comprehensive guide explores the intricacies of marine radar, its components, types, functions, and the technological advancements that have revolutionized its use.

Introduction

Marine radar is a vital navigational aid that helps detect and track objects such as other vessels, landmasses, buoys, and navigational hazards. It operates by emitting radio waves that reflect off objects, with the reflected signals processed to determine the object's distance, direction, speed, and course. This information is crucial for safe navigation, especially in poor visibility conditions such as fog, rain, or nighttime.

Historical Background

The development of radar technology dates back to the early 20th century, with significant advancements during World War II. Initially used for military applications, radar technology was adapted for maritime use to enhance navigation safety. The first marine radars were large and cumbersome, but technological advancements have made them more compact, efficient, and user-friendly.

Types of Marine Radar

Marine radar systems are broadly classified into two types based on the frequency bands they operate in: S-band radar and X-band radar.

• S-band Radar: Operating at a frequency of around 3 GHz, S-band radar is capable of penetrating rain, fog, and sea clutter more effectively. It offers longer-range detection and is ideal for tracking large targets such as ships and landmasses. S-band radar is particularly useful in adverse weather conditions.
• X-band Radar: Operating at a higher frequency of around 9 GHz, X-band radar provides higher resolution and sharper images. It is more sensitive to small targets like buoys and fishing boats. However, it can be more susceptible to weather conditions. X-band radar is commonly used for collision avoidance and navigation in congested waterways.

Main Components of Marine Radar

A typical marine radar system consists of three main components:

1. Antenna: The antenna emits radio waves and rotates continuously to cover a 360-degree area. It receives the reflected signals from objects in its path. There are two primary types of antennas used in marine radar:
• Open Array Antenna: Known for its higher gain and better performance, especially in distinguishing closely spaced targets.
• Radome Antenna: Enclosed in a protective dome, it is more compact and suited for smaller vessels.
2. Transmitter/Receiver Unit: This unit generates the radio waves emitted by the antenna and processes the returned signals. It calculates the range, bearing, speed, and course of detected targets.
3. Display Unit: The display unit shows the radar picture on a screen, which can be either a cathode ray tube (CRT) or a liquid crystal display (LCD). Modern radar systems often use high-resolution LCD screens for clearer images and enhanced user interaction.

Functions of Marine Radar

Marine radar performs several critical functions that enhance navigational safety and operational efficiency:

• Detection and Tracking: Marine radar identifies and tracks various objects, including other vessels, landmasses, buoys, and navigational hazards. This capability is vital for situational awareness.
• Navigation Aid: Radar provides essential information that aids in navigation through restricted visibility conditions. It helps mariners plot courses, identify safe passages, and avoid hazards.
• Collision Avoidance: One of the primary functions of marine radar is to prevent collisions at sea. Radar provides bearing and distance information about other vessels, enabling navigators to take evasive action when necessary.
• Automatic Radar Plotting Aid (ARPA): ARPA systems enhance radar functionality by automatically tracking the movement of multiple targets. ARPA calculates the closest point of approach (CPA) and the time to closest point of approach (TCPA), helping navigators assess collision risks and make informed decisions.
• Search and Rescue Operations: Marine radar is crucial in search and rescue missions, helping locate distressed vessels or individuals in the water.

Technological Advancements in Marine Radar

The marine radar industry has seen significant technological advancements, improving its functionality and usability:

• Solid-State Technology: Traditional magnetron-based radars are being replaced by solid-state radar technology, which offers better performance, reliability, and lower maintenance requirements.
• Broadband Radar: Broadband radar systems provide higher resolution and better target discrimination, especially at close ranges. They are particularly effective in detecting small targets and navigating in congested areas.
• Integration with Other Systems: Modern marine radar systems are often integrated with other navigational tools, such as Electronic Chart Display and Information Systems (ECDIS) and Automatic Identification Systems (AIS). This integration provides a comprehensive view of the navigational environment.
• User-Friendly Interfaces: Advances in user interface design have made marine radar systems more intuitive and easier to operate. Touchscreen displays, customizable interfaces, and advanced plotting features enhance the user experience.
• Environmental Adaptability: Modern radars can adjust their settings based on environmental conditions, optimizing performance in various weather and sea states.

