RADAR Flashcards
What is RADAR
Radio Detection And Ranging
Principle of operation of radar?
Echo ranging principle ( SPEED x TIME) / 2
What shape is a radar and why
Rectangular, so that the leading edge of the transmitted and returned pulse is vertical
X band characteristics
3cm 9-10 GH/z
High resolution picture for precise Navigation
(Better definition of land)
More accurate bearings
S Band characteristics
10cm 3GH/z
Sacrifices High resolution for relative freedom from Sea and Rain clutter
Improved range
What does the transmitter do?
Produces short, high powered pulses of RF
Modern Transmitters are sited at or near the site of the scanner to minimise the waveguide length
Function of the Scanner/ Antenna
Propagates radio energy in a highly directional manner. Should be sited to avoid blind sectors
Function of the Receiver
Amplifies the weak pulses and produces them as video pulses for eventual display. Usually sited close to transmitter
What does the Display do?
Screen (plan view)
Should be sited with and near nav bridge equipment
Function of Time synchronised circuits
syncs transmission with the method of display and generation of timing marks on the display
Waveguides
Copper pipe for Radio energy.
Dimensions are critically related to wavelength in use
Long waveguide means high loss of energy
What determines the Pulse length
Determined by range scale in use
What determines the Pulse Repetition frequency?
Determined by range scale in use.
Can be fine tuned once at desired range, for need of function.
Short ranges have a high PFR
Long ranges have a low PFR
Principal of Range discrimination
Ability to detect discriminate between two targets on the same bearing and closely spaced in range
2 targets on the same bearing must be separated by half a pulse length
Bearing discrimination
Ability of the radar to discriminate between two targets at the same range but closely spaced in bearing
Narrow horizontal beam width is required for bearing discrimination
Greater the length of the scanner, the narrower will be the beam width
What is the Sea clutter function?
Suppresses the gain, applies this to the area near to the ship. Never extends the whole range
Also known as Sensitivity Time Constant (STC)
Only to be used if sea conditions require it
What is Side lobe error and at what range is it more likely?
Dick and balls
Side Lobes extending from the main beam catch targets close by, displaying them at the same range, extending out from the main target
Usually with large targets at close range
What is Radar interferance, and at what range is it more pronounced?
Near other radar instillations within the same frequency
Curved or spiralling dashed lines which change in shape and position with each revolution of the trace
Can be minimised by the interference rejection function. Slightly modulates the frequency. This should be changed back after use.
Most pronounced in Long ranges
Equations for range of targets on radar
Range = speed x Time/ 2
Range = speed x Time / ( 2 x 1852 )
Speed value for radar
Speed = 300m/Us1(Speed of light)
How do we check and correct heading misalignement
take visual bearings at different points on the compass, compare these with RELATIVE compass bearings. Any difference in radar bearings is the error.
Bearings vs ranges in accuracy
Ranges are more accurate, bearings suffer from half beam width error.
Half beam width error, due to the width of the pulse the painting of a target is not the centre line from which the bearing should be measured from.
What two inputs should you check when setting up a stabalised radar?
Heading input (Gyro)
Speed input (STW/SOG)
How do we set up Brilliance, Gain, Tuning
Brilliance: Increase to a level which allows the OOW to see the display clearly
Gain: adjust so that a lightly speckled background of clutter can be seen
Tuning: adjust for best response from a small target on the edge of the display
What is Rain Clutter on a radar and how dows it work
Cuts through range clutter and interference.
Does this by trying to cut off the trailing edge so that just the leading edge is seen.
Also known as FAST TIME CONSTANT (FTC)
5 features in determining a good radar conspicuous object
1.Material
2. Aspect
3. Size (height and breadth above horizon)
4. Texture of surface
5. Shape
What are multiple echoes
When an object close by, be it a ship or a building, sends the pulse back before the receiver is open.
This results in multiple false echoes, extending behind the original echo. can happen multiple times at close range
What is an Indirect Echo
Pulse bounces off an object on board own ship before hitting the target.
The radar thinks the target is in the direction of the obstruction, but at the range of the target.
What is a Second Trace Error?
False targets may appear on the same bearing but up to half the distance, i.e. target is at a greater range than the range scale in use but is still displayed on PPI
Increasing range scale (therefore reducing PRF) and visual observations will help
Occurs due to extra super refraction on very fine days
Occurs commonly and is hard to detect
SART
X band 3 cm radar, Used for detection of casualties in SAR situations
True vector
Indicates the direction the target is moving (heading on a sea stabilised radar)
Factors to consider when setting CPA/ TCPA alarm on radar
Sate of visibility
Traffic Density
Manoeuvrability of vessel
Proximity of navigational hazards
Masters standing orders
In order, 6 pieces of information ARPA gives about a target
- present range
- present bearing
- Predicted CPA
- predicted TCPA
- Calculated True course
- Calculated True speed
Time gap between target acquisition and reliable information
3 minutes based on steady state tracking
What happens when two targets are moving close to each other, and what action should the OOW take
Their tracking gates may overlap. the radar may get confused and stop tracking one target with the other on given its vector.
