Explain why ranges are more accurate than bearings when plotting a ship’s position
Bearings suffer from half-beamwidth error. When the edge of the beam strikes an object it paints an echo on the radar display, but due to the width of the pulse this is not the centre line of the beam which the bearing measurement should be taken from. Ranges are not affected by this error
Define range discrimination
The ability of the radar to discriminate between two targets on the same bearing and closely spaced in range
Define bearing discrimination
The ability of the radar to discriminate between two targets at the same range and closely spaced in bearing
How would you check and correct for heading misalignment?
Take visual relative bearings of objects at different points of the compass. Compare these with the relative radar bearings. Any difference is the error in heading marker alignment. Apply this correction in the software function for this purpose (a pass code may be required).
When setting up, what would you check before transmitting with the radar?
The radar scanner should be clear of personnel and obstructions (safety)
What two inputs would you check when setting up a stabilised radar?
- Heading input
- Speed input
Explain how to set up the following controls:
- Brillance
- Gain
- Tuning
- Brilliance - Increase to a level which allows the OOW to see the display clearly
- Gain - Adjust so that a lightly speckled background of clutter can be seen
- Tuning - Adjust for best response from a small target near edge of the display
State the wavelength and frequency of the X-band and S-band radars
X-band 3cm 9GHz
S-band 10cm 3GHz
List two advantages of X-band over S-band radar
- More definition of land
- More accurate bearings
Picks up signals from Sart
Can be used to track squals
List two advantages of S-band over X-band radar
- Better range of detection
- Better at seeing in and through precipitation
Briefly explain how the rain clutter control affects the display
It acts on the echo by trying to cut off the trailing edge so that just the
leading edge is seen. It applies this to the whole of the range of display.
Briefly explain how the sea clutter control affects the display
It suppresses gain. It applies this to the area near to the ship and increases the effect as the control is turned up. It never extends to the whole range.
What five features would you consider when determining a good radar conspicuous object?
- Material of the object
- Aspect
- Size (Height and breadth above radar horizon)
- Texture of the surface
- Shape
Describe with the aid of a sketch the cause of Multiple Echoes and the effect on the radar display
A radar pulse bounces off a target at close range, with enough power in it to allow it to bounce back and forth several times, painting an echo each time received.
Describe with the aid of diagrams the cause of Indirect Echoes and the effect on the radar display
The radar pulse bounces off some obstruction on the ship before hitting a target, with the echo being received by the same route. The radar thinks the target is in the direction of the obstruction, but at the distance of the target.
Describe with the aid of diagrams the cause of Side Lobes and the effect on the radar display
Side lobes electromagnetic radiation leakage near to the scanner. If they pick up targets in this area, extra echoes will be painted in the direction of the lobes.
Describe with the aid of diagrams the cause of Blind and Shadow Sectors and the effect on the radar
display
Blind sectors are areas where the radar transmissions are completely obscured and no echoes will be found. Shadow sectors are areas of reduced sensitivity where radar may pick up some targets.
Use a sketch of a radar display to show how a Parallel Index line can be used to monitor the track of a
vessel
What should be checked when parallel indexing and position fixing?
- Heading marker alignment
- Gyro error and gyro input
- Alignment of radar with gyro input
- Positive identification of navigational objects of interest
What is a SART used for and what type of radar would you expect to see its transmissions?
A Search and Rescue Transponder is used for locating a casualty.
It can be seen on a 3cm (X-band) radar
Describe the use of a True Vector
It indicates the direction a target is moving in (heading on a sea stabilised radar)
Describe the use of a Relative Vector
It gives indication of risk of collision if in the direction of own ship
List five factors to take into account when setting the CPA/TCPA alarm on the radar
- The state of visibility
- Traffic density
- Manoeuvrability of the vessel
- Proximity of navigational hazards
- Master’s standing orders
List, in order, the six pieces of information ARPA is required to give about a target
- Present Range
- Present Bearing
- Predicted CPA
- Predicted TCPA
- Calculated True course
- Calculated True speed