Some of the main features of marine radar with ARPA integration are:

• Range scale: This is the maximum distance that the radar can cover. It can be adjusted by using the range key on the keyboard or by selecting from a menu on the screen. The range scale determines the size and resolution of the radar picture.
• Range rings: These are concentric circles that divide the radar screen into equal intervals. They help to measure the range of a target by counting the number of rings between the center of the display and the target echo.
• Variable range marker (VRM): This is a dashed circle that can be moved by using a scroll wheel or a trackball. It gives more accurate range measurements than range rings by touching the inner edge of the target echo.
• Electronic bearing line (EBL): This is a straight line that extends from the own ship’s position to any point on the screen. It gives more accurate bearing measurements than  compass rose by aligning with any target echo.
• Parallel index line (PI): This is a dashed line parallel to EBL that indicates how far off course or off-track own ship is from its intended course or track.
• Heading marker: This is an arrow at 0°T on top of EBL that shows own ship’s heading relative to true north.
• Course over ground (COG) vector: This is an arrow at own ship’s position that shows own ship’s course over ground relative to true north.
• Speed over ground (SOG) vector: This is an arrow at own ship’s position that shows own ship’s speed over ground relative to true north.
• True motion mode: This is a mode where own ship moves across the screen while targets remain stationary relative to true north.
• Relative motion mode: This is a mode where own ship remains stationary at the center of the screen while targets move across it relative to own ship.

Here are some tips on how to use marine radar effectively:

• Adjust gain control: Gain control adjusts the sensitivity of the radar receiver. It should be set so that background noise is just visible on screen without obscuring weak echoes from small targets or distant targets.
• Adjust sea clutter control: Sea clutter control reduces unwanted echoes from the sea surface caused by waves or swell. It should be set so that sea clutter does not interfere with target detection near the horizon or close range.
• Adjust rain clutter control: Rain clutter control reduces unwanted echoes from precipitation caused by rain or snow. It should be set so that rain clutter does not interfere with target detection in areas affected by weather conditions.
• Select appropriate range scale: Range scale should be selected depending on prevailing circumstances and conditions such as traffic density, proximity to the coastline, visibility etc. A longer range scale provides advance warning of approaching targets while shorter range scale provides better resolution for close-range targets.
• Use VRM and EBL for accurate measurements: VRM and EBL provide more accurate measurements than fixed range rings and compass rose for target’s range and bearing respectively. They also help to determine if there is a risk of collision by checking if the bearing remains constant with decreasing range.
• Use PI line for course keeping: PI line helps to keep own ship on its intended course or track by showing how far off it deviates from it due to wind, current etc. It also helps to estimate closest point of approach (CPA) with other vessels by showing how much clearance there will be between them at crossing situation.

 COMMON PROBLEMS ON MARINE RADAR

Marine radar is not without its challenges and limitations. Some of the problems that can affect the performance and accuracy of marine radar are:

• Clutter: Clutter refers to unwanted echoes or noise on the radar screen that can obscure or confuse the real targets. Clutter can be caused by various factors, such as rain, snow, fog, sea waves, birds, insects, interference from other radars or electronic devices, etc. To reduce clutter, the radar operator should adjust the gain control (sensitivity) so that only the relevant echoes are visible on the screen. The operator should also use filters or suppressors to eliminate specific types of clutter.
• Blind zones: Blind zones are areas where the radar cannot detect targets due to physical obstructions or limitations of the antenna. For example, blind zones can occur behind tall structures (such as masts or funnels), below or above the horizon (due to earth’s curvature), or close to own ship (due to minimum range). To avoid blind zones, the operator should use different range scales or switch between X-band (shorter wavelength) and S-band (longer wavelength) radars if available.
• False echoes: False echoes are misleading signals that appear on the radar screen but do not correspond to real targets. False echoes can be caused by various factors, such as reflection from land features (such as mountains or buildings), refraction from atmospheric layers (such as inversion or ducting), multipath propagation (when radio waves bounce off more than one surface), etc. To identify false echoes, the operator should compare them with visual observations or other sources of information (such as AIS or VHF).
• Shadow sectors: Shadow sectors are areas where a target is hidden from view by another target that is closer to own ship. For example, a small boat behind a large ship may not be visible on radar due to shadowing effect. To avoid shadow sectors, the operator should use different bearing lines or electronic bearing lines (EBLs) to measure the relative bearings of targets from own ship.
• Sea return: Sea return is a type of clutter that occurs when radio waves reflect off sea surface due to rough weather conditions or high wind speed. Sea return can mask small targets near own ship or create false targets at longer ranges. To reduce sea return, the operator should adjust sea clutter control (STC) which reduces sensitivity at short ranges.

 

Conclusion

Marine radar is an essential tool for maritime navigation, providing critical information that ensures the safety and efficiency of vessels at sea. From detecting and tracking objects to aiding in collision avoidance and search and rescue operations, marine radar plays a pivotal role in modern maritime operations. Technological advancements continue to enhance its capabilities, making it an indispensable asset for mariners worldwide.