“ARPA”
(AUTOMATIC RADAR PLOTTING AID)
Basically an ARPA is a computerized radar plotting system, which can perform radar plotting manually or automatically according to the operators' choice.
RADAR Plotting - Modern radar now-a-days are equipped with an electronic plotting feature known as ARPA this feature makes plotting directly on the radar display very convenient and replaced the need for the reflection plotter or plotting sheet.
“Principal ARPA System”
• Computer based radar system have been used on board naval vessels since the 1950’s but not until the end of the 1960’s did systems for merchant maritime use come onto the market
• These systems were well received because of new, more economical technology and good business conditions for shipping.
• It is not by chance that the transition from conventional radar to ARPA has been compared to the pre- and post-radar era onboard ships. We can expect some of the same problem as in that first transition if proper and thorough training is not received.
• When it works properly, ARPA is a fantastic tool in the hands of a qualified navigator with proper training. In the hands of untrained personnel, it is a dangerous tool, which can add to the cause of collision.
• One main requirement for all users of ARPA is to have good knowledge of the principles of manual plotting in order to understand the information given by the instrument. On the same principles, it is necessary to obtain maximum usage of the equipment in a safe and responsible manner.
• Since the first computerized radar system was introduced up to this present time many different ways of presenting the information has been developed, produced and delivered, today, regardless of graphic presentation.

• All ARPA systems must be able to present target information in form of both relative and true vectors. Both types of vectors should be time adjustable, in addition to displaying targets information graphically, all ARPAs also display targets information digitally on the traffic display or on a separate screen.

* Since the first in addition a number of graphical symbols are used fro different purposes. • Defining Stationary targets
• Indicating Navigational Marks
• Sailing Routes
•Pointing out targets that causes alarms

“ARPA Display Information”
• Warning about the technical problem of the ARPA
• Defining stationary targets
• Indicating navigational landmarks
• Sailing routes
TV
TV
• Pointing out targets that cause alarms etc.
• True speed vector indicates he targets speed and course.
RV
RV

• Relative speed vector indicates targets relative course and speed.
TH
TH

• Track History should be provided on request to indicate the targets pass track history.
PPC
PPC

• Point of possible collision is the point at which a collision could take place.
PAD
PAD • Predicted Area of danger is the area to be avoided based on CPA and TCPA setting and relative target speed.

* MANUAL ACQ.
MANUAL ACQ.
AUTO ACQ.
AUTO ACQ.
MANUAL
MANUAL
Most ARPA system today has 2 to 3 type s of acquisition for plotting

* Manual mode means that the operator by a means of a pointing device place a acquisition symbol on top of an ARPA echo and presses the MANUAL bottom to tell the computer to register time and position
• After a few minutes the operator must re position the mouse on top of the echo and press the acquisition again, the computer will now calculate target information and present to the operator in form of a symbol of target vector.
• Manual acquisition means that the operator by a means of a pointing device place a acquisition symbol on top of an ARPA echo and presses the MANUAL ACQUISITION bottom * After a some time normally less than 60 seconds the computer presented the operator with the calculated target information. * Before ARPAS can be operated in Automatic Acquisition Mode. The search criteria must be specified * The operator should remember that most ARPA systems are more sensitive in manual mode that in automatic mode. This is particularly important under difficult operational conditions such as during restricted visibility. * Most ARPA systems can operate in several acquisition modes , when operating the ARPA system in manual or manual acquisition mode the operator decide all the time how many echoes should be plotted. Since the capacity of the tracking system are limited the operator should make it a working habit to cancel target that are no longer required * When operating the ARPA in automatic acquisition mode, the operator must be aware of the following tracking system limitations * Normally the sensitivity of the ARPA tracking system is r educed when operating in Automatic Acquisition Mode.

* GUARD RING - When the Guard Ring Philosophy is used by the ARPA tracking system, echoes can escape acquisition because they are detected by the radar at a range closer than the distance to the inner guard ring or the echoes remain between guard rings. * SEARCH AREA - When the search Area philosophy is used, echoes can escape acquisition because they are outside the specified area or too many echoes are picked up, resulting in system overload.

