USE RADAR TO ENSURE SAFE NAVIGATION

Fix Vessel’s Position By Radar;

RANGE AND BEARING TO A SINGLE OBJECT:

Preferably, radar fixes obtained through measuring the range and bearing to a single object should be limited to small, isolated fixed objects which can be identified with reasonable certainty. In many situations, this method may be the only reliable method which can be employed. If possible, the fix should be based upon a radar range and visual gyro bearing because radar bearings are less accurate than visual gyro bearings. A primary advantage of the method is the rapidity with which a fix can be obtained. A disadvantage is that the fix is based upon only two intersecting position lines, a bearing line and a range arc, obtained from observations of the same object. Identification mistakes can lead to disaster.Two or More Bearings

Generally, fixes obtained from radar bearings are less accurate than those obtained from intersecting range arcs. The accuracy of fixing by this method is greater when the center bearings of small, isolated, radar-conspicuous objects can be observed. Because of the rapidity of the method, the method affords a means for initially determining an approximate position for subsequent use in more reliable identification of objects for fixing by means of two or more ranges.

TANGENT BEARINGS:

Fixing by tangent bearings is one of the least accurate methods. The use of tangent bearings with a range measurement can provide a fix of reasonably good accuracy. Two or More Ranges

The most accurate radar fixes result from measuring and plotting ranges to two or more objects. Measure objects directly ahead or astern first; measure objects closest to the beam last. This procedure is the opposite to that recommended for taking visual bearings, where objects closest to the beam are measured first; however, both recommendations rest on the same principle. When measuring objects to determine a line of position, measure first those which have the greatest rate of change in the quantity being measured; measure last those which have the least rate of change in that quantity. This minimizes measurement time delay errors. Since the range of those objects directly ahead or astern of the ship changes more rapidly than those objects located abeam, measure objects ahead or astern first.

Record the ranges to the navigation aids used and lay the resulting range arcs down on the chart. Theoretically, these lines of position should intersect at a point coincident with the ship’s position at the time of the fix. However, the inherent inaccuracy of the radar coupled with the relatively large scale of most piloting charts usually precludes such a point fix. In this case, the navigator must carefully interpret the resulting fix.

Aids To Radar Navigation and Safety;

Various aids to radar navigation have been developed to aid the navigator in identifying radar targets and for increasing the strength of the echoes received from objects which otherwise are poor radar targets.

RADAR REFLECTORS:

Buoys and small boats, particularly those boats constructed of wood, are poor radar targets. Weak fluctuating echoes received from these targets are easily lost in the sea clutter on the radarscope. To aid in the detection of these targets, radar reflectors, of the corner reflector type, may be used. The corner reflectors may be mounted on the tops of buoys or the body of the buoy may be shaped as a corner reflector. A radar wave on striking any of the metal surfaces or plates will be reflected back in the direction of its source, i. e., the radar antenna. 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 insure receiving strong echoes at the antenna.

Radar Beacons

While 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 radarscopes of ships within range of these beacons. There are two general classes of these beacons: racon which provides both bearing and range information to the target and ramark which provides bearing information only. However, if the ramark installation is detected as an echo on the radarscope, the range will be available also.

Racon:

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 automatically superimposed on the ship’s radar display. The signal may be emitted on a separate frequency, in which case to receive the signal the ship’s radar receiver must be capable of being 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. just beyond the position of the radar beacon or as a Morse code signal displayed radially from just beyond the beacon. Racons are being used as ranges or leading lines. The range is formed by two racons set up behind each other with a separation in the order of 2 to 4 nautical miles. On the PPI scope the “paint” received from the front and rear racons form the range. Some bridges are now equipped with racons which are suspended under the bridge to provide guidance for safe passage. The maximum range for racon reception is limited by line of sight.

Ramark:

Ramark is a radar beacon which transmits either continuously or at intervals. The latter method of transmission is used so that the PPI can be inspected without any clutter introduced by the ramark signal on the scope. 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 series of dashes, a series of dots, or a series of dots and dashes.

