Watchkeeping - RADAR

Question 1

What is the difference between X band and S band radar?

Answer 1

X-Band RADAR transmits a 3cm wavelength on a 9GHZ frequency. In fair weather it will provide
better definition of targets. It will also detect an activated search and rescue transponder.

S-Band RADAR transmits a 10cm wavelength on a 3GHZ frequency. Better for long range use
and will provide better definition of targets when rain and sea clutter present.

Question 2

What errors and limitations exist in RADAR systems?

Answer 2

• Multiple echoes which are caused by at short range, strong reflector and own ship bouncing the signal back and forth between them, the echoes
  will appear as an echo or line of echoes leading away from the target.
• Indirect echoes may be present as the RADAR signal bounces of own vessel’s superstructure of reflective surface ashore. An indirect echoe will
  appear to be on a different bearing from true target but at approximately the same distance.
• Blind and shadow sectors may exist dependent on the position of the RADAR antennae installed in reference to the ship’s structure.
• As the RADAR antannae rotates, in addition to the main radiation lobe, side lobe radiation will escape either side and may present true targets
  with echoes. This is mostly present at short range with strong reflective targets.
• Radar should be positioned at a height in reference to its vertical beamwidth to ensure maximum efficiency in reference to close range targets.
• The RADAR data will only be as accurate as its inputs.

Question 3

What is the difference between bearing resolution and range resolution?

Answer 3

Bearing resolution is the ability to detect more than 1 target which are which are in simlar bearing proximity to one another, the ability to do so
depends upon the horizontal beamwidth of the radio wave. This is why X-band RADAR will provide better definition of targets in fair weather.

Range resolution is the ability to detect more than one target that may be closes by in respect to distances and is dependent upon pulse length.
A long pulse length will provide larger targets but may merge 2 targets one.

Question 4

What are the 3 main orientations of RADAR and what will they display?

Answer 4

• North up where the orientation is that of a nautical chart which creates less confusion when using in conjunction with ECDIS or paper charts.
• Head up -
• Course up -

Question 5

What RADAR settings would you employ for navigating ocean/coastal navigation?

Answer 5

Ground stabilised for fixed points to remain stationary and true course over ground.

Question 6

What RADAR settings would you employ for collision avoidance

Answer 6

I would setup the the RADAR as relative motion, relative vectors with relative trails. Sea stabilised to ensure of accurate understanding of
aspect. Appropriate use would be made of both X-band and S-band systemsdependent on the conditions.
All relevant targets to be selected and full use of ARPA/VRM’s & EBLS’s cross referenced with visual bearings if possible.

Question 7

What is the difference between Ground Stabilised and Sea Stabilised RADAR?

Answer 7

Ground stabilised COG and SOG is obtained through GPS. Fixed objects will remain stationary on the RADAR display.

Sea stabilised course and speed is input through the Gyro and Log. Fixed objects will appear to have a course and speed equal to the
reverse course and speed of own vessel.

Question 8

What are the limitations of ARPA?

Answer 8

Inproper tuning relevant to the present conditions.
Incorrect inputs either from GPS or GYRO or LOG.
Delay in accurate data after a course or speed change.
Target swap may occur when two targets are within close proximity of each other.
Target data will only be available if either selected or enter an auto acquisition zone.

Question 9

Why is it better to use sea stabilised RADAR for collision avoidance?

Answer 9

It will provide the interrogated vessels aspect which may not be accurate when using ground stabilised when influenced by wind
and tide.

https://tstarmet.com/free-study-aids/navigation/radar-stabilisation/

Question 10

In what circumstance may sea stabilised RADAR be of detrimental
use for collision avoidance?

Answer 10

In coastal waters, rivers and areas with large large amounts of stationary objects, excess clutter will be produced by the displayed trails.

https://tstarmet.com/free-study-aids/navigation/radar-stabilisation/