15 Miguens_14_4 Radar para Evitar Colisões

Radar for Collision Avoidance at Sea

Introduction to Relative Motion

The radar is a crucial tool for preventing collisions at sea, particularly when visibility is limited. It allows for the early detection of other vessels. The radar image, whether stabilized or not, presents a relative motion display. In this display, the vessel remains fixed at the center of the screen, and all targets are displayed with their motion relative to our vessel. While the use of true motion radar displays is increasing, relative motion displays offer significant advantages for collision avoidance.

Understanding Relative Motion

The relative motion of a ship, in relation to another that is also moving, is defined by:

  • Direction of Relative Motion (DMR): The direction in which the other vessel appears to be moving relative to your own.

  • Velocity of Relative Motion (VMR): The speed at which the other vessel appears to be moving relative to your own.

Each form of motion can be represented by a vector, indicating direction and speed.

Practical Example: Two-Ship Scenario

Consider two ships, Alpha and Bravo. Alpha is on a course of 000^{\circ}, while Bravo is on a course of approximately 030^{\circ}. The key is to observe how Bravo appears on Alpha's radar screen and vice versa at three different instances.

  • Instant 1:

    • Bravo appears on Alpha's screen at a bearing of 080^{\circ} and a distance of 4 nautical miles.

    • Alpha appears on Bravo's screen at the reciprocal bearing of 000^{\circ} and the same distance of 4 nautical miles.

  • Instant 2:

    • Bravo appears on Alpha's screen at a bearing of 034^{\circ} and a distance of 3.8 nautical miles.

    • Alpha appears on Bravo's screen at the reciprocal bearing of 314^{\circ} and the same distance of 3.8 nautical miles.

  • Instant 3:

    • By observing the positions of Alpha and Bravo at this instant, one can determine their relative motion.

Determining DMR and VMR

To determine the DMR and VMR, plot the positions of the target vessel (e.g., Bravo) on your radar screen at different times and connect the points. This line represents the relative motion.

Calculation

  • DMR: The direction of the line connecting the plotted positions.

  • VMR: Calculated by considering the relative distance covered and the corresponding time interval. For example, if the relative distance between positions B1 and B3 is 11 nautical miles, and the time interval is 1 hour, then the VMR of target Bravo is 11 knots.

VMR = \frac{\text{Distance Relative}}{\text{Time Interval}}

Conditions for Relative Motion

Relative motion exists only when the absolute motions (true motions) of the vessels differ in:

  • Speed

  • Direction

  • Both Speed and Direction

If two ships are traveling in the same direction at the same speed, they are stationary relative to each other and will maintain the same position on the radar screen. The method of relative motion can be used to calculate the DMR and VMR of another vessel using a maneuvering board. While not directly relevant to practical exams, it aids in visualizing situations. However, ARPA systems can calculate this more easily.

Key Definitions: CPA and TCPA

  • Closest Point of Approach (CPA): The minimum distance to which your vessel will pass a target if both vessels maintain their current course and speed.

  • Time to Closest Point of Approach (TCPA): The time it will take to reach the CPA.

    • A positive TCPA indicates that the vessels are approaching each other.

    • A negative TCPA indicates that the vessels are moving away from each other.

ARPA systems, when interfaced with GPS, can also calculate the Course Over Ground (COG) and Speed Over Ground (SOG) of the target.

Practical Application with ARPA

Consider a radar display interfaced with a gyro and GPS:

  • The display is stabilized, with True North at the top.

  • Own ship's heading is 276^{\circ}.

  • Three targets are visible, each with a displayed CPA and TCPA.

Example:

  • Target 3: TCPA is 25 minutes, CPA is 0.4 nautical miles.

  • Targets 1 & 2: TCPA is shorter (6 minutes for Target 2 with a CPA of 0.8 nm and 1 minute for target 1 with a CPA of 1 nm), requiring immediate attention.

Automatic Radar Plotting Aid (ARPA)

ARPA systems reduce the time required for manual radar plotting and solving relative motion problems. They also decrease the risk of human error. ARPA systems provide:

  • True course

  • Speed

  • CPA elements of detected targets

  • Audio and visual alarms for contacts on a collision course.

ARPA calculates target elements as soon as the target is detected, regardless of the range scale set on the Plan Position Indicator (PPI). For instance, a contact acquired at 17 miles will have a complete solution computed in approximately 2 minutes. When a radar detects a target, it initially knows only the distance and bearing. Over time, with new distance and bearing readings, the ARPA calculates the target's DMR and VMR. Interfacing with ship instruments allows ARPA to calculate the target's true speed over ground and course over ground. However, for collision avoidance, DMR and VMR are most important

Advantages of ARPA

  • Automatic Acquisition and Processing: ARPA automatically acquires and processes radar echoes, simultaneously examining up to 200 echoes, and updating their data with each sweep.

  • Echo Display: Displays the closest echoes (up to 40 in some systems) with their respective vectors.

  • Instant Information: Provides instantaneous information on echo elements such as relative motion, position, course, speed, and CPA.

  • Collision Risk Alarm: Systems have collision risk alarms based on the set CPA distance, independent of the PPI range scale.

  • Elimination of Human Error: Reduces human error in mechanical plotting tasks and target element calculations.

  • Time for Maneuvering: Provides the navigator with more time to maneuver and indicates the results of simulated maneuvers.

ARPA can simulate maneuvers to assess the outcome in relation to other targets, such as altering course to pass behind another vessel.