* When the operator is acquiring the target what he it actually doing is telling the ARPA to search a target in that position since a target is hit by several radar sweeps the area pointed should have several target size range and bearing. This area is called Tracking gate or tracking window. * The number of sweeps digitized in each tracking gate depends on the tracking philosophy used by the actual ARPA manufacturer. Several sweep is always required. * In order to start digitizing the analogue radar echo, the echo must have a certain size or amplitude. Another important requirement is that the echo is not lost by too many scans during a specified time, as this will result in rejection of defining the echo as a possible target and no further processing will be executed. * As soon as the ARPA tracking system defined a target as a “maneuvering target” the fast filter is connected, this means that the updating of the information presented to the operator represents the average target data calculated from the last 3 to 5 scans depending of ARPA manufacturer’s software configurations. In other words, it takes time before the ARPA detects a maneuver, once the maneuver is detected the ARPA will update the information more often, but the information will always be behind the real situation. * The information on the ARPA is always delayed in relation to “real Life” and this fact must be taken into account at all times. * Only a few samples are used for calculations of target data during target maneuvering. Still what you see in the ARPA is history.

* Familiarization with the target presentation on your ARPA is very important. Use the ARPA in good visibility to understand better the system limitations. VECTOR VIEW PAD VIEW

* The ARPA should clearly warns the operator of Lost targets

• Reasons for losing targets
• Target moved outside radar range
•Target Lost in Clutter
•Target entering Blind Sector
•The target is a submarine

• The operator should make it a habit to try to figure out the reason for a lost target as this can help to understand the ARPA system limitations In the ARPA manual technical conditions for triggering a lost target should be given Often this is related to a fixed number of “no hits” during specified number of scans * The ARPA system starts up procedure varies from system to system however, the following points outline the minimum of what must be checked before ARPA is operated in basic mode, i.e. for traffic surveillance. Overlooking one or more of these points may cause serious consequences.

• Switch on the ARPA and check that the required radar in connected and properly adjusted. • Check that the ships corrected course is feed up into the system, i.e. Correct the gyro course if necessary. • Check the radar antenna alignment is correct, if not correct it. • Check that the required log is selected (the log should provide speed through water, no correction for drift and current). • Select required radar mode, normally True motion course Up or True Motion North Up should be used for traffic surveillance purposes. • Select required range. Vector length and collision warning criteria. • Familiarize yourself with the ARPA manufacturers recommended start up procedures and other recommendations.

• If a navigation features are available on your ARPA and you intend to use them, the following additional points must be checked. * Date and time should be displayed correctly on the ARPA information system * Own ship position input must be kept correct on the ARPA at all times, otherwise all positions calculated by the ARPA will be incorrect * In coastal area, navigation check-points should be marked on the ARPA in order to assist the navigator in detecting a possible positioning error as soon as possible. * Special requirements pointed out by the ARPA manufacturer.

• The main purpose of the ARPA is to provide the navigator with the best possible overview of the traffic situation at all times, in order to do so nowadays ARPA can display any traffic situation which is easy to understand mode. * Today's navigator prefers the True motion course up true vector sea stability mode.

* The ARPA picture is stabilized at 240°, own ship’s heading is 240°, speed 15 knots, selected vector length is 12 minutes.

* Same traffic situation but the ship change course to 30° to starboard observe that not rotation that ARPA display has change. Observe that radar picture remains stabilized on the intended course during avoiding actions. * True motion course up is without a doubt the best presentation for traffic surveillance and for maintaining overview of the situation around the ship at all times. * True motion, course Up, true Vector is the closest we can get to what is referred to as positive “Instrumentation”. That is, the instrument should provide the simplest and best possible overview of the information provided to the operator at all times. * Whenever CPA / TCPA information is needed this can be graphically easily obtained by pressing the spring loaded relative vector button. Target CPA/TCPA is also available in alpha numerical format on the screen.

* Same situation as before the course change was executed to course up but the ARPA is stabilized relative to North. When this mode is used the navigator has to turn the image 180 in his head in order to correct the direction of the image, * Many navigators don’t like the North stabilized picture, especially sailing on southerly courses, as they get the feeling of sailing in the wrong direction from this ARPA presentation

* This presentation is available to some ARPA system. However it is not recommended. As long as Course Up or North Up is available. The main reason being that the picture is very much distort is being caused by the ships yawing and course changes

* An un-stabilized True Motion presentation should be regarded as a last resort, i.e. an emergency presentation to be used when stabilized presentation are failing for one reason or another.

* The relative motion north up is good presentation for navigation, however is cannot recommended for traffic surveillance as it is not giving any information about calculating target aspect.

* The relative motion presentation is difficult to interpret correctly during dense traffic requiring frequent course alterations, taking about traffic surveillance by ARPA, this presentation should only be used when true motion is not available.

* The relative motion head up can be defined as the ARPA emergency mode. Only recommended when everything else is failing. Not recommended and not suitable for traffic surveillance at all.

