RADAR and ARPA plus Flashcards
- Write the equation a radar would use to calculate range of an echo and state the speed value (5)
- Explain why ranges are more accurate than bearings when plotting a ship’s position (5)
Bearings suffer from half-beamwidth error. When the edge of the beam strikes an obiect 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.
Bearing are inaccurate due the horizontal beam width.
Radar ranges are obtained pointing the beam on a precise spot.
- Define range discrimination (2)
The ability of the radar to discriminate between two targets on the same bearing and closely spaced in range
It’s affected from pulse length: Short Pulse -> good range discrimination
- Define bearing discrimination (2)
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? (5)
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? (2)
The radar scanner should be clear of personnel and obstructions (safety)
- What two inputs would you check when setting up a stabilised radar? (2)
Heading imput
speed imput
- Explain how to set up the following controls:
Brillance (2)
Gain (2)
Tuning (2)
Brilliance - Increase to a level which allows the OOW to see the display clearly (2)
Gain - Adiust so that a lightly speckled background of clutter can be seen (2)
Tuning - Adjust for best response from a small target near edge of the display (2)
- State the wavelength and frequency ofthe X-band and S-band radars (4)
X-band 3cm 9GHz
S-band 9cm 3GHz
- list two advantages of X-band over S-band radar (2)
More definition of land
More accurate bearings
- list two advantages of S-band over X-band radar (2)
Better range of detection
Better at seeing in and through precipitation
- Briefly explain how the rain clutter control affects the display (3)
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 (3)
It suppresses gain. It applies this to the area near to the ship (in the center of the radar) 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? (5)
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 (4)
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 (4)
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
It may occur as a result of object external to the ship.
Describe with the aid of diagrams the cause of Side Lobes and the effect on the radar display (4)
Side lobes are escape of electromagnetic radiation 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 (6)
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 ~–.– (5)
PI is a line running parallel to teh ship course at a fixed distance.
The display must be Compass Stabilised and may be either Relative or True motion.
Use 3nm range scale
- Indicate on the display above what would happen as the vessel gets set off track by tide (2)
- Explain how the OOW would regain track and then remain on track (3)
Alter course (into the tide) until the vessel regains track with the Pl off the headland.
Adiust heading to make good the required groundtrack.