RADAR

(i) Vertical beam width (VBW)

• It is the vertical angle at the scanner contained between upper and lower edges of the radar beam.
• In commercial marine radar, VBW is between 15° and 30°.

(ii) Horizontal beam width (HBW)

• It is the horizontal angle at the scanner contained between leading and trailing edges of the radar beam.
• In commercial marine radar, HBW is between 0.6° and 2°.

(iii) Pulse Length (PL)

• It is the time taken for a pulse to leave the scanner i.e. interval between leading edge of the pulse leave the scanner and when the trailing edge of the pulse leave the scanner.

(iv) Pulse Repetition Frequency (PRF)

• It is the number of pulses sent through the scanner in one second.
• In commercial marine radar, PRF is between 500 and 4000.

(v) Wavelength

• It is the distance between two adjacent crests of sound waves.
• It is about 3 cm in X-band RADAR & 10 cm in S-band RADAR.

There are two different methods of stabilization in RADAR

• So if gyro fails, RADAR works in ground stabilized mode. True motion display of RADAR is used. Course is fed from GPS i.e COG & SOG.
• And, if GPS fails, RADAR works in sea stabilized mode. Relative motion display of RADAR is used. Course is fed from Gyro & Speed is taken from log i.e STW.

OAW triangle is used for manual Radar Plotting.

• It is used as a basic radar plot to determine CPA, TCPA of target, Course & speed of target.
• The three sides of triangle are:-  OW- own vessel course & speed      WA- target vessel course & speed      OA- apparent motion of target

★ Steps to be followed for RADAR plotting:- 

• Take 2 or 3 set of bearing and range of the target at some interval and note it down.
  Example:- 1800- 060°×8.0 NM, 1806- 059°×7.0 NM, 1812- 057°×6.0 NM
• Plot the 1st range × bearing-name it ‘O’, then 2nd range and bearing & then 3rd range and bearing- name it ‘A’ on the radar plotting sheet.
• Now, join all three points & extends the line upto near the center. This is the line of relative approach of the target towards our vessel which we have assumed at the center of the sheet.
• To find the CPA- Draw a line perpendicular to the line of approach & where the perpendicular line cuts the line of approach- name it ‘C’ and measure the distance of this line from the scale in the radar plotting sheet, i.e CPA – 0.8 NM
• To calculate TCPA- since we know that from O to A I.e. 2 NM covered in 12 min ; so know, we will measure the distance CA I.e let’s say 5.8 NM. Now with simple mathematics, calculate the time the target would take to cover 5.8 NM. TCPA will be (12÷2)×5.8= 34.8 minutes.

• To calculate course & speed of target- Plot own vessel course 045° and cut it on distance 2.4 NM- name it ‘W’ (course- 045°, Speed- 12 knots, 2.4 NM in 12 min), this is OW.      Now join W & A. WA is the course & speed of the target vessel. With parallel ruler check the course & with the scale measure the distance WA- this is the distance covered by the target in 12 minutes- with simple mathematics, calculate the distance it would travel in 60 minutes to get the speed.

• It is the angle at the target contains between heading of the target & own ship bearing from the target.
• It is the relative bearing of own vessel from the target vessel.
• In simple words, the aspect of a target describes how much of the target vessel’s profile, we will be able to see.

• If the aspect is 90°(R), this means we will able to see the complete port side profile of the target vessel.
• If the aspect is 0°, this means we will be able to see front profile of the target vessel.

★ Steps to be followed for aspect plotting:- 

• Join own ship position (which is at the center of the potting sheet) to the target vessel last position ‘A’.
• Extend the course line of the target vessel (line WA).
• The angle between these two line is the aspect of the target vessel.
• The aspect of the vessel is written as 0° to 180° red or green.
• If we are on target’s vessel port side or we are able to see the red light of the target, the aspect will be ‘RED’ & vice versa.

Automatic Radar Plotting Aid

• It is an computerized addition to basic RADAR which carry out radar plotting quickly and accurately.
• It provides facilities which include tracking of targets, automatic and manual acquisition, true motion & relative motion display, trial manoeuvres.
• ARPA is able to automatically track, process, simultaneously display and continuous update information on at least 20 targets.
• ARPA is able to give read out of a target within 30 seconds.
• It also provides various alarms which include CPA/TCPA alarm, course change of tracked vessel alarm etc.