Radar and ARPA Flashcards
What are the different errors and 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
