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Aids To Radar Navigation

Trihedral reflector

Most radar reflectors are variations on the 3-sided corner reflector, also known as a corner cube or a trihedral reflector.

The principal echo from a trihedral reflector will be strongest when its “pocket” is oriented directly towards the radar.

As the trihedral reflector is rotated off this axis in any direction, the echo becomes weaker, and drops by half (-3 dB) at an angle of 12° to 20° from the axis of symmetry, depending on its specific shape.

With increased rotation, the return continues to drop to almost zero as one of the three sides approaches an edge-on attitude to the radar.

When one edge is exactly edge-on, there will be a strong but narrow return, caused by the other two edges acting as a dihedral (2-sided) reflector, or one side acting alone as a flat plate reflector.

These returns can be very strong, but so narrow in angle as to have little value.

Octahedral Reflectors

The classic octahedral reflector is made of three planar circles or squares of metal intersecting at right angles, forming eight trihedral reflectors.

In the usual “catch rain” position, one trihedral will face up and one down, and the remaining six are arrayed around a circle, three oriented 18° above the equator, and three 18° below.

This optimizes the return from the “pockets”, and avoids the nulls or gaps as best as is possible, but only at a 0° angle of heel.

Considerations of heel angle has led to the “double catch rain” position (see figure), with one planar surface oriented vertically along the vessel’s axis, and the other two planes ±45° from the vertical.

This is not the ideal with no heel angle, but moves towards the “catch rain” position as the boat heels.


Radar navigation aids help identify radar targets and increase echo signal strength from otherwise poor radar targets.

Buoys are particularly poor radar targets. Weak, fluctuating echoes received from these targets are easily lost in the sea clutter.

To aid in the detection of these targets, radar reflectors, designated corner reflectors, may be used.

These reflectors may be mounted on the tops of buoys. Additionally, the body of the buoy may be shaped as a reflector.

Each corner reflector, shown, consists of three mutually perpendicular flat metal surfaces.

A radar wave striking any of the metal surfaces or plates will be reflected back in the direction of its source. Maximum energy will be reflected back to the antenna if the axis of the radar beam makes equal angles with all the metal surfaces. Frequently, corner reflectors are assembled in clusters to maximize the reflected signal.

Although radar reflectors are used to obtain stronger echoes from radar targets, other means are required for more positive identification of radar targets.

Radar beacons are transmitters operating in the marine radar frequency band, which produce distinctive indications on the Radar displays of ships within range of these beacons.

There are two general classes of these beacons:

Racons, which provide both bearing and range information to the target, and

Ramarks which provide bearing information only.

A racon is a radar transponder which emits a characteristic signal when triggered by a ship’s radar.

The signal may be emitted on the same frequency as that of the triggering radar, in which case it is superimposed on the ship’s radar display automatically.

(The signal may be emitted on a separate frequency, in which case to receive the signal the ship’s radar receiver must be tuned to the beacon frequency, or a special receiver must be used.)

In either case, the PPI will be blank except for the beacon signal.

However, the only racons in service are “in band” beacons which transmit in one of the marine radar bands, usually more often in the 3centimetre band though the 10 centimetre ones are also in use.

The racon signal appears on the PPI as a radial line originating at a point just beyond the position of the radar beacon, or as a Morse code signal displayed radially from just beyond the beacon.

A ramark is a radar beacon which transmits either continuously or at intervals. The latter (Racon) method of transmission is used so that the PPI can be inspected without any clutter introduced by the ramark signal on the display.

The ramark signal as it appears on the PPI is a radial line from the center.

The radial line may be a continuous narrow line, a broken line, a series of dots, or a series of dots and dashes.

Radar beacons (Racon);

A receiver-transmitter device associated with a fixed navigational mark which, when triggered by a radar, automatically returns a distinctive signal which can appear on the display of the triggering radar, providing range, bearing and identification information.