Use Parallel Indexing in Radar Navigation;

§ To check the direction of the heading marker while using an off-centre display.

§ To obtain the bearing of a target while using an off-centre display.

§ To obtain the bearing between two targets.

§ To obtain distance between two targets.

§ To obtain the CPA range quickly (RM display only).

§ To obtain the course and speed of a target quickly.

Parallel index is the art of maneuvering a ship to a desired position, or along desired track, in such a manner that the entire maneuver is carried out while watching the PPI only. The chart is consulted before hand, and a little pre-computation may be done, but no fixes are plotted on the chart because continuos fixing is done on the PPI with the help of parallel index. Allowances for current and wind are made, as when necessary, during the maneuver, by inspection of the ship’s progress on the PPI. There are a lot of techniques are available for using parallel index but mainly I can tell passing distance techniques and course alteration techniques.

THE SYSTEM FOR AUTOMATIC PLOTTING:

Radars With Semi And Full Automatic Plotting Capabilities;

The availability of low cost microprocessors and the development of advanced computer technology during the 1970s and 1980s have made it possible to apply computer techniques to improve commercial marine radar systems. Radar manufactures used this technology to create the Automatic Radar Plotting Aids (ARPA). ARPAs are computer assisted radar data processing systems which generate predictive vectors and other ship movement information.

The International Maritime Organization (IMO) has set out certain standards amending the International Convention of Safety of Life at Sea requirements regarding the carrying of suitable automated radar plotting aids (ARPA). The primary function of ARPAs can be summarized in the statement found under the IMO Performance Standards. It states a requirement of ARPAs....“ in order to improve the standard of collision avoidance at sea: Reduce the workload of observers by enabling them to automatically obtain information so that they can perform as well with multiple targets as they can by manually plotting a single target” .As we can see from this statement the principal advantages of ARPA are a reduction in the workload of bridge personnel and fuller and quicker information on selected targets.

A typical ARPA gives a presentation of the current situation and uses computer technology to predict future situations. An ARPA assesses the risk of collision, and enables operator to see proposed maneuvers by own ship. While many different models of ARPAs are available on the market, the following functions are usually provided:

1. True or relative motion radar presentation.

2. Automatic acquisition of targets plus manual acquisition.

3. Digital read- out of acquired targets which provides course, speed, range, bearing, closest point of approach (CPA, and time to CPA (TCPA).

4. The ability to display collision assessment information directly on the PPI, using vectors (true or relative) or a graphical Predicted Area of Danger (PAD) display.

5. The ability to perform trial maneuvers, including course changes, speed changes, and combined course/ speed changes.

6. Automatic ground stabilization for navigation purposes.

ARPA processes radar information much more rapidly than conventional radar but is still subject to the same limitations. ARPA data is only as accurate as the data that comes from inputs such as the gyro and speed log.

Over the past 10 years, the most significant changes to the ARPA systems has been in their design. The majority of ARPAs manufactured today integrate the ARPA features with the radar display. The initial development and design of ARPAs were Stand- alone units. That is they were designed to be an addition to the conventional radar unit.

All of the ARPA functions were installed on board as a separate unit but needed to interfaced with existing equipment to get the basic radar data. The primary benefits were cost and time savings. This of course was not the most ideal situation and eventually it was the integral ARPA that gradually replaced the stand- alone unit. The modern integral ARPA combines the conventional radar data with the computer data processing systems into one unit. The main operational advantage is that both the radar and ARPA data are readily comparable.

Operating a ARPA System:

Types of display of ARPA

From the time radar was first introduced to the present day the radar picture has been presented on the screen of a cathode ray tube. Although the cathode ray tube has retained its function over the years, the way in which the picture is presented has changed considerably. From about the mid-1980s

The first raster-scan displays appeared. The radial-scan PPI was replaced by a raster-scan PPI generated on a television type of display. The integral ARPA and conventional radar units with a raster-scan display will gradually replace the radial-scan radar sets.

