Radar and ARPA Part 1 Flashcards
Explain why ranges are more accurate than bearings when plotting a ship’s position
- Bearings suffer from half-beamwidth error.
- 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 differentiate between two targets on the same bearing and similar ranges
- LONG pulse gives POOR range discriminiation
- SHORT pulse gives GOOD range discrimination
Define bearing discrimination
The ability of the radar to differentiate between two targets at the same range and similar bearing
- WIDE horizontal beam width gives POOR bearing dsicrimination
- a NARROW horizontal beam width gives GOOD bearing discrimination
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.
- if error in heading marker alignment.
- Apply 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 - gyro
Speed input - from log
- Explain how to set up the following controls?
Brilliance
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
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.
What five features would you consider when determining a good radar conspicuous object?
M.A.S.T.S
- Material of the object
- Aspect
- Size (Height and breadth above
radar horizon) - Texture of the surface Shap
- Shape which is better cube or sphere?
Describe with the aid of a sketch the cause of Multiple Echoes
( Ship diagram)
- 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 a sketch the cause of Multiple Echoes and the effect on the radar display (Radar diagram)
- 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 ( ship Diagram)
- The radar pulse bounces on 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 Indirect Echoes and the effect on the radar display ( Radar diagram )
- The radar pulse bounces on 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
- The 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 a sketch the cause of Blind and Shadow Sectors and the effect on the radar display
Blind sectors - radar transmissions are completely obscured and no echoes will be found.
Shadow sectors - areas of reduced sensitivity where radar may pick up some targets.
Describe relative motion
Predominantly radar
- The land moves
- We stay fixed to the screen
Describe true motion
Ecdiss display
The land stays fixed
We move across the screen
Define ARPA
Automatic radar plotting aid -
* Is a processor that looks for movement of selected target
Define Box Aquisition Phase
- When a target is initially aquired (manually or automatically)
- The ARPA will process the box Aquisition phase
How long does ARPA need for steady tracking
2 - 3 minutes
Define trial mounever mode
- This allows you to see what effect a change in course and or speed with or without delay.
- very useful in heavy traffic situations