A receiver- transmitter device in the maritime radio navigation service which transmits automatically when it receives the proper interrogation, or when a transmission is initiated by a local command.  The transmission may include a coded identification signal and/or data.  The response may be displayed on a radar PPI, or on a display separate from any radar, or both, depending upon the application and content of the signal.

Transponder is a device which when properly interrogated can provide for – ship target identification and echo enhancement with the proviso that such enhancer should not significantly exceed that which could be achieved by passive means on the radar PPI of an interrogating ship or shore station;

Transponder are used to meet the following operational requirements

identification of certain classes of ships (ship-to-ship)

identification of ships for the purposes of shore surveillance.

search and rescue operations.

identification of individual ships and data transfer.

establishing positions for hydrographical purposes.

In making a landfall and in harbour approaches a problem of navigation is the identification of lighthouses and lightships marking either hazards or approaches to buoys channels.  In conditions of poor visibility many vessels may congregate in harbour approaches, and positive identification of a single known mark - light vessel or buoy may enable a channel to be identified among many ship echoes.

A racon is required to transmit a signal, the “response” each time it receives a pulse from the radar set which may have any frequency within the 200 mHz wide band.

The response of the in-band racon must be of a frequency that can be received and processed by th same radar.

The Racon may receive the radar pulse via a broad band emitter covering the full 200 mHz in order that it may be picked by the radar receiver, tuned to a particular frequency within the band.

The usual method is to alter the frequency of the response a function of time and repeat this periodically in a saw-tooth fashion.

As a result the radar will pick up the response only at those moments that the transmitted frequency is within the bandwidth the radar received; at other moments the response will not received.

A slow sweep in-band Racon, the frequency sweep takes place at slow rate (2-4 mHz) and the process is consequently repeated long intervals of time (50 - 100 s). 

In that case the response is not received at every sweep of the radar beam over the Racon; fact the period that no information is received may be too long for navigational purposes.

This disadvantage is overcome in a fast sweep Racon where the frequency of the response is altered very fast, so that it sweeps through 200 mHz in 5 - 7 microsec.

The problem does not present itself with Fixed Frequency Racon,as the response is always transmitted at the same frequency and is picked up by a special receiver at the radar.

The slow sweep in-band Racon response can consist of a single pulse, usually of some 45 microsecond duration for every radar pulse received.

The radar pulses recur at the rate of the PRF and the triggered responses which integrate to provide a radial paint on the PPI beyond the target, 3.6 n. miles long for a 45 micros response.

This paint starts at a short distance from the target which is determined by the overall system delay of the Racon.

The indication on the PPI is a flash towards the edge of the screen the bearing of the beacon and at a range some 300 metres great than the true range of the Racon.

The response may be coded to consist of a number of consecutive pulses of predetermined duration, thus providing a characteristic paint on the PPI for the purpose of identification.

Most radar sets are equipped with a differentiator circuit for the purpose of suppressing rain clutter.

The differentiator reacts up sudden alterations of the incoming signals and accentuates the suppressing constant echoes with a slow rise time.  The pulse of slow sweep Racon lasts at least several microseconds even when coded and it is purposely given slow rise time.  Its characteristics therefore resembles those of rain echoes and can be suppressed by actuating the differentiator.  This is of advantage when interference is caused by side lobe triggering at short range, as the interference can be removed at will by means of the differentiator.

Obviously, if a slow sweep in-band Racon is hidden in rain echoes, the differentiator may remove both the rain and the Racon from the screen.

IALA recognises a requirement for 3 types of Racon

Long range Racon, for the purpose of enhancing the echo of and identifying a landfall mark at a maximum range of 25 n.miles. This Racon would use a sweep duration of 90- 120 seconds.  The Racon antenna can have a narrow vertical angle of divergence and so constructed as to have a high gain in the approach sector.

Medium range Racon, for the coastal navigation and identification of navigation marks at ranges of 8 to 15 n. miles.  The sweep duration should be 60 -90 seconds.

