HSSC Introduction to Radar

This is a very brief introduction to radar without going into the niceties of plotting on graph paper or a chart what an actual set of conditions would look like on radar.

Radio Detecting And Ranging

Radar is the method of locating an object by measuring how long a pulse of energy takes to go to that object and come back.  To determine the distance listen for the echo to come back and halve the time because, as said, the energy has go there and come back.

For humans and bats, with sound travelling at the speed of sound of 343.2 m/s (1,126 ft/s) or 1,236 kilometres per hour (768 mph), this is roughly one kilometre in three seconds or approximately one mile in five seconds.  Radar uses microwave electromagnetic energy pulses in much the same way, but here the speed is that of light, or 300 km/s, or 3*108 m/s rather than sound.

The radio-frequency (rf) energy is transmitted to the reflecting object.  A small portion of the reflected energy returns to the radar set.  This returned energy is called an ECHO, just as it is in sound terminology.  Radar sets use the echo to determine the direction and distance of the reflecting object.  BUT, the target must be able to reflect the incoming beam, so small ships must carry a radar reflector.

In conventional radar sets, the outgoing microwave energy is pulsed and the radar listens for the echo in the non-pulsed part of the cycle.

Figure 1: Radar principle: The measuring of a round trip time of a microwave
Source: http://www.radartutorial.eu/01.basics/rb04.en.html

How far can radar see?

Radio waves, like light, travel in straight lines.  The curvature of the earth creates a “hill” over which the wave cannot see.  The way of calculating the distance is the same as calculating the dipping distance of a light house.

The dipping distance, M, is where you can just see the light over the horizon.  The same applies to radar.
Dipping distance M miles = 2.08 × (√ (height of eye h metres + √ height of light H metres).

Figure 2 – Dipping distance



Lights – distance off when rising or dipping (Miles)
Height of Eye metres
Light height    1    2    3    4    5    6    7    8    9    10
10    8.7    9.5    10.2    10.8    11.3    11.7    12.1    12.5    12.8    13.2

12    9.3    10.1    10.8    11.4    11.9    12.3    12.7    13.1    13.4    13.8

14    9.9    10.7    11.4    12    12.5    12.9    13.3    13.7    14    14.4

16    10.4    11.2    11.9    12.5    13    13.4    13.8    14.2    14.5    14.9

18    10.9    11.7    12.4    13    13.5    13.9    14.3    14.7    15    15.4

20    11.4    12.2    12.9    13.5    14    14.4    14.8    15.2    15.5    15.9

22    11.9    12.7    13.4    14    14.5    14.9    15.3    15.7    16    16.4

24    12.3    13.1    13.8    14.4    14.9    15.3    15.7    16.1    16.4    17

26    12.7    13.5    14.2    14.8    15.3    15.7    16.1    16.5    16.8    17.2
Source: http://www.btinternet.com/~keith.bater/dipping_distance.htm

In the example, the maximum distance to see a ship 18 m tall from a radar placed on the mast at a height of 7 m above sea level is 14.3 miles.  The multiplier 2.08 seems to vary according to which source is referenced.  Another reference uses 2.21. (http://books.google.co.uk/books )

Height is king so far as distance is concerned, BUT, beware placing the radar too high that it affects the stability of the boat.  Typically, it is placed ⅓rd to ½ the way up the mast, and in front of it.  Do not place it on the spreaders because that would give a large shadow when the radar beam was pointing at the mast each time the aerial rotates.  On power boats do not place it next to the TV satellite dish!  Ideally the radar should be placed on the centre line of the ship.

What is the smallest object you can distinguish?

Get the biggest transmitter possible because beam width is a function that is inversely proportional to the length of the aerial array – the longer the array, the narrower the beam.  A narrow beam passing two objects that are close to each other should distinguish them as two objects where a wider beam, still covering the first as it hits the second will only show one elongated reflection.

Figure 1: Antenna pattern in a polar-coordinate graph

Antenna Pattern
Source: http://www.radartutorial.eu/06.antennas/an05.en.html

The beam has a main spur, the one that is important, and has side lobes.  The side lobes are unwanted.