* Relative motion – Head Up is the presentation which gives the shipping industry the phrase “Radar assisted collision”. Off course, the radar did not cause he many collision, the collision took place because the navigator did not understand the information provided by the ARPA and does not have sufficient time to do proper plotting.

* All radar ARPA presentation presented so far can be classified so far as pure radar ARPA presentation. Overtime it is strongly recommended only to use pure ARPA presentation as impure or mixed presentation can get you into real trouble. A mixed presentation is actually what is presented in the screen during critical situation. This can also be used but use it correctly during short intervals * Mixed presentation should be used with utmost care; several collisions can be traced back to the use of mixed radar/ARPA presentation over the past 15-20 years.

* When using ARPA for traffic surveillance purposes own ship speed through the water is highly recommended as input to the ARPA system, no corrections should be made for current or drift the main reason for this is as this is the only way in acquiring target calculated aspect.

* Avoid using speed input from Doppler log or GPS as this normally gives speed over ground. Fine for navigation, useless for traffic surveillance.

“Risks of over reliance on ARPA” * Raster scan ARPA display “Lock Up” * Mixing trial and real time information * Wrong speed input or overlooking type of speed input in the ARPA * No correction for gyro course error before input to ARPA * Misinterpreting of display symbols may caused severe problems * Operating long periods in “impure presentations” may cause serious consequences * Exclusive reliance on ARPA will sooner or later give you’re a problem. * Remember that ARPA is only a navigational aid and that its limitations including those of its sensor. Makes exclusive reliance on ARPA dangerous * When operating an ARPA system in auto acquire mode, visual observations of the ARPA screen is not only required, but also very much a question about good seamanship and common sense. * The majority of ARPA systems manufactured today provide an automatic acquisition feature. The feature may reduce the navigator's workload during busy period and thus contributing positively to safer sailing. * However the operator should be aware of the fact that most ARPA system are less sensitive in auto acquisition mode than in manual acquisition mode. This is one good reason not to rely on the new target warning system only but in regular intervals visually observe the ARPA screen to make sure that all targets are acquired. * Wrong speed input to an ARPA system working in True motion True vector mode can seriously affect target calculated aspect. * All ARPA operators must at all times be aware of what speed is used on their ARPA and the accuracy of this speed.

Calculated results as given by different speed input sources

* It is not by chance that the transition from conventional radar to ARPA has been compared to the pre-and-post radar era on board ships. We can expect some of the same problems as in that first transition if proper and thorough training is not received.

* When it works properly, ARPA is a fantastic tool in the hands of a qualified navigator with proper training. In the hands of untrained personnel, it is a dangerous tool, which can add to the cause of collision.

ASPECTS
The aspect is defined red on the port side and green on the starboard side. Aspect is measures from dead ahead to 180o targets relative bearing of own vessels taken from the target.

RADAR PLOTTING
Plotting has two main purposes: * It can show whether danger of collision exist, how close we will pass the target and how much time there is left before this will take place. * Approximate determination of the course and speed of the other vessel, so that sensible avoiding action can be taken when needed. * Plotting can be done with Head Up or North Up display. Regardless of radar selected presentation it is advantageous to plot with North Up. * True plotting gives a natural and easily understood picture of the course and events. It can be done directly in the chart if the scale is large enough to give a clear picture J.

Heading – is defined as the direction in which the bow of the vessel is pointing expressed as angular distance from north.
Relative Bearing – if the relative bearing of an approaching target remains the same over time, collision danger is observed.
True Bearing – are mainly used for position fixing.
Bearing – on a ship bearing can be relative or true. In connection with traffic surveillance. Relative bearing are often used.

CPA – must not be mixed with point where the target crosses own ship heading. Often referred as BCP (bow crossing position or point).
TCPA – is the time estimated, as measured along the echo line from the present position to the closest point of approach

RADAR PICTURE
The radar picture is a plain picture of the ship’s surroundings as seen from the scanner
The use of radar to assist in navigation can be divided into three categories. * Making landfall * Coastal navigation * Pilotage
Racons used by the maritime industry are only triggered by x-bands radars

ARPA VIEW

ERROR SOURCE IN MANUAL PLOTTING * Possible Error source * Error in Bearing * Error in range * Error in between plots * Low relative speed * Error in own ship speed * Error in own ship heading * Error caused by own ship yawing * Error due to own ship maneuvering

Standard Symbol

Manual acquisition and plotting: a cross shall be used as the cursor for manual acquisition on an ARPA and ATA and for plotting on an EPA Area for Auto Acquisition: If facilities are provided for suppression of acquisition, continues lines shall be used to define the limits outside which auto acquisition is suppressed

