Watchkeeping (Radar)

Q. In the relative motion stabilized radar, what is the effect of current on target course, speed, CPA & TCPA?
In a relative motion display, the information regarding the course and speed is not fed. The ownship is stationary point at the centre of PPI. As long as the current affects the ownship and target in a similar way, there will be no error in above data.

Q. What will you get? The targets course and speed over water or over ground?
By plotting, the course and speed over water is obtained.

Q. How will you know that there is current?
In a normal OAW triangle, plotted with respect to a stationary target, say beacon, AW is the set and drift. The current goes from A to W.

Q. By plotting a stationary target, can you find out course made good and distance made good?
Yes, in the OAW triangle, WO is the course steered and distance over water whereas, AO is the course and distance made good.   

Q. What initial things would you do, when you pick up a target o the radar screen for the first time in restricted visibility?
A watch keeper, while keeping a navigational watch, in restricted visibility, while dealing with a target must find out:

  1. If the target is stationary; or
  2. if the target is mobile.

If the target is mobile then

  1. Whether the target is coming down on reciprocal or crossing courses?
  2. Is on an overtaking course w.r.t. own ship?
  3. If there is risk of collision or close quarter situation then what is most appropriate action by own ship?
  4. What may be the most probable action by target?

Q. When there is more than one target as seen on the radar screen in restricted visibility, how do you deal with the situation?
If own ship is dealing with more than on target, then the following must be found:

  1. Which of the target must be given priority? Thus, a vessel with less TCPA must be given priority over the vessel with more TCPA.
  2. The product of TCPA and CPA may give an index to decide the priority number.
  3. Which of the target are not likely to close in for considerable period?
  4. If there is risk of collision or close quarter situation then what is most appropriate action by own ship?
  5. If the action is taken for a vessel, will it support or be compatible with the action to be taken for the vessel on lower priority?
  6. What may be the respective probable actions by the targets?

Q. The three targets acquired are as shown in the picture. In what order will you give a target priority over the others?

In a situation where the targets are manually acquired, one may want to reject one or more targets. This he will do based on the parameters like TCPA, distance, aspect, CPA and relative bearing. But this may also depend on situation. Another factor that will decide the priority is if the action taken for prior targets favors the action to be taken for later.
In above situation, considering the TCPA, target 2 should be given priority but considering the relative bearing and the CPA, target 1 should be given priority. The action in term of alteration is common for both. Both are likely to alter to their starboard. Although the CPA for target 3 is 0 but TCPA is 38 minutes. Alteration to starboard anyway would be very clear and CPA anyway would increase further.

Q. The four targets acquired are as shown in the picture. In what order will you give a target priority over the others?
A prudent watch keeper may place the above targets in the order of priority viz. (3), (6), (5), (4) for the reasons stated below:

Q. What is the role of ARPA in improving the standard of collision avoidance at sea?
The importance of ARPA is to improve the standard of collision avoidance at sea. It effectively reduces the workload of observers by enabling them to automatically obtain information so that they can perform well with multiple targets. The ease is even better than, as when manually a single target is plotted.

Q. What data is provided by a modern ARPA?
A modern ARPA normally, should provide the following:

  1. Relative vectors
  2. True vectors
  3. Points of Collision
  4. Predicted areas of danger
  5. Trial maneuvers
  6. Operational warnings
  7. Rejection boundaries
    Other data being – History tracks, Digital data output, Navigation lines & limits, Equipment warnings, etc.

Q. How will the relative vector of a target which can collide, appear? What about the stationary target? |
A target on the collision course will give a vector whose extension will pass through own ship.
Relative vectors on targets which are stationary, will represent the reciprocal of own ship’s ground track.
Targets observed from centre of PPI and target on:
a. collision course;
b. stationary.

Q. How will the true vectors of following targets appear on a true motion display?
a. reciprocal course;
b. stationary.

Q. In the true motion mode, what true vector will be shown by a fixed target such as navigational mark, when course steered and speed over water is applied to own ship?
The true vectors will appear on fixed targets representing the reciprocal of set and drift affecting own ship.

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