CWO Zeezeilschool | RYA Training Centre | Jachtverhuur | Est. 2000

Lesson 14 RADAR

Radar is an electronic navigation tool that allows us to determine the distance and direction to coastlines, buoys and ships. The radio radiation that bounces off targets and is received by the antenna can be seen on the radar screen as echoes.

Position on the radar screen

The position of the ship is in the middle of the screen, with the exception of the "offset option" which is also called "true motion", the radar is then linked to log and compass. The image freezes and the ship sails across the screen.


Individual targets such as ships, buoys, oil platforms are easier to see than e.g. low coastlines. The size of echoes is not a good measure of the size of the target.


The range indicates the area monitored by the radar.

Range rings

These are the rings with which distances to targets can be determined.


Electronic bearing Line is a line to make bearings on targets.


The variable range marker is a adjustable circle to determine the distance to a target.

Guard sectors, alarms and watch mode

Modern radars have the option to set an area so that when a target enters that area, a signal sounds. It should be noted that this should not replace crew watchkeeping.

radar guardzone

Heads-up versus North-up mode

Heads-up means the boat is pointing to the top of the screen. This setting is often used when the RADAR is used to recognize collision courses
North-up means that the top of the screen is North (there is a link to the electronic compass). This setting is often used when the RADAR is used for navigation purposes. The RADAR screen can be "projected" directly on the nautical chart, without having to rotate the image in your mind. The advantage of this is also that with course changes (for example, swinging on high waves) the bearings / ranges on the screen do not change.

Relative vs. true bearings

Relative bearings are relative to the ship's course. True bearings are relative to the north. The latter is possible if you choose North-up.

Stabilized vs. unstabilized radar

Yacht radars are unstabilized. The boat is in the center of the screen.


All radars should warm up for 1 to 2 minutes.

Stand-by mode

Most radars have a standby mode, so that the device remains ready for use, without much power consumption and without broadcasting. 45% less power is used, normally 33 W to 18 W in standby mode.


Adjusts the brightness of the screen. Depending on whether it is day or night or sunny, we use that function.

Tuning (tuning bar)

"In tune" means that the targets are sharply displayed on the screen. Modern radars have automatic frequency control (AFC).


With the Gain function, the displayed echoes can be enlarged. This is the main tuning function for the radar and usually needs to be adjusted for large range changes. With Gain we can omit weak echoes, so that only the important echoes remain on a clear screen with few echoes. But too much gain means that even weak targets are not visible on the screen.

Sea clutter (AC sea of ​​STC)

Large waves are also echoed on the screen. High waves can make the screen cluttered, so that small targets are no longer clearly visible. Use this function as little as possible. Targets at a short distance can disappear. The sea clutter filter works up to 4 Nm. Turn off the function at calm sea. In rough seas and small ranges, the echoes become one. In these cases, this function should be increased until the echoes separate from each other.

Rain clutter (FTC)

Also rain showers or snow can be seen on the radar. That can be very useful, for example, to avoid a squall. But it can also prevent you from seeing weak echoes from ships. This filter allows targets to be visible that would otherwise not be visible due to rain or snow. The FTC filter works across the entire display.

Echo stretch

This option can extend echoes. E.g. to find a small echo. This function is normally disabled.

Interference rejection

Other radars can also interfere with our radar. This function can filter the disturbance from other radars. We will be able to observe the disturbance especially on busy waterways.

Zoom and offset (shift)

Offset or shift allows us to shift the center of the screen from our own position so that we can view a specific area. Return the screen to normal position after use to avoid confusion.

Heeling angle

It is important for sailing ships that sail at an angle that the antenna remains straight.

6 minute rule

With many calculations, it is useful to take 6 minutes as the time interval.
For example: How many Nm have we sailed in 6 minutes at a speed of 5 knots?
Distance traveled in Nm = speed in knots X the number of minutes / 60 minutes = 5 X 6/60 = 0.5Nm. If we take 6 minutes as the default time interval, we can use this formula: Distance traveled in Nm = Speed ​​/ 10
Example: If we want to calculate how fast our target moves if it has moved 2.5 Nm in 6 minutes, we can use the following formula: Speed ​​= 10 x the distance = 25 knots.

ARPA and Marpa

ARPA stands for Automatic Radar Plotting Aid. For example, ARPA calculates the point that a target comes closest to your ship, where it happens, what time it happens, etc. To better understand ARPA, you can make the case below. The advantage of ARPA radars over manual calculations is that they provide faster and better information and we can monitor several ships at the same time. Marpa stands for mini arpa, which can also be found on yachts.

Cases Preventing collisions

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Case 1 Radar

Our own course is 000 degrees and our ground speed is 5 knots.

Suppose we gauge a target on the radar at 10.00 and find Bearing 40 degrees and range 4 Nm.

At 10.06 we probe the target again and find: bearing 20 degrees, range 3Nm.

We plot these two targets in the radar transfer plotting sheet.

The first we label R and the second plot we label m.

If we draw the line from R to M, we find the speed of relative motion (SRM) and the direction of relative motion (DRM).

