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Radar on board

What is Radar?

Radar is an electronic navigation aid that allows us to determine the distance and direction to coastlines, buoys and ships. The radio radiation that bounces back against “targets” and is received by the antenna we see on the radar screen as echoes.

Position on the radar screen

The ship’s position is in the centre of the screen, except for the “offset option” this is also called “true motion” the radar is then linked to log and compass. The image is stationary and the ship is moving across the screen.


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


The range indicates the area observed with the radar.

Range rings

These are the rings used to determine distances to targets.


Electronic bearing Line is the bearing line to take bearings on targets.


The variable range marker is a ring that allows us to determine distances to a target.

Guard sectors, alarms, and watch mode

Many modern radars have the ability to set an area so that if a target enters that area a signal sounds. It should be noted that this should not replace watchkeeping by crew.

radar guard zone

Heads-up versus North-up mode

Heads-up means that the sail line is the ship’s keel line. This setting is often used when the RADAR is used for collision avoidence.
North-up means that the sail line is north (there is then a link to the electronic compass). That setting is often used when the RADAR is used for navigation. This is because you can “project” what you see on the RADAR screen directly onto the nautical chart, without having to rotate the image. The advantage is also that in case of course changes (e.g. swinging on high waves), the bearings to targets on the screen do not change.

Stabilised vs. unstabilised radar

Radars of yachts are unstabilised. The boat is in the centre of the screen.


All radars need to warm up for 1 to 2 minutes.

Standby mode

Most radars have a standby mode to keep the device ready for use, without using much power and without transmitting. 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 the targets are sharply displayed on the screen. Modern radars have “automatic frequency control” (AFC).


The Gain function can be used to increase the echoes displayed. This is the main tuning function for the radar and usually needs to be adjusted for large changes in range. Gain allows us to omit weak echoes, leaving only the important echoes on a clear screen with few speckles. But too much gain means that even weak targets are not visible on the screen. Gain should be adjusted when large targets are close and when rain clutter is used.

Sea clutter (AC sea or STC)

Large waves are also displayed as echoes on the screen. High waves can make the screen cluttered, making small targets difficult to see. Use this function as little as possible. Targets at small distances may disappear. The sea clutter filter works up to 4 Nm. Turn the function off in calm seas. In rough seas and at small ranges, the speckles become one. In these cases, increase this function until the echoes separate.

Rain clutter (FTC)

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

Echo stretch

This option can extend echoes. E.g. to find small echoes. Normally, this function is off.

Interference rejection

Other radars can also interfere with our radar. This function can filter the interference from other radars. Especially on busy waterways, we will be able to observe the interference.

Zoom and offset (shift)

Offset or shift allows us to shift the centre of the screen away from our own position to view a particular area. Return the screen to normal position after use to avoid confusion.

6-minute rule

Among the many calculations, it is convenient to take 6 minutes as the time interval.
For example: How many Nm did we travel in 6 minutes at a speed of 5 knots?
Distance travelled 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 standard time interval then we can use this formula: Distance travelled in Nm = Speed / 10
Example: If we want to calculate how fast our target sails when it has moved 2.5 Nm in 6 minutes, we can use the following formula: Speed = 10 x distance = 25 knots.

ARPA and Marpa

ARPA stands for Automatic Radar Plotting Aid. ARPA calculates, for example, the point 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 multiple ships at the same time. Marpa stands for mini-arpa, which can also be found on yachts.

Cases Preventing collisions

To create the following cases, you will need the Radar plotting sheet.

Case 1 Radar

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

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

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

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

We label the first one R and label the second plot 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 appropriate scale on the left-hand side of the radar transfer plotting sheet. In this case, it is the scale rising to 6Nm.

DRM: 259 degrees

Drawing the line all the way through, we find the RML, the relative motion line, which predicts how the target will move across our radar screen.

This line is equal to the DRM: 259 degrees

relative motion line

If we then draw a perpendicular line on this relative motion line to the centre 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 or CPA. This calculation is very similar to the calculation for an ETA.
The formula is this:
TCPA = time at M, plus 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 are going to enter the values into the formula.
10.06 + (1.5 / 15kn) x 60 = 10.12


Now we can also calculate the true speed and true motion of the target. We do that by plotting our own speed (in 6 minutes) to R. 5 knots, therefore, 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 line segment is the true motion of the target.

