Radar Flashcards
Navigational equipment Required for a vessel 2999GT?
Magnetic Compass
Gyro Compass
Means of taking a bearing
Echo sounder
GPS
LOG
Rudder angle indicator
Radar, both 3 and 9 (what?)
ECDIS
AIS
Signaling lamp
Met Instruments
Search light
EPIRB
SART
BNWAS
LRIT
What is X band and S Band
X band 3cm 9GHz
Shorter pulse, better definition, better for navigation, taking bearings etc
Detects SARTs!
S band 10cm 9GHz
Longer pulse, more energy, better for detecting targets and collision avoidance
Better in Rain
Physically larger scanner
Short Pulse vs Long pulse
Short pulse, higher frequency of pulses
Better definition of return
Lower energy may mean small/distant targets do not get picked up
Long pulse, lower frequency of pulses
Worse definition, blobbier image
Better chance of picking up weak targets
How does sea clutter work
A/C Sea, (STC) Sea control, also called Sensitivity Time Constant
suppresses reflections from waves near own ship. It reduces the sensitivity of the receiver from the centre out.
The proper setting should be such that the clutter is suppressed, and echoes become distinguishable.
If the control is set too high, both sea clutter and true echoes will disappear from the display
How does Rain Clutter Work
Called Precipitation clutter A/C Rain
The rain control suppresses the reflected echoes from rain, hail and snow to clear the display.
Via video processing it does not display the leading-edge returns.
On the X-band radar, because of its short pulse length, the echoes from legitimate
contacts can become lost in the echoes from precipitation, called rain clutter. When rain clutter masks the display, adjust this control to break up the clutter and distinguish echoes.
Adjust the control so that the clutter just disappears; too much A/C rain action may shrink or erase the echoes from legitimate targets.
Difference between shadow and blind sectors
both caused by obstructions on the radar’s horizon
Could be masts, antennas or land masses
Shadow is partially obstructed, causing reduced sensitivity in that area
Blind is fully obscured, no targets will be detected in this area, only multipath echoes (False Echoes)
What characteristics effects the detection of radar targets
MAST
Material - Metal is best
Aspect - The angle can reflect the radar pulse up/down instead of back to scanner
Size - Larger is better, particularly the height of the object
Texture - Very smooth/flat will be harder to detect
This also applies to land/coast. Cliffs good, low lying sand/beaches will be inconsistant
Types of Radar error/limitation
Side lobe effect - spiil from scanner causing smear
Spurious echoes
Radar interference
Indirect echoes - caused by obstructions
Multiple echoes - 2nd/3rd bounce
Shadow and Blind sectors
Radar horizon - dependant on height of scanner to bow
Checking that any video preocessing effects are applied corrrectly or removed to get the true picture
Setting up radar for use
Check for obstructions, workers aloft, permit to work
Brilliance - Adjust for day/night, turn up until the time-base trace is just visable
Range - Suitable for area/traffic
Gain- start at 0, turn up until light speckling appears, then down until its just gone
Tuning - tune for best display. works best with a target, if not, sea clutter, or even your own wake,
Clutter - start from 0, bring up for best display
Re-adjust after 10 mins, when the kit has warmed up
Then every hour, to match the changing conditions
What would you check on the radar before departure
‘Performance monitor’ function, before sailing and every 4 hrs
VRM checked against range rings
EBL checked against ships compass of a conspicuous object
Check Ships head marker by comparing true ships head vs reported ships head on the Radar display
Check the displayed ships head marker is aligned with the For-aft line of the ship, by comparing the True compass bearing of a small distant target, vs the relative bearing on the display
What is the built in radar test called, what is it?
The performance monitor function
Should be done before sailing and then every 4 hours
Produces a plume on the display, showing the power of the magnetron
If the plume is shrinking, the magnetron is losing power
Modern radars do a self test
Magnetrons need replacing every 10,000 hours
Good radar practise/ limitations of radar
Good Radar Practice/Limitations of Radar
* The quality of the performance of the radar needs to be checked regularly: A performance monitor if fitted should be used for this purpose.
* Misalignment of the heading marker, even if only slightly, can lead to dangerously misleading interpretation of potential collision situations.
* Small vessels, ice and other floating objects such as containers may not be detected by the radar.
* Video processing techniques should be used with care.
* Echoes may be obscured by sea or rain clutter.
* Masts or other structural features may cause shadow or blind sectors on the display.
When to use Sea stabilised/ground stabilised
What stabilises it?
Ground/SOG for pilotage/navigation, better fore seeing movement in relation to land/bouys etc, to see the effect of tide
Sea/STW for collision avoidance, gives true representation of aspect, tide is ignored/effects both equally
Can be by doppler log or GPS
Head up, North up, course up differences
Head up, default setting, reverts to this if loses gyro input
matches up to the view out the window
unstabilised, as ship yaws, targets will appear to move as well, difficult to determine risk of collision
North up, Gyro stabilised
Better represents the chart
Course up, gyro stabilised
Better represents the view out the window
True motion vs relative motion
True motion, you will move accross the screen, land will appear fixed
Requires accurate speed and heading info of your own vessel to give accurate speed and heading of another
Relative motion, you will be stationary, in center or offset, land will pass you
