Radar, ARPA and PI's Flashcards
What are the different errors of a radar?
Indirect Echoes
- Radar beam is reflected off an obstruction before reaching the target
- Echo will appear on a different bearing/range
- Visually there will be no target
Second Trace Echoes
- Sub refraction, when there is cold air over a warm sea, the beam refracts up, range decreases by 20%
- Super refraction, warm air over a cold sea, beam refracts down, range increases by 40%
- Ducting, a band of warm stagnant air, the beam travels very long distances, land from very far away appears much closer, this is very rare
Multiple Echoes
- Two surfaces will reflect the beam before it reaches the receiver
- Echoes appear on the same bearing at different ranges, closest one is always the true object
- Reduce gain to fix
Interference Echoes
- When two radars are operating on the same frequency in the same vicinity
- Will appear as a spiralling pattern, can interfere with other targets
- fix by using interference suppression button on the radar
Side Lobe Echoes
- When the weaker side beams adjacent to the main beam produce an echo, usually with strong targets at close range
- Appears as multiple echoes at the same range but different bearings
- Fix by reducing the gain
Blind and Shadow Sectors
- When the radar beam is blocked by the vessel’s superstructure so no echoes or only very weak ones will appear
- These sectors should be printed and posted next to the radar
How would you set up a radar for departure?
- Check scanners are clear and no PTW’s for them
- Turn on, radar may need to warm up if starting from cold
- Turn all settings to zero, double-check monkey island
- Transmit
- B, G, T (user manual if required to do it manually), Rain, Sea
- Pulse length
- Appropriate range
- Accuracy of VRM and EBL
What is changing when you are altering the rain and sea clutter?
- Reduces the amplification of signals returning to the receiver but smaller targets can be lost in the process
- Rain affects the whole screen, Sea just around the ship
What are the two types of radar? Which is mandatory? What is each one better at?
- X-band, 9 GHz 3cm and S-band, 3GHz 10cm
- X-band is mandatory for >300 GT and all pax ships because it interrogates a SART
- X-band is better for target acquisition and discrimination
- S-band better for target detection in poor weather because of its longer wavelength
What mode of operation is best for collision avoidance?
- Relative motion, larger look ahead for targets and weather
- Relative vectors, a clear indication of CPA
- True trails, gives an indication of the aspect
- Sea stabilised, a clear indication of the aspect
How many targets can an ARPA track and display info of simultaneously?
40
What visual and audible alarms does ARPA have?
- Comes within minimum CPA set by the operator
- If the target is lost
- If a target comes within a certain range or transits a certain zone
What are the errors of ARPA?
Target Swap
- When two targets are close together ARPA may not be able to tell them apart and their ARPA data can swap
Target Glint
- A large target at close range which is rolling the ARPA may think that it’s moving
Wait Time for ARPA
- The early less accurate data provided ARPA
Sensor Errors
- Will affect WO, in turn, skewing WA, therefore, changing the aspect which could alter how you deal with the situation
Accuracy
- Industry-standard is up to 0.3m
Close Range effect
- Rule 7
What information does ARPA produce?
In 1 min
- relative course (3°)
- relative speed
- CPA (0.3NM)
In 3 mins
- TCPA (1 min)
- True course
- True speed
What are the limitations of a radar?
Clutter
- Interfere with radar display (rule 6 part B) and small targets can get lost in the clutter
- Rain and sea can be adjusted however must be aware that you could also lose small targets this way
Blind and Shadow Sectors
- When the radar beam is blocked by the vessel’s superstructure so no echoes or only very weak ones will appear
- These sectors should be printed and posted next to the radar
False Echoes
- These are the different errors of a radar
Heading Misalignment
- Cross-check with the line of the berth
- Compass Error
Considerations when determining a PI reference point
- Distance of object and likely radar range scale to be used at that time
- Drying heights that may affect the coastline and how it appears on the radar display
- Is the object fixed and radar conspicuous
- When no fixed objects are available the light vessel can be used but consideration must be given to the fact that it is not fixed
Precautions when using PI techniques as per MGN 379
- Does not remove requirement for regular position fixing
- PI only indicate if vessel is left or right of tack not its progress along the track
Checks when using PI techniques as per MGN 379
- Identity of fixed objects
- Radar, overall performance
- Gyro error and accuracy of heading alignment marker
- PI lines are correctly positioned and on a suitable display
- Accuracy of variable range markers, cursor and fixed range rings
Accuracy of Radar
- 1°
- Range rings accurate to 30m or 1% of scale in use
What are the limitations of ARPA?
- Dependant on effciency and set up of radar
- Inaccuracy in speed and heading inputs could lead to inaccrurate ARPA data and misinterpretation of situation
- Obejcts obsurced by rain and sea clutter
- Takes time to produce accurate information
