RADAR Flashcards
Pulse length
Pulse length is determined by the design of the modulator circuit.
A radar typically has more than one pulse duration and is switched automatically as the displayed range increases or decreases.
When very short ranges are selected on display, the transmitted pulse length is
made very short, typically 0.07μ seconds.
Conversely, when the long-range is selected, long pulse duration is employed, typically lμ second.
Some radars use three switched pulse
durations, short, medium and long, in accordance with the range selected on display.
Carrier Frequency
Carrier frequency is the radio frequency on which the transmission is made.
Marine radar equipment is manufactured to operate in either one of two frequency bands.
‘X’ Band (3 cm) 9 GHz
‘S’ Band (10 cm) 3 GHz
‘X’ Band (3 cm) 9 GHz
Better resolution, preferred for navigation
‘S’ Band (10 cm) 3 GHz
Better in rain, longer range
Sea Clutter
A/C Sea (STC): Sea control, also called STC (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 echoes will disappear from the display.
When there is no sea clutter visible on display, turn the control fully counter clockwise.
Precipitation Clutter
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, the echoes from legitimate contacts can become lost in the echoes from precipitation, called rain clutter, because of its short pulse length. 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.
Detection characteristics of radar targets
The ability of the radar set to produce a paint on the screen which represents a target depends largely upon the amount of energy that the target can reflect back to the scanner.
“MAST”
Material - Good conductors of electricity usually make good reflectors of radar signals. Eg Steel
Aspect - describes how the target is presented to the radar signal.
Size - the larger a target, the better the chances of detection. (height > width)
Texture - rough-surfaced targets will almost always give better returns than smooth targets of poor aspects.
Setting up a radar
check that the radar is free to rotate
“BIG RANDY GIRLS TAKE COCK”
BRILLIANCE -
RANGE -
GAIN - ensure that weak echoes will be seen.
TUNING -
CLUTTERS - sea, precipitation
Errors of radar
Errors of Radar
• Side Lobe effects
• Spurious Echoes and Effects
• Radar Interference
• Indirect Echoes
• Multiple Echoes
• Shadow Sectors
• Blind Sectors
• Radar Horizon
Pre departure radar checks
The performance of the radar should be checked using the Performance Monitor before sailing and at least every four hours whilst a radar watch is being maintained.
VRM Checks against range rings, ERBL cross-check against compass.
Good Radar Practice
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 a dangerously
misleading interpretation of potential collision situations.
• The radar may not detect small vessels, ice and other floating objects such as
containers.
• 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.
Performance Monitor
The performance monitor provides a check of the performance of the transmitter.
Being limited to a check of the operation of the equipment, the performance monitor does not provide any indication of performance as it might be affected by the propagation of the radar waves through the atmosphere. Thus, a good check on the performance monitor does not necessarily indicate that targets will be detected.
When the performance monitor is used, a plume extends from the centre of the PPI.
The length of the plume, which depends on the strength of the echo received from the echo box in the vicinity of the antenna, indicates the performance of the transmitter and the receiver.
The length of this plume is compared with its length when the radar is known to be
operating at high performance.
Radar Displays
•Un-stabilised means no compass input. Relative bearings.
• Stabilsed means gyro fed in.
•H-up un-stabilised by any external heading input uses that it was installed forward- facing. Alterations of heading are shown by the entire image rotating away from the turn. The bearing will be relative to the ships head.
•C-up display is stabilised by an external heading input; small alterations of headings are shown by the heading marker swinging side to side. The bearing can be taken relative or compass bearings.
• N-up image is stabilised, and it aligns with a chart or chart plotter. This is the
standard setting.
•True motion and relative motion - True motion moves across the screen.
•Sea and ground stabilised. Log input.
•Sea stabilised effects of tide and wind on your vessel are accounted for any free-
floating or stationary object, or moving targets will be shown with their true heading calculated via ARPA. For collision avoidance, it must be sea stabilised.
•Moving vessels have their course and speed calculated by ARPA.
•Stationary free-floating objects appear stopped.
•Anchored vessels navigation marks appear to move in the opposite direction of the combined effect of tide and current.
•Ground stabilised GPS feed in or doppler operating in BT ARPA could be wrong as it
uses your course and speed to give out course and speed of other vessels. Can be
used for pilotage or navigation but not collision avoidance.
Sea and ground stabilised - which log input
What is a Magnetron
This devices converts the extra high tension electrical energy into microwave radio energy. The
Being a physical rather than electronic unit, magnetrons have a limited lifespan.