As said above, the longer the antenna, the narrower the beam.  For example, from the Raymarine technical specification of their RD418D and RD424D radar units, the horizontal beam width of their 18 inch unit is 4.9° and the 24 inch the beam width is 3.9°.  The width of the beam is that part where the power is ½ that of maximum – it falls off away from the centre.  Both have a vertical beam width of 25°.  Both transmit at 9405 +/– 25 MHz which puts them in “X” band.

Some antennas are highly directional; that is, more energy is propagated in certain directions than in others.  The ratio between the amount of energy propagated in the desired direction compared to the energy that would be propagated if the antenna were not directional (Isotropic Radiation) is known as its gain.  When a transmitting antenna with a certain gain is used as a receiving antenna, it will also have the same gain for receiving.

Heading line
One of the issues with radar is to ensure that the beam position displayed on the radar screen lines up with the boat’s heading.  Swing the boat to get the fluxgate compass sorted, and get the heading line sorted by aiming the bow of the boat at an object about 1 mile ahead.  Then adjust the heading line.  I have been on a boat where the target for this exercise was the marker buoy for a lobster pot.  Setting the radar display “course up” or “head up” (see below) would be beneficial.

North up, head up, course up?

Which is best?

North Up

As the name suggests, North Up displays the screen in the same manner as placing the chart on the chart table, so North will be at the top of the screen.  Because this is the same as the chart, plotting positions might be easier.  The radar displays the ship’s heading with a heading line, so should the course deviate from what was intended it should be come evident quite quickly.  This is not the case with Head Up or Course Up.

Head Up

The heading of the ship is at the top of the screen, and orientation is such that when you poke your head above the parapets, what you see on the screen will show in the same relative position.  What it does not do is give any indication that the ship has swung off course – the helmsman should be concentrating on the compass.

Course up

As with Head Up, but the display on the screen is damped for the swing of the boat.  Again the advantage is that when you poke your head above the parapets, what you see on the screen will show in the same relative position, but the disadvantage is that any unintended course alteration does not show on screen.

How do you know there is going to be a collision?

Check different ranges, eg, maximum down to about 3 miles or less, to see what is coming.  If you stick to just one range you will miss what is just outside the “viewfinder” and heading straight for you.

In exactly the same way as in visual collision detection – if the relative angle between you and the target remains the same, then a collision will occur if either you or the other boat do not take action.

To check this, plot the angle and distance of the “target” at, say 5 or 6 minute intervals.  Use the variable range marker (VRM) and electronic bearing line and (EBL) and check if the target is moving down the EBL.

With Head Up and Course Up you remain in the centre of the screen, but your progress is “up the screen”.  In North Up you move across the screen on the course you are following.

What does the display look like?

Radar Display – showing “North Up”
Source: http://www.fas.org/man/dod-101/navy/docs/es310/radarsys/radarsys.htm

The ship’s heading will be displayed with a “heading line” at 106°.


What does it look like when it all goes wrong?

The following radar shots are taken from MAIB report on the collision and sinking of the Etoile des Ondes fishing boat that was run down by the Alam Pintar. The fishing boat sank, one crew member was lost and the Alam Pintar did not stop or report the accident.

The radar screen shots were taken from a third party vessel’s VDR, and are copied from the MAIB report that may be found at http://www.maib.gov.uk/cms_resources.cfm?file=/Etoiles_Alam_Report.pdf 

figure 3: Radar screen shot* showing Etoile des Ondes on Alam Pintar’s port bow

figure 4: Radar screen shot* showing Etoile des Ondes fine on Alam Pintar’s starboard bow



figure 5: Radar screen shot showing Etoile des Ondes still ahead of Alam Pintar with a vector to the north



figure 17: Screen shot (enhanced) showing Etoile des Ondes left astern of Alam Pintar




What did it all look like when plotted as courses?









What else can radar be used for?