Short range Racon, for ranges up to 6 n. miles and for use in inshore waters.

A faster sweep rate if necessary in order to provide more frequent information and a sweep duration of 60 seconds or less is recommended.

This equipment, often destined for buoys, must have a wide vertical angle of divergence and therefore be fitted with a low gain antenna.

Marine Racon utilised a coding unit which can give a choice of eight three digit morse signals. i.e. the letters G.R.O.W.D.U.S.K. followed by a normal racon flash towards the edge of the screen.  When two Racons are sited so close to each other that each triggers its neighbour, a virtual continuous transmission will be the result.  This is a situation that should be avoided.

If a radar equipment and a Racon are installed in close proximity to each other such as may be the case in a lightvessel, mutual. interference and possible damage to the equipment may be expected when they operate at the same time.  This may be overcome by the application and complicated equipment.

When a Racon is mounted on a buoy, its position should be monitored; shipping must be warned and the Racon emission switched off by remote control if the buoy is known to be off station.



A RAMARK is a beacon that provides bearing information only.

It comprises an unsychronized transmitter of which the frequency is swept over the marine band so that its transmission can be received and displayed by any commercial radar set operating in the band for which the beacon is designated (X-band or S-band).

Consequently it is an in-band beacon.

Whenever the emitted frequency is within the radar receiver bandwidth the radar will pick up and display the signal each time the antenna is directed at the RAMARK.

In this way a duty cycle can be introduced whereby periodic information is provided at longer or shorter intervals depending on the sweep rate and the receiver bandwidth, as in the case of slow-sweep in-band Racon. The transmission may be coded by pulse or by frequency modulation in order to provide identification; this appears on the PPI as a succession of dots or dashes.

The possibilities of coding are thus very limited.


Interference can be caused by a Ramark at close range when its emission is picked up by the side lobes of the radar antenna.  This results in a wide sector of paint on the PPI which can in an extreme case cover the entire screen and obliterate all echoes.

This interference is much more severe than that experienced with Racon, the latter being confined to ranges greater than that of the target and being limited by the length of the response.  A long duration Ramark duty cycle, either obtained through a slow sweep rate or by intermittent, transmissions may be employed to ensure that there is a minimum acceptable period of time that the PPI is clean.  With a slow sweep rate long pulses may be used in which case, the interference may be removed by the differentiator.

However, at moments that Ramark information is needed the differentiator must be switched off, and the interference then experienced will persist and spoil the radar picture for some time after the differentiator is switched on again, because of the afterglow properties of the radar screen.

The Ramark signal on the PPI originates from the ship’s own position on the screen.  When within range of several such beacons this position becomes the hub of a number of spokes and this makes the picture very crowded in the area close to the vessel which is so crucial for its safety.  Because Ramark suffers from some serious disadvantages and only provides bearing information its application has been limited.

Swept frequency radar beacon

A radar beacon in the maritime service which is capable of transmitting a warning signal, automatically, to any radar-equipped ship in its vicinity - the beacon will be triggered automatically by the transmissions of any radar operating in the appropriate radar band the return signal is to be displayed on the PPI of the triggering radar.

Swept frequency radar beacons are used only for the following purposes; it should not be used to enhance the detection of marine craft.

Ranging on and identification of positions on inconspicuous coastlines.

Identification of position on coastlines which permit good ranging but are featureless.

Identification of selected navigational marks both seaborne and land based.

Landfall identification. as a warning to identify temporary navigational hazards and to mark new and uncharted dangers.

Fixed frequency radar beacon :

A radar beacon in the maritime radio navigation service which is capable of responding automatically to any radar-equipped ship in its vicinity, and which returns a signal on a fixed frequency which can be displayed on the PPI of a suitably configured radar - the beacon will be triggered automatically by the transmission of any radar operating in the radar band, the signal may be displayed continuously, either separately or super-imposed on the radar picture, or may be switched off, at the option of the operator.