In this case, that is: 280 degrees

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

true bearing and course

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

Determine the following:

Distance to CPA
Direction of true motion
Speed of true motion

elaboration case 2 radar


Bearing to CPA 185 degrees
Distance to CPA 2.9 NM
SRM: 2 Nm in 6 minutes, i.e. 20 knots
DRM: 95 degrees
True motion: 83 degrees
Speed true motion: 20 knots
Minutes to CPA: Distance from M = 2.1nm / 20kn * 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

Determine 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, i.e. 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 and collision prevention, namely navigation. With radar, we can determine the direction and distance to a mapped land point, which we can plot on the chart. The variable range marker (VRM) and Electronic bearing line (EBL) make this very easy. Probably one of the most important navigation equipment on board is the radar. GPS can provide a more accurate position than the radar. But often, on clear water, we don’t need that accuracy. A radar can warn us of collisions. The GPS (especially with an electronic chart plotter) is better at position determination. A normal position determination can be to plot the GPS position in the chart and from that position in the chart determine the distance and direction to a recognisable point. Then checking this with the radar. We also check whether the depth sounder indicates the expected value. This will also make it easier for us to interpret the radar screen. So we go from radar to chart 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 steep high coast. A RACON is an ideal target. It provides 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 (root height antenna + root height target). Higher targets can indeed be seen beyond the horizon.

Determining fix with the Radar

Determining the fix using the radar is done as follows, when the Radar is set in North-up mode.
1. Set the EBL (Electronic Bearing Line) to the landmark, e.g. a lightship or RACON. In this way, a bearing is made similar to a compass bearing. We can plot the obtained Line of Position on the map.
2. Determine the distance to the landmark with a VRM (Variable Range Marker) and draw the distance in, on the LOP (Line of Position) on the map, thus finding your MWS.

When the radar is set in Heads-up mode, we use the radar to probe relative to our heading. Of course, this bearing must then be converted to a true bearing relative to north before it can be plotted on the chart.

Questions & Answers

Question 1: When do you use North-up mode on the radar?

a: In collision avoidence
b: navigating

Question 2: In heads-up mode, if we make a relative radar bearing of 40-degree on a beacon on the port side at a true course of 220 degrees, what is the true bearing?

a: 200 degrees
b: 260 degrees
c: 180 degrees

Question 3: If we make a bearing on a ship on a true 290-degree course on the radar in heads-up mode at 45 degrees to starboard and a few minutes later at 15 degrees to starboard…

a: Then the other ship goes in front
b: Then the other ship goes behind
c: Then there’s a collision course

Question 4: If we on a true 290-degree course on the radar in heads-up mode make a bearing on a ship at 45 degrees over starboard and we want to see that ship with our own eyes, in which compass direction should we look for that ship if we want to observe it with our eyes?

a: 335 degrees
b: 225 degrees
c: 345 degrees

Question 5: In the previous question, will you see more of the side or will the ship come more straight towards you than you would expect just after your radar bearing?

a: No difference between radar screen and reality
b: More of the bow
c: More of the side

Question 6: Use the radar plotting sheet for questions 6 to 9. On a course of exactly 0 degrees in the Heads-up mode you will see an echo at 46 degrees over starboard with a distance of 1.62 Nm. 6 minutes later the echo is at 64 degrees with a distance of 1.18 Nm. Your own speed was 4 knots. What was the DRM of the other ship?

a: 190 degrees
b: 160 degrees
c: 210 degrees

Question 7: What was the SRM of the other ship?

a: 4 knots
b: 6 knots
c: 8 knots

Question 8: What was the true course of the other ship?

a: 190 degrees
b: 211 degrees
c: 232 degrees

Question 9: What was the speed of the other ship?

a: 2.3 knots
b: 4.3 knots
c: 0 knots

Question 10: You are sailing on map 1801.6 from Blankenbergen to Hoek van Holland. You will see the Morse code T on your radar screen. An EBL indicates that the true bearing is 311 degrees. A VRM indicates that the distance is 6.7 Nm. What is your position?

a: 51 51.2N and 003 42.4E
b: 51 50.2N and 003 48.4E
c: 51 51.2N and 003 48.4E