I am currently studying for my Ocean Yachtmaster Certificate. The syllabus requires that we learn about all sorts of different aspects of navigation, safety, boat handling, and much more. So this post is about all I need to know about Radar! For the Ocean Yachtmaster certification, you need to have a general understanding of the following things:
- Principles of radar.
- Components of a radar set.
- Radar navigation.
- Collision Avoidance.
Radar stands for – RAdio Detection And Ranging Radar was primarily used for the detection of other vessels and collision avoidance, and as the technology advanced, it became valuable for navigation and fixing positions.
Radar operates on an echo principle. A stream of radio energy pulses is transmitted in a narrow beam from a highly directional aerial called a ‘scanner’. These pulses may be reflected from objects in the path of the beam – called ‘targets’ and returned to the scanner as ‘echoes’. The speed of travel of the pulses is 300 million metres per second! By timing the interval between transmission and reception of each pulse the radar can determine the range of the target using the time/speed/distance formula.
Echoes are displayed on a screen – sometimes called a Plan Position Indicator (PPI) Your own vessel is in the middle of the display and targets around the vessel are displayed in plan form so that their bearing and range can be determined.
Radars can be interfaced with other navigational instruments – for example the compass and GPS receiver. The information from these other instruments can be incorporated in to the radar display.
You can adjust the range scale to different distances. There is also a heading line indicating the direction in which the vessel is heading. If linked to a compass it can also show the compass heading of the vessel. You can select North Up or Head Up display options.
The vessel remains stationary in the display and the targets move past the vessel – relative motion.
Radar can detect targets which may not be visible to the eye – making it useful for the detection of targets including boats and the coastline in poor visibility and at night.
Many weather conditions, including fog, heavy seas and rain can affect its operation and efficient detection of targets. Radar views the coastline from a sea level perspective – not a birds eye view – so the coastline may appear different to what the chart shows. This can make identification of areas difficult.
Smaller scanners on small boats limit the accuracy of the bearings and this can mean they have limited navigational use.
Radar can only correctly reproduce the nearest side of the coastline at sea level – never points inland or up high.
Poor targets such as low lying land or wooden & fibreglass boats do not reflect the radar signal very effectively, and therefore might not show up. Conical shapes also don’t produce a good echo. It can also not see over its horizon.
Radar can be used to either obtain bearings from your position or distances off/range or a certain target. If the radar is linked to a compass, then the bearing of an object will be a compass bearing and can be utlised accordingly. If it is not linked to a compass, then the bearing is relative to the head of the vessel and needs to be converted in to a compass course before it can be plotted on a chart.
Most radars on small boats are not stabilised with a compass. A properly calibrated radar can accurately measure the distance from any object visible as a target. Radar range can be read from the display by means of fixed range rings or calibration rings – or a Variable Range Marker (VRM), or as a cross shaped cursor which can be moved around the display.
Make sure you positively identify the target that the radar is using with that on your chart! You can then set your drawing-compass to the same charted distance in nautical miles as the measured radar range and plot a position circle on your chart. Combine this with a bearing and you have a position fix!
If the bearing of a ship remains the same on your radar screen over a period of time, then there is a risk of collision.
Clutter (also termed ground clutter) is a form of radar signal contamination. It occurs when fixed objects close to the transmitter—such as buildings, trees, or terrain (hills, ocean swells and waves)—obstruct a radar beam and produce echoes. The echoes resulting from ground clutter may be large in both size and intensity. The effects of ground clutter fall off as range increases usually due to the curvature of the earth and the tilt of the antenna above the horizon. Without special processing techniques, targets can be lost in returns from terrain on land or waves at sea. Clutter is used by the military to jam radars by the use of chaff. Chaff is small reflective material used to hide troop, ship, or aircraft movements by creating many returns and overwhelming the radar’s receiver with spurious targets.
More clutter will usually appear on the screen upwind.
Discrimination – that is the capacity of the radar to distinguish between two targets fairly close together.
CPA – Closest Point of Approach & TCPA – Time to Closest Point of Approach
Jackie made a comment below about CPA & TCPA. This is a method of working out how close a target is going to get to your course and how long it is going to take. Here is a link to the way you can work it all out.
Another thing that Jackie mentions in her comment is Parallel Indexing. This technique involves creating a line on the screen that is parallel to the ship’s course, but offset to the left or right by some distance. This parallel line allows the navigator to maintain a given distance away from hazards. Here is another link to some more information on how to set up parallel indexing.
Thanks for those tips Jackie! Sounds like some more useful things to know!
Here is a link to an article from Cruising World with some great tips about radar usage, new types of radars available, their limitations and where to mount them.
So do you have radar on your cruising boat? Do you enjoy using it? I don’t currently have radar so I would love to know!