One vector should be deleted from this target
Target should be re acquired
Why is sea stabilised mode important for collision avoidance?
If there is current present the sea stabilised mode will more accurately show the heading of targets on the display and therefore aspect. Important for ColRegs which are mainly based on aspect.
What is Attenuation?
Which radar suffers more heavily from attenuation?
X or S band?
The scattering and absorption of the energy in a radar beam as it passes through the atmosphere. Attenuation is greater at the higher frequencies or shorter wavelengths.
X band suffers more heavily in heavy rain
How many targets can an OOW expect to have selected for ARPA?
20-40
characteristics of Head Up Orientation
Heading marker fixed to 000
Information relative to own vessels movement
“Unstablilised Course up”
In case of Gyro failure, Radar will revert back to Head up mode as default
Will be effected by ‘yaw’ and smear targets across the screen
Characteristics of Course up Orientation
Similar to head up, but will take yawing into account with a cleaner image
Similar to head up but the heading marker will make small changes instead of the entire screen
User must ‘reset’ the display after an alteration of course
Characteristics of ‘North up’ Display
Compass stabilised, 000 is at the top of the screen and the heading marker is positioned relative to true north
Land and targets stay fixed and heading marker moves during a turn with no ‘smearing’
This view also corresponds best with the Chart and ECDIS/ECS and therefore better for Navigation.
Can take more time to become accustomed to especially when headed South.
3 Main factors determining range
Vertical Beam Width (VBW)
Height of Antenna
Pulse length
How to setup a RADAR
- Check for Obstructions
- Turn on Radar (allow magnetron to warm up)
- Press transmit (turn controls to 0)
- Brilliance
Range
Gain
Tune ( Auto tuning )
Clutter ( rain and sea )
How does a radar determine bearing?
It continuously sends pulses throughout a 360 degree arc of the horizon, and compares received signals to a fixed heading marker
How is True bearing determined?
Through a compass stabilised radar, converting relative bearings to the Ships head, to True
(In TRUE MOTION mode)
What is the function of the magnetron?
Sited in the Transmitter, it determines the Radio frequency of the pulse
What is the function of the transmitter?
The Transmitter will generate the pulse
Trigger controls the PRF
A modulator will define the power, length and shape of the pulse
A magnetron will determine the radio frequency of the pulse
What is the purpose of the waveguide?
Channels the pulse from Transmitter to T/R Cell and then to Antenna and then from Antenna to T/R Cell and on to Receiver
Ensure minimal loss of energy
Function of the Duplexer?
Acts as a one way gate valve
Opens and closes for transmitting and receiving
Also aids to protect electronic equipment from high energy levels on the TX side reaching the RX side
Function of the Antenna?
Radiates the pulse out into space in a highly directional manner through 360⁰ of arc
Catches return echos
Most common type is ‘Slotted Waveguide’. Slots are precision engineered to IMO Standards
What is the Function of the Receiver/Amplifier?
Receives signals and amplifies them to send to the display
Rain and sea clutter are also a function of the receiver
Protected from high energy by the TR Cell
Function of the display?
Indicates the presence of detectable objects to the user
Heading marker displayed to indicate the fore and aft direction of the vessel
What determines Maximum Detection Range?
Height of Antenna
Target characteristics
Refraction
Pulse length
How often should we run the performance monitor function
Once per watch and record
What are blind and shadow sectors?
Blind and shadow sectors are fixed areas that the radar’s view is partially (shadow) or totally obstructed (blind).
Vessels should have a blind and shadow sector card posted next to the radar display clearly indicating where these areas are.
Likely causes are any objects visible to the radar’s horizon such as:
Funnels, Masts, Cranes and super structure/Bow structure, etc.
How can we find our Shadow/Blind Sectors?
In the instillation Manual, in the Yellow booklet provided
By turning up the gain and looking for these sectors when operating the radar.
How do we Tune our radar?
Adjust until we have the best visual picture
NOT AUTO for the sake of the exam
How do you know if it is correctly tuned?
In coastal nav?
At sea?
At sea with no swell?
Tune to a conspicuous object/point of contact
At sea, tune to the sea clutter
If no swell, turn vessel to create wake and tune to the vessels own wake
Pre departure checks of a RADAR
Seen in MGN 379
Visual check for damage/obstruction
PM function
VRM against rings
EBL of a point, compare this to compass bearing of said point
Check heading marker, fore and aft centreline of the ship
(pick a target as close to the edge of the display as possible. Check this against the compass bearing of said object
Check heading marker alignment against compass course
Then check….BRGTC
ARPA alarms
Target has breached user-set CPA and TCPA
Tracked target is lost
An input failure (gyro, speed etc)
A target enters a guard zone (one that is not already tracked)
Picture freeze