In this example:

SRM: 1.5Nm per 6 minutes, or 15 knots. You can measure the speed with a compass (distance between R and M) and read it on the correct scale on the left side of the radar transfer plotting sheet. In this case, that is the scale that goes up to 6Nm.

DRM: 259 degrees

If we draw the line all the way, we find the RML, the relative motion line, that predicts how the target will move across our radar screen.

This line is equal to the DRM: 259 degrees

Relative motion line

Next, if we draw a right angle on this relative motion line to the center of the radar transfer plotting sheet, we find the closest point of approach (CPA).

We can measure the Bearing to the CPA.
350 degrees

We can measure the distance to the CPA.
2.5 Nm

We can also calculate the Time of CPA. This calculation is very similar to the calculation for an ETA.
This is the formula:
TCPA = time at M, plus the travel time from M to CPA. We also call this travel time MCPA, minutes to CPA. The MCPA is distance / speed x 60 minutes.
Now we will enter the values ​​in the formula.
10.06 + (1.5 / 15kn) x 60 = 10.12

CPANow we can also calculate the actual speed (true speed) and true direction (true motion) of the target. We do this by plotting our own speed (in 6 minutes) to R. 5 knots, becomes 0.5 Nm in 6 minutes. At the end of our own velocity vector, we note E.

Next, we draw a line from e to m. the direction of that segment is the target's true motion.

In this case it is: 280 degrees

The length of the line from e to m is the actual speed: 1.6Nm in 6 minutes is 16 knots.

true speed of motion and true direction of motion

Case 2 Radar

North-up: range 6 NM
Our course is 0 (t) and our speed is 5 knots SOG
R at 15.20: 240 degrees, 5nm
M at 15.26 222 degrees, 3.7nm

Specify the following:

Distance to CPA
Direction of true motion
Speed ​​of true motion


Bearing to CPA 189 degrees
Distance to C


Bearing to CPA 189 degrees
Distance to CPA 3.1 NM
SRM: 1.9 Nm in 6 minutes, so 19 knots
DRM: 100 degrees
True motion : 81 degrees
Speed ​​motion: 18.5 knots
Minutes to CPA: Distance from M = 2Nm / 19 * 60 = 6.3 minutes
TCPA = 15.26 + 6 minutes = 15.32

Case 3 Radar

Heads-up: range 3 NM
Our SOG is 5 knots
R at 05.10: 228 degrees, 2.5 nm
M at 05.16 310 degrees, 1.5 nm

Specify the following:

Distance to CPA
True motion
Speed ​​of true motion
Minutes to CPA


CPA 262 degrees
Distance to CPA 1.1 NM
SRM: 1.2 Nm in 6 minutes, so 12 knots
DRM: 172 degrees
True motion: 170 degrees
True speed: 6 knots
Minutes to CPA: Distance from M = 1.1 Nm / 12 * 60 = 5.5 minutes
TCPA = 05.16 + 6 minutes = 05.22

Radar and navigation

Radar has a second function besides preventing collisions, which is determin your position. With the radar we can determine the direction and distance to a charted point, which we can draw on the chart. The variable range marker (VRM) and Electronic bearing line (EBL) make this very easy. The radar is probably one of the most important navigation equipment on board. GPS can provide a more accurate position than the radar, but often we do not need that accuracy on the water. A radar can warn us about collisions. The GPS (especially with an electronic chartplotter) is better in terms of position determination. A normal position determination can be to plot the GPS position in the chart and determine the distance and direction from that position in the chart to a point to be recognized. Then check this with the radar. We also check whether the depth sounder indicates the expected value. This also makes it easier to interpret the radar screen. So we go from radar to map and back again to keep a good overview.

Land identification

Identifying landmarks is easier with tall, steep, clearly defined objects with a unique shape. Such as an oil platform or small island with a steep high coast. A RACON is an ideal target. It gives a Morse code on the Radar screen towards the edge of the radar screen.

Radar horizon

The following formula can be used to calculate the maximum range of a radar:
Maximum radar range in Nm. = 2.1 x (square root height antenna + square root height target). Higher targets can be seen beyond the horizon.

Determine a fix with the Radar

The determination of the fix using the radar is as follows, if the Radar is set in the North-up mode.
1. Place the EBL (Electronic Bearing Line) on the landmark, for example a lightship or RACON. In this way, a bearing is made similar to a compass bearing. We can draw the obtained Line of Position in the map.
2. Determine the distance to the landmark with a VRM (Variable Range Marker) and write the distance on the LOP (Line of Position) on the map, thus finding your fix.

When the radar is set in the Heads-up mode, we use the radar to make bearings relative to our course. Of course, that bearing must first be converted into a true bearing relative to the north, before that bearing can be drawn into the chart.

Determination of the fix by multiple bearings

Determining a fix by means of two radar bearings on two landmarks actually works the same as a cross bearing that we can make with a bearing compass. Radar bearings are less accurate than compass bearings, because targets are often stretched on the radar screen.

Determining the fix with a number of Range Rings

Determine the distance to two or 3 landmarks with the VRM (Variable Range Marker).
Using a pencil drawer dividers, draw the circles in the map with the targets as the center of the circles.
Where the circles meet is our fix. Distances are more reliable than bearings using RADAR.

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