KUP 20: Ability to operate and to interpret and analyse information obtained from radar and ARPA performance<gwmw style="display:none;"></gwmw>
KUP 20 Ability to operate and to interpret and analyse information obtained from radar and ARPA performance, including
1) factors affecting performance and accuracy,
2) setting up and maintaining displays,
3) detection of misrepresentation of information, false echoes, sea return, etc., racons and SARTs
Learning objectives:
The learning objective is to develop the ability to operate radar and ARPA systems effectively and to accurately interpret and analyze the information they provide. This includes understanding factors that affect radar and ARPA performance and accuracy, setting up and maintaining displays, and detecting misrepresentation of information, such as false echoes, sea return, racons, and SARTs, to ensure safe and effective navigation.
Content
1. Operate, interpret, and analyze radar and ARPA information
Plotting positions and fixing are fundamental techniques used by mariners to determine their precise location on a nautical chart. These techniques enable mariners to maintain accurate situational awareness, make informed navigational decisions, and ensure safe passage through waterways. Let’s explore the importance of plotting positions and fixing in maritime navigation and the methods employed to achieve accurate results.
Plotting Positions:
Plotting positions involves transferring observed or calculated positions onto a nautical chart. It allows mariners to visualize their vessel’s location in relation to navigational aids, hazards, and other chart features. Positions can be determined using various methods, including visual fixes, dead reckoning (DR), celestial observations, or electronic positioning systems.
Visual Fixes:
Visual fixes are determined by taking bearings of identifiable objects or landmarks from the vessel. By using a compass or electronic bearing device, mariners measure the angular difference between the object and a known reference point. These bearings are then transferred onto the nautical chart, and intersecting lines are plotted to obtain the vessel’s estimated position (EP). Visual fixes are particularly useful in coastal navigation, where prominent landmarks or navigational aids can serve as reliable reference points.
Dead Reckoning (DR):
Dead reckoning is a technique for estimating a vessel’s position based on its course, speed, and elapsed time. By projecting the vessel’s course line on the chart from a known position (fix), mariners can estimate their position by advancing the course line using the vessel’s speed and elapsed time. While DR positions may not be as accurate as fixes, they provide a continuous reference for navigation between known positions.
Celestial Navigation:
Celestial navigation involves using celestial bodies, such as the sun, moon, stars, and planets, to determine position. By measuring the altitude of celestial bodies at specific times, mariners can calculate their latitude and longitude. Celestial navigation is particularly useful for long-distance voyages or when electronic positioning systems are not available or reliable.
Electronic Positioning Systems:
Modern electronic positioning systems, such as the Global Positioning System (GPS), provide mariners with accurate and real-time position information. GPS receivers use signals from multiple satellites to determine latitude, longitude, and elevation. Mariners can transfer their GPS-derived positions directly onto the nautical chart, ensuring precise plotting.
Fixing:
Fixing is the process of determining a vessel’s exact position by intersecting lines of position (LOPs) obtained from different sources, such as visual fixes, electronic positioning systems, or celestial observations. By drawing multiple LOPs on the nautical chart, mariners can pinpoint their vessel’s position at the intersection of these lines. Fixes provide the most accurate and reliable reference points for navigation, enhancing safety and facilitating course planning.
It is important for mariners to continuously update their positions as they progress along their intended route. Regular plotting, fixing, and cross-checking with other navigational methods allow mariners to monitor their progress, detect any deviations from the planned course, and make necessary adjustments to maintain safety and accuracy.
By employing these techniques, mariners can confidently navigate through challenging waters, avoid hazards, and reach their destinations with precision. Regular practice and proficiency in plotting positions and fixing contribute to safe and efficient maritime navigation, ensuring the well-being of vessels, crew, and the environment.
2. Factors affecting performance and accuracy
The performance of a radar system can be judged by the following [1] :
- (1) the maximum range at which it can see a target of a specified size,
- (2) the accuracy of its measurement of target location in range and angle,
- (3) its ability to distinguish one target from another,
- (4) its ability to detect the desired target echo when masked by large clutter echoes, unintentional interfering signals from other “friendly” transmitters, or intentional radiation from hostile jamming (if a military radar),
- (5) its ability to recognize the type of target, and
- (6) its availability (ability to operate when needed), reliability, and maintainability.
The limitations of marine radar are affected by the following [2]:
(1). Range Discrimination
The RADAR can clearly distinguish two targets on the same bearing with a slight difference in ranges as two separate targets.
The factor which governs range discrimination is PL. Pulse length makes all targets appear larger in radial depth by an amount equal to half PL in meters. If the gap between the two targets is less than ½ PL in meters, then these targets would appear as one target on the PPI.
As per IMO Performance standards for RADAR, two small targets on the same bearing and separated by a distance of 40 meters on 1.5 miles range scale (placed at 50 to 100% of PPI), shall appear as two separate targets on the PPI.
(2). Bearing Discrimination
A RADAR’s ability to distinguish two targets on the same range is slightly different in bearings as two separate targets.
The factor that governs bearing discrimination is HBW, which causes all targets to appear larger in Azimuth by equal to half HBW on either side, i.e. one full HBW. So if the angle at the Electronic Centre made by the inner corners of two targets is equal to or less than HBW their paints would merge on the PPI and they would appear as one big target.
Bearing discrimination is expressed in degrees and as per Performance Standards for Navigational RADAR (IMO), it should not exceed 2.5º.
(3). Factors Affecting Minimum Range
The minimum detection range of the RADAR set depends on
a. The Pulse Length
b. The De-ionisation Delay
c. The VBW And The Height Of The Scanner
d. The Wavelength
(4). Factors Affecting Maximum Range
a. Height Of Scanner
b. Power Of The Set
c. Wavelength
d. Pulse Repetition Frequency
e. Pulse Length
f. VBW & HBW
g. Receiver Sensitivity
h. Nature Of Target
i. Weather Effect
j. Anomalous Propagation
k. Sea and swell
5. Range Accuracy
As per Performance Standards for Navigational RADAR (IMO), the error in the range of an object obtained using range rings or the VRM, should not exceed 1% of the maximum range of the scale in use, or 30 m whichever is greater.
The range accuracy of RADAR depends on:
a. Correct synchronization between the transmission of the pulse and the commencement of the trace. b. Uniformity And Rectilinearity of the time base.
c. The Scale Of Size Of The Tracing Spot
d. Height Of Scanner
6. Bearing Accuracy
As per Performance Standards for Navigational RADAR (IMO), the RADAR bearing of an object whose echo appears on the edge of the display should be capable of being measured with an accuracy equal to, or better than +/- 1º.
Factors that govern bearing accuracy are:
a. Correct alignment between the heading marker and the scanner.
b. Correct alignment bet. The heading marker and the bearing scale.
c. Gyro error, if any when the display is Gyro-stabilised.
d. Rectilinearity of the trace.
e. Beamwidth distortion.
f. Scale the size of the spot.
3. Setting up and maintaining displays
How best we can set a radar to display the targets perfectly? The following