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understanding radar
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understanding radar

by Andrew Gliganic

RADAR, or Radio Detection and Ranging, is one of the most useful electronic navigation aids carried aboard  small vessels.  This instrument  provides information which, if interpreted by a trained operator, can prevent collision, determine position, and can be used to tell the course and speed of  other vessels in the area.

Because RADAR can do so much for the navigator, be aware that if your vessel is so equipped, your liability to prevent collision may be altered and a greater burden may be placed upon you.  Generally, a vessel with a functioning RADAR set may be obligated to have it operating while underway, and the navigator is expected to be trained to use it correctly.  It is NOT the purpose of this module to provide that training, only to familiarize the navigator with it's capabilities.  It is recommended that professional, hands-on training be obtained by every RADAR operator. 


RADAR is based on the fact that radio waves travel at a constant speed, namely the speed of light.  RADAR sets transmit brief pulses of radio waves which travel outward in essentially straight lines.  When these waves strike a reflecting surface, like another vessel or the shoreline, some of the energy is returned to the receiver.  By measuring the time difference between the transmission and receipt of these waves, the RADAR set can calculate an object's distance.  It also notes the relative direction of the object and graphically plots this information on the screen.

RADAR signals travel along the "line of sight."  The theoretical maximum range of RADAR depends on the height of the RADAR scanner and the height of the target.    The higher the scanner and/or the target, the farther the RADAR range would extend.  Of course not every RADAR set has the power to reach this maximum range.  Be sure to read the manual.  The RADAR horizon, or the distance it's signal will travel before being blocked by the Earth's surface, is actually slightly farther than the geographical or visual horizon.  This is because atmospheric conditions can bend the RADAR waves over the horizon.



The screen is set up as a Plan Position Indicator (PPI) where the RADAR equipped vessel is at the centre of the screen and the ship's heading faces up.


Fig. E2

In Fig. E2  you can see a situation where the RADAR equipped patrol vessel is steaming towards a larger ship.  The patrol vessel's PPI shows it's own position as a cross hair in the centre of the display.  Straight up on the display is the direction it's heading, whatever direction that might be.  The rings are Range Rings, circles of known diameter used for quickly measuring distance.  The large green blob at the top of the display is the target echo.  Note that an echo shown on the PPI may not correspond in size with the relative size of the target, that is it may appear larger or smaller than the actual object you're looking at.  Consider the edge of the echo that's closest to your vessel as the actual position of the target.  Finally, most modern RADAR sets will give you the Range and Relative Direction of the target, in this case by placing the cursor over the target with the trackball.  Remember that you can convert this relative bearing to an actual bearing by adding your ship's compass heading to the relative bearing and adjusting that number for compass error.  If the resulting number is over 360, just subtract 360 and the remainder is the target's true bearing from your position.  

Of course the RADAR scanner is constantly rotating, so you're getting a 360 picture of what's going on around you.  The image is refreshed with each revolution of the scanner.  Assuming that your vessel is in motion on a constant heading, stationary objects around you will look like they're moving down the PPI at the same speed as your vessel, and on a course opposite yours.  If another vessel were travelling along side you and  at the same speed, it's blip would appear stationary on your PPI.  Vessels ahead on courses that would cross your vessel's bow would appear to travel across the PPI, but remember that the courses and speeds of these vessels as they appear on you PPI are not their actual course and speeds.  It's all relative to what your vessel is doing.  Trained and experienced RADAR operators can quickly calculate their course and speed by plotting right on the RADAR screen with a grease pencil,

but remember that if a target  appears to be closing on you and remains on the same bearing, there is a danger of collision with your vessel.  

Not all targets reflect RADAR waves efficiently.  Surfaces that tend to deflect or absorb the radio energy will not produce a good target on the PPI.  Gradually sloping beaches and  small fiberglass boats are a couple of examples.  Reflective surfaces oriented perpendicular to the direction that the signal is coming from produce the best targets.  Large metal vessels, steep shorelines, and anything with a RADAR reflector on it usually make excellent targets.  

For more information on RADAR;

  • Read the manual.  This document should be kept aboard the vessel.  It explains the proper adjustment and operation of your particular set.

  • Attend a professional, USCG approved RADAR course.  If you have a captain's license, you can take the CG test and get the RADAR endorsement, required for high end licenses and most un-inspected tow boat skippers.

  • Consider purchasing a RADAR simulation program.  There are some very good ones out there, and they are great for learning how to interpret RADAR displays.  

  • Read a book.  Chapman's has a good chapter, and of course Bowditch and Dutton's will tell you everything there is about this and any other subject.