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 input
Speed input
- 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 of the 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.
- What should be checked when parallel Indexing and position fixing? (4)
Heading marker alignment
Gyro error and gyro input
Alignment of radar with gyro input
Positive identification of navigational obiects of interest
- What Is a SART used for and what type of radar would you expect to see its transmissions? (3)
A Search and Rescue Transponder is used for locating a casualty.2
It can be seen on a 3cm (X-band) radar 1
- Describe the use of a True Vector (2)
It indicates the direction a target is moving in (WA in the WOA triangle), on a sea stabilised radar will give STW and CTW
In ground stabilised mode true vectors will give COG and SOG.
- Describe the use of a Relative Vector (2)
It gives indication of risk of collision if in the direction of own ship (OA in the WOA triangle)
It’s a prediction of the target’s relative motion and provides a more clear information about CPA and TCPA
- list five factors to take into account when setting the CPA/TCPA alarm on the radar (5)
The state of visibility
Traffic density
Manoeuvrability of the vessel
Proximity of navigational hazards
Master’s standing orders
- How many targets can the OOW expect to be able to track at one time? (3)
Between 20 and 40 depending on the size of vessel
- List, in order, the six pieces of information ARPA is required to give about a target (6)
Present Range 1
Present Bearing 1
Predicted CPA 1⁄2
Predicted TCPA 1⁄2
Calculated true course 1⁄2
Calculated true speed 1⁄2
- After acquiring a target how long should the OOW wait before being given reliable information based on steady state tracking? (2)
3 Minutes
- Explain what can happen when two acquired targets are moving close to each other on the display and what action the OOW should take. (5)
As two targets get close to one another their tracking gates may overlap The radar may get confused and stop tracking one target with the other one given its vector
One vector should be deleted from this target The target should be re-acquired
- The display below shows an acquired target with a 10 min relative vector which indicates that it will pass astern by 0’.2. It is 6’ away.
If own vessel is on a course of 040 x 12 kn what roughly is the track of the target? (4)
Use the rel. vector to plot an OAW triangle
Calculate WO
WO = 12 x 10/60 = 2’ Plot WO
WA is roughly 2’ long WA= 2 x 60/10 = 12
Target track approx. 080, speed 12 kn
- What would be your action in the above situation if you can visually see the other vessel, give your reasons (4)
This is a crossing situation under (1) rule 15 of the ColRegs with risk of collision. Owrt vessel is stand ori. As the stand on vessel under rule i7, keep course and speed. Continue to monitor track of other vessel.
- Why is it important to use the radar in Sea Stabilised mode for collision avoidance? (3)
If there is a current present the sea stabilised mode will more accurately show the heading of targets on the display and therefore aspect. This is important for applying the ColRegs which are mainly based on aspect.
Define PRF
Pulse Repetition Frequency is the number of pulses transmitted in one second
How do you set up a radar?
Switch off:
- Rain clutter
- Sea clutter
- Interference rejection
- Adaptive gain
When radar has warmed up, adjust, in order:
- Brilliance
- Contrast
- Range
- Gain
- Tuning
- Then: rain and sea clutter,IR, and/or adaptive gain
Describe Sea Stabilisation mode
The Sea stabilisation mode uses compass heading and STW as inputs.
Fixed object have a true vector that is current and/or leeway is affecting your own ship
It should be used for collision avoidance in areas where land is not the major factor in navigation.
Describe Ground stabilisation mode
This mode uses SOG and COG as inputs, so it’s not suitable for collision avoidance but may be useful where land is major feature, like narrow channels or rivers.
Describe ARPA benefits
- Reduces work load
- target tracking is automatic
- Continuous updated inf and rapid situation evaluation
- reduce risk of collision
What additional factors with a radar should be taken into account when determining safe speed (Rule 6)
- The charateristics, efficiency and limitations of the radar
- Any constraints imposed by the radar range scale in use
- effects on the radar detection of sea state, weather and other sources of interference
- The possibility that small vessels, ice and other floating objects may be not detected at an adequate range
- the number, location and movement of detected vessels
- more exact assesment of visibilitu may be possible
Pros and cons X-band radar?
- Better range and bearing discrimination
- Better at picking up targets
- picks up SART
- 3 cm beamwidth 9GHz
- more sea clutter response
Not good in bad weather or long range
Pros and Cons S-band radar
- better in bad weather
- better in long range
- 9cm beamwidth 3Ghz
CONS - Doesn’t pick up SART
- Poor range and bearing discrimination
Describe hotizontal beam width
It is the angular width of the beam, usually from 0.5° to 2°
X band -> 3 cm beamwidth
S-band -> 10 cm beamwidth
Narrower the beam, better the definition
List the factors that determine the practical detection range of the radar
- Height of scanner
- Height and reflective properties of targets
- Atmospheric conditions
- Pulse length
List 3 main factors that affect minimum detection range
- Height of scanner above sea level
- Pulse length
- Vertical beam width of the scanner
- physical obstruction
- receiver paralysis
Describe the minimum range of radar
It is the ability ofthe radar to detect and display echoes near the radar. It is important in narrow channel or in reduced visibility
What does the Ground stabilised show?
- Stationary targets moves with the tide
- Moving targets move. Trail -> ground track
- Land -> stationary on display
What does Sea stabilised shows on the display?
- Stopped targets stationary on the display
- Moving targets move. Trail -> Water track
- Land moves contrary to the tide
How can the radar overlay confirm correct input?
- matching charted features, an off set would indicate GPS error
- rotation would indicate heading input error
Describe the function of radar antenna (scanner) and associated aerial system
The radar must display the range and bearing of targets lying within 360° at a constant speed, so it is refferd to as a scanner, rather than an antenna.
Radar aerials can be slotted waveguide type and can be sitted everywhere.
the latest types don’t use waveguides at all and the signal is transmitted from panels on the rotating aerial.
Describe how bearing is determined by the azimuth of scanner
Describe the errors from Second trace errors
Caused by energy from one pulse being reveived after the next pulse has been sent. It’s possible to get a second trace echo on the screen even the true target is outside the range
Describe the Trial moanoeuvre function
Can be used to carry out tests proposed course/speed changes.
Can be manually or automatically interrupted after a set time.