Target being tackled during the initial state: A broken square around the target indicates the target under acquisition and initial stage of tracking before steady-state tracking

Course and Speed Vector: Indicating the targets predicted true or relative motion, which may have a fixed time scale or time-adjusted scale. The vector origin is to be defined by a small dot or the center of a circle, the circle shall have at least 2 mm in diameter. The position of the own ship shall always be indicated by a dot

Past position of target on ARPA: At least four equally time-spaced past position to be shown on request as dots on an ARPA. Plot position shall be identified by an associated plot number adjacent to the initial plot and subsequently adjacent to the vector origin. On EPA the past plot position may not be equally time-spaced, and are not shown astern of ones own ship
Course and speed graphics: The graphics can take the form of a shape such as a hexagon or ellipse. The vector origin is to be defined by a small dot or the center of a circle of at least 2mm in diameter.

Target entering guard zone: A flashing equilateral triangle, apex down, shall be used to mark the target. A guard zone shall consist of continuous lines bounding the area chosen by the navigator CPA / TCPA Warning: A flashing equilateral triangle, apex top shall be used to mark the target; in addition, the target may be flashed

Lost target warning. A diamond shall flash. The diamond shall be formed from two equal triangles

Data Requirement: A square is to be used as a symbol to mark the data reading target

Ground Referencing: Symbols 3, 4 and 9 annotated with the letter R and if more than one target is used, with R1, R2, R3, etc.

Track target without vector or graphic: A clear visible dot (such as a negative video or different color) over the target which is distinguishable from, symbols 4A, 4B and 5

Trial Maneuver: The letter “T” shall flash at the bottom of the display

Test target superimposed on the life picture: The letter “X” at the bottom of the display and on the test target shall flash. During a trial maneuver the letters “XT” shall appear at the bottom of the display

SUPPLEMENTARY SYMBOLS

Target with a speed moving less than 1 knot

Fixed target: mark a stationary target

Navigation Buoy: marks the area of a navigation buoy.

Off-shore installation: marks the area of an off-shore installation

Anchorage: specify Anchorage areas boundaries

Standard map Line: marks map information, for example a coastline

Dashed map Lines: mark map information, for example a 10 meter depth contour

Heading Line: marks own ship heading line

Electronic bearing Line: used to select bearing to selected objects/ position Special Areas: specify Special area boundaries.

Danger Zone: specify danger area boundaries

Prohibited Area: specifies prohibited area boundaries.

Reference Position: Mark active reference position used to calculated relative range and bearing of a selected object.

Principal ARPA System WARNINGS

Audible and flashing warning activated whenever a traced target violates the pre-set collision criteria.

Audible and flashing warning activated whenever the system no longer can track a target.

Audible and flashing warning activated when a pre-set limit is violated .

Principal ARPA System OPTIONS

Access to Navigation features, i.e. map, fairways, positioning, etc.

Selection of information layers.

Access to adjustment of system parameters.

Principal ARPA System ARPA MODE

ARPA display a stabilized on ships ordered course.

ARPA display un-stabilized, when ships heading pointing to the top of the screen.

ARPA display stabilized with North to the top of the screen.

Selection of relative motion selection.

Selection of true motion presentation.

True Target Vector Presentation

Remove the Vector from the Target

Selection of Relative Target Vector presentation

Select Default Vector

Reduce target Vector Length

Increase Target Vector Length

Principal ARPA System DISPLAY

Reduce Range Display

Increase Range Display

Select Default Range

Place the Sweep Center in the position of the Marker

Reset Sweep center with heading marker through the center of the screen

Switch the Pointer ON / OFF

Principal ARPA System TRACKING

Displays calculated Target data

Activate target history feature

Activate radar positioning feature.

Manual acquisition of target

Activate automatic acquisition of target Cancel tracking of selected target

Principal ARPA System TRIAL

Change trial course to Port

Change trial course to starboard

Switching between pre-set turn rates

Activate second maneuver feature

Increasing Trial Speed Setting

Reducing trial Speed Setting Specified trial time maneuver Setting required trial time to maneuver

Switch the trail feature ON and OFF

Increasing trial speed setting

Reduce trial speed setting

Principal ARPA System SYSTEM CONTROL

Warning about the technical problem of the ARPA Keeps the ARPA ready for use in a short notice

Sets the ARPA On and Off

Regulates the brightness of the picture

Shape the over all radar picture

Sets the correct frequency

Adjust the focus display

Reduce the effect of echoes from sea

Reduce the effect of echoes from rain