“Marine Radar” Components, Working, Characteristics, Limitations and IMO Standards
What is Marine Radar (Radio Detection and Ranging) ? It is an Antenna fitted on the monkey island (Navigational bridge), which rotate continuously to detect ships, landmass and Navigational hazards in the vicinity, it is an aid to Navigation by which we can determine range and bearing of other targets. It is used for Position fixing and Collision avoidance.
- Scanner or Aerial
- Display Unit
How Marine Radar works:
Transmitter emits Electromagnetic energy in the form of Pulses, through the Scanner at a specific frequency, called Pulse Repetition Frequency (PRF) or Pulse Recurrence Rate (PRR). These Pulses travel at the speed of light (300 000 km/s or 186 000 mi/s) and when they strike any target in their path, they reflected back as echoes. These echoes received by Receiver goes to the Display Unit and show up as a bright spot in the screen of display unit. Display unit is just a screen in which we can see the processed image of the received echoes (i.e. targets) and their is a circular area in the screen in which the targets are visible this circular area is called Plan Position Indicator (PPI). This is how Marine Radar works.
1.Vertical Beam Width (VBW):It is a Vertical angle at the Scanner contained between the upper and lower edges of the radar beam. There are chances if VBW is too small target can be missed and if VBW is too large the Radar energy would spread in a larger section and then the spot on the display unit will be very faint.
2.Horizontal Beam Width (HBW): It is an horizontal angle at the scanner contained between the leading and trailing edges of the radar beam. Echoing from a target commences when the leading edge of the beam touches the target and continues until the trailing edge of the beam left the target. Normally Radar rotates in a clockwise direction so the first edge in the moving direction will be Leading edge and later will be Trailing edge.
3.Pulse Length (PL): It is the total time taken for the Pulse to leave the scanner, basically it is the time period between the leading edge and the trailing edge to come out from the Scanner. Pulse length is controlled by transmitter. It is expressed in micro seconds or meters.
4.Pulse Repetition Frequency (PRF): PRF is the number of pulses emits through the scanner in one second. High value of PRF will result in more clear picture.
5.Wavelength: Wavelength of Marine Radar directly affects its performance. Wavelength for X band is about 3 cm (9.2 GHz to 9.5 GHz) and Wavelength for S band is about 10cm (2.9 GHz to 3.1 GHz). It depends on two important factor Attenuation and Diffraction.
It is the ability of a Radar to clearly distinguish two targets at same bearing and slightly different ranges as two different targets on the PPI. It depends on the Pulse Length.
Is the ability of Radar to clearly distinguish two targets on the same range and slightly different bearings as two different targets on the PPI. It depends on Horizontal Beam Width (HBW).
3.Factors Affecting Minimum Range:
- Pulse Length
- De-ionisation delay
- VBW and Height of scanner
4.Factors Affecting Maximum Range:
- Height of scanner
- Power of the radar set
- Pulse repetition frequency
- Pulse length
- VBW and HBW
- Receiver sensitivity
- Nature of target
- Weather effects
- Anomalous propagation
- Sea and Swell
- -Correct synchronization
- -Uniformity and recti linearity
- -Scale of size of the tracing spot
- -Height of scanner
- Correct alignment between the heading marker and the scanner
- Correct alignment between the heading marker and the bearing scale
- Gyro error, if any when the display is gyro stabilised
- Recti linearity of the trace
- Beam width distortion
- Scale size of the spot
IMO performance standard for Navigational Radar:
- Vertical beam width: The Radar should function without deterioration in performance when the vessel is rolling or pitching up to +- 10 degrees.
- Range Discrimination: Two small similar objects on the same bearing, separated by 40 meters in range, should be separately indicated when using a range scale of 1.5 M or less when they lie between 50% and 100% of the range scale in use.
- Bearing Discrimination: Bearing discrimination should not exceed 2.5 degrees.
- Minimum Range: The minimum detection range, with a scanner 15m high shall not exceed 40m. Targets between 40m and 1M should be displayed without resetting any control other than range selector.
- Range Accuracy: The error in the range of an object, obtained using range rings or the VRM, should not exceed 1% of the maximum range of the scale in use, or 30m, whichever is greater.
- Bearing Accuracy: The Radar bearing of an object, whose echo appears on the edge of the display, should be capable of being measured with an accuracy equal to, or better than +-1 degree.
- Scanner: The scanner must rotate at a constant RPM of not less than 20 (and also stop and start) in relative wind speed up to 100knots.
- Range Sector: The range scales to be available are 0.25, 0.50, 0.75, 1.5, 3, 6, 12 and 24 Nautical miles.
- Performance Monitor: The Radar set must have means of ascertaining the level of its overall performance.
- Horizontal Beam Width: When two vessels are at same range, but slightly different bearing, they should appear as two distinct targets on the PPI. (0.6 degree to 2.0 degree).
Carrying Radar is a requirement by SOLAS (Chapter V- Safety of Navigation). Radar makes the life easier for officers, it is a great navigational aid. Officer who is using radar should have thorough knowledge about Radar. Radar have certain limitations officer should keep that in mind while navigating so the officer of the watch should not be relied on radar they should use it as an aid to Navigation.
Now you know How Marine Radar works and What it is. If their is anything you need to add or have any doubt please let us know in the comment section.