Marine Radar(Simplified)

            Marine RADAR 

Intrduction :

The marine radar is an equipment  that is perhaps used the most on the ship’s bridge by the OOW in carrying out a safe navigational watch.

A mandatory aid to navigation, the radar is used in identifying, tracking (with integrated ARPA) and positioning of vessels (including one’s own vessel) among other things in order to adhere to the COLREGs so as to safely navigate a ship from one point to another.

The marine radar is classified under the x-band (10 GHz) or S-band (3GHz) frequencies. The x-band, being of higher frequency is used for a sharper image and better resolution whereas the S-band is used especially when in rain or fog as well as for identification and tracking.

With the help of ship radar, accidents can be prevented at sea with the use of the various inherent functionalities of the radar (determining the CPA and the TCPA, EBL, VRM etc).

However, even while the ships are docked in the port, with the help of these radars, the coast guard, VTS and the other authorities can use them to monitor the traffic in the small radar range.

The ship radar has a screen (referred to as the Plan Position Indicator) that displays all the targets that are present within the radar range. Since all the objects are clearly visible on the screen, navigating and monitoring the position of the ship becomes really feasible, hence the term ‘aid to navigation’

.Main features of RADAR

The main features of the marine radars can be explained as follows:

• The parabolic radar antenna transmits and receives electromagnetic waves; as far as a target being displayed is concerned, that is basically the wave that bounced off a certain object that paints itself on the PPI (Plan Position Indicator)
• The frequency and the time taken by the flashes to return (reflections) to the radar receiver of the ship helps to find out whether the route of the boat can be continued with or not. The transmission and receiving of the pulse travels twice the distance in going and hitting the target and back; therefore, the target displayed on the PPI is basically halved with regard to its range
• On the PPI, the reflections can be seen so that identifying the actual distance of the objects can be even easier. The same paint on the PPI can be also be checked for determination of the bearing of the target

Working Principle :

The term “Radar” is an acronym for Radio Detection and Ranging”. The marine radar works on the basic principle of electromagnetic waves. The radar antenna sends the high-speed electromagnetic waves to establish the location, which is the distance, the velocity and the direction the wave travelled along with the altitude of the object, moving or stationary.

The radar system out electromagnetic waves as a high-speed, signal which will travel several miles in the direction of radar facing. If there are no objects in the direction of the wave, the radar screen will show blank.

If there is an object, which will reflect the wave back to the radar, the computer set up of the radar will determine the distance between the ship and the object along with its location. Hence, it can be said that the radar basically reads two things – The position of the object and the direction of the same.  

Position of the object: The radar antenna is continuously rotating on top of the ship sending and receiving signals. Hence, the radar is sweeping the signals all around the ship. As the radar pulse waves are reflected by an object, it will travel in the same direction and will be received by the radar detecting the position of the object. When the pulses are received back, the computer screen will log the position.

Distance from the ship: As the radar antenna is constantly sending and receiving signals from the object on the way of the electromagnetic signal, the received signal is sent back to the computer unit, which calculates the time taken for the signal to reflect back to the radar. Once the computer knows the time, it will calculate the distance by using the speed and time formula.

Uses of Marine RADAR

1. To calculate range and bearing of a target and thereafter use the information to determine speed, course etc
2. Integration with other shipboard equipment (such as ECDIS) to derive precise data
3. Navigating own vessel and her course with regard to collision avoidance
4. Fixing the ship’s position using terrestrial objects such as lighthouses, buoys etc
5. Differentiating between targets in high traffic density areas
6. Determination of the weather, to an extent
7. Use by VTS in controlling coastal traffic
8. Usage of features such as parallel indexing to ensure safe navigation
9. Alleviating workload on the OOW on the bridge
10. Used extensively in pilotage that covers the above aspects

Components of radar 

Transmitter 

Antenna system

Receiver

Display unit

Limitations of radar

Vertical beam width (VBW)

Horizontal beam width (HBW)

Range discrimination 

Bearing discrimination

Pulse repetition frequency (PRF)

It is the number of pulses transmitted per second . for marine radars PRF is between 500 and 4000. they have 3 or 4 values that change automatically with the range scale.

lower the PRF better the maximum range

maximum range is inversely proportional to PRF.

Anamalous propagation 

sub refraction 

super refraction 

ducting

Spurious echoes (False echoes)

side lobe echoes

indirect echoes

multiple echoes

second trace echoes

Spoking 

Unwanted lines appearing on the PPI due to dirty contacts or heavy sparking of motor or mechanical equipment in the vicinity.

Starring

Unwanted dotted lines which appear on the PPI due to interference of another radar in the vicinity,operating on the same bandwidth.

Subscribe for regular updates

Good luck.