Search and rescue.  A Search and Rescue Transponder (SART) may be triggered by any X-Band (3 cm) radar within a range of approximately 8 n miles, but be aware SART will only respond to an X-Band (3 cm) radar.  It will not be seen on S-Band (10 cm) radar.

Navigation – locating position

Radar can be used for navigation, but if the beam width is wider than a harbour entrance it will not pick up the entrance until the beam width is narrower than the entrance being sought, and that might be too close in.

Radar can be used to get a 3-point fix, either by EBL (Electronic Bearing Line) or by VRM (Variable Range Marker).

The RYA book “An Introduction to Radar” recommends the Mk I eyeball wherever possible.  When using radar, note that churches don’t show up too well, and identifying prominent geographical features is key.  When using the EBL, it will show relative to the ship’s heading, so that must be converted to true with correction for variation and deviation.  When using VRM draw arcs from the features reflecting at the ranges given.  In both, measure the fastest changing bearing or distance last.

Other “types” of radar

Radar now comes in “digital” that increases the sensitivity, and by using colours, can distinguish targets hidden by heavy rain.  There is also a version of radar known as “Broadband” that transmits and receives continuously (rather than pulsed), but is not good at ranges greater than about 5 miles.  Also, they don’t fire up RACONS (RAdar BeaCON radar transponders are commonly used to mark maritime navigational hazards) that, when fired up, give an echo on the radar screen in morse code of the letter of the beacon.  For example, Nab Tower is “T” as is the mid-Channel EC2 Racon.  This would show up as a single dash on the screen.

See and be seen

For another ship to see you, you have to present a target from which their beam can reflect.

There is a new standard for radar reflectors to ensure a certain minimum performance.  That standard is ISO 8729-2 that requires a 7.5m2 RCS (radar cross section) at 10 degrees for motor vessels and 20 degrees for sailing vessels.

The traditional reflector is the collapsible octahedron, the next best is a variation of it with the reflector encased in a plastic container so it does not foul the fore sail.  The third is the powered reflector.  From the technical information on an Echomax Active X powered reflector, it has an area of 20.80m2.  Echomax works on the basis of amplifying an incoming X-band signal before sending it back out.  BUT, it does require power and unless it is the XS version, it will only transmit back on X-band and not be seen by ships using S-band.




Collision avoidance – collision regulations

Rule 19 : Conduct of Vessels in Restricted Visibility




There is no stand on or give way vessel in restricted visibility


Rule 19 : Conduct of Vessels in Restricted Visibility

(a) This rule applies to vessels not in sight of one another when navigating in or near an area of restricted visibility.
(b) Every vessel shall proceed at a safe speed adapted to the prevailing circumstances and condition of restricted visibility. A power driven vessel shall have her engines ready for immediate maneuver.
(c) Every vessel shall have due regard to the prevailing circumstances and conditions of restricted visibility when complying with the Rules of Section I of this Part.
(d) A vessel which detects by radar alone the presence of another vessel shall determine if a close-quarters situation is developing and/or risk of collision exists. If so, she shall take avoiding action in ample time, provided that when such action consists of an alteration in course, so far as possible the following shall be avoided:
(i) An alteration of course to port for a vessel forward of the beam, other than for a vessel being overtaken;
(ii) An alteration of course toward a vessel abeam or abaft the beam.
(e) Except where it has been determined that a risk of collision does not exist, every vessel which hears apparently forward of her beam the fog signal of another vessel, or which cannot avoid a close-quarters situation with another vessel forward of her beam, shall reduce her speed to be the minimum at which she can be kept on her course. She shall if necessary take all her way off and in any event navigate with extreme caution until danger of collision is over.







Collision Regulations 19

Further reading:

RYA An introduction to Radar – the RYA’s complete guide, ISBN 978-1-905104-10-9

RYA one day radar course questions and answers, ISBN 1905 104 162

Collision Regulations

Wahkuna and Vespucci
MAIB Wahkuna Report

Etoile des Ondes and Alam Pintar

Other references:



With many thanks to Robert Falk.

09:33 02/03/2011

Portsmouth Tides