The development of commercial marine radar entered a new phase in the 1980s when raster-scan displays that were compliant with the IMO Performance Standards were introduced.

The radar picture of a raster-scan synthetic display is produced on a television screen and is made up of a large number of horizontal lines which form a pattern known as a raster. This type of display is much more complex than the radial-scan synthetic display and requires a large amount of memory. There are a number of advantages for the operator of a raster-scan display and concurrently there are some deficiencies too. The most obvious advantage of a raster-scan display is the brightness of the picture. This allows the observer to view the screen in almost all conditions of ambient light. Out of all the benefits offered by a raster-scan radar it is this ability which has assured its success. Another difference between the radial-scan and raster-scan displays is that the latter has a rectangular screen. The screen size is specified by the length of the diagonal and the width and height of the screen with an approximate ratio of 4:3. The raster-scan television tubes have a much longer life than a traditional radar CRT. Although the tubes are cheaper over their counterpart, the complexity of the signal processing makes it more expensive overall.

Raster-scan PPI:

The IMO Performance Standards for radar to provide a plan display with an effective display diameter of 180mm, 250mm, or 340mm depending upon the gross tonnage of the vessel. With the diameter parameters already chosen, the manufacturer has then to decide how to arrange the placement of the digital numerical data and control status indicators. The raster-scan display makes it easier for design engineers in the way auxiliary data can be written.

Monochrome and Color CRT:

A monochrome display is one which displays one color and black. The general monochrome television uses white as the color. This however is not an appropriate color for the conditions under which a commercial marine radar is viewed. Unlike a television screen, marine radar displays tend to be viewed from the shorter distance and the observer has a greater concentration on the details of the screen and therefore is subject to eyestrain. For this reason the color most common to monochrome raster-scan applications was green. The green phosphor provides comfortable viewing by reducing eye strain and stress. The color tube CRT differs from its monochrome counterpart in that it has three electron guns, which are designated as red, green, and blue.

Controls:

HM OFF

Temporarily erases the heading marker.

ECHO TRAILS

Shows trails of target echoes in the form of simulated afterglow.

MODE

Selects presentation modes: Head- up, Head- up/ TB, North- up, Course- up, and True Motion.

GUARD ALARM

Used for setting the guard alarm.

EBL OFFSET

Activates and deactivates off- centering of the sweep origin.

BKGR COLOR

Selects the background color.

INDEX LINES

Alternately shows and erases parallel index lines.

X2 ZOOM

enlarges a user selected portion of picture twice as large as normal.

CU, TM RESET

Resets the heading line to 000 in course- up mode; moves own ship position 50% radius in stern direction in the true motion mode.

INT REJECT

Reduces mutual radar interference

RANGE RINGS

Adjusts the brightness of range rings.

DISPLAY CONTROLS - PLOTTING KEYPAD:

ORIGIN MARK

Show and erases the origin mark (a reference point).

VECTOR TRUE/ REL

Selects true or relative vector.

VECTOR TIME

Sets vector length in time.

RADAR MENU

Opens and closes RADAR menus.

E- PLOT, AUTO PLOT MENU

Opens and closes E- plot and AUTO PLT menus.

NAV MENU

Opens and closes NAV menu.

KEYS 0- 9

Select plot symbols. Also used for entering numeric data.

CANCEL

Terminates plotting of a specified target or all tracked targets.

ENTER

Used to save settings on menu screen.

TARGET DATA

Displays the acquired target data.

TARGET BASED DATA

Own ship’s speed is measured relative to a fixed target.

AUTO PLOT

Activates and deactivates the Auto Plotter.

TRIAL

Initiates a trial maneuver.

LOST TARGET

Silences the lost target audible alarm and erases the lost target symbol.

HISTORY

Shows and erases past positions of tracked targets.

MARK

Enter/ erase mark.

CHART ALIGN

Used to align chart data.

VIDEO PLOT

Turns the video plotter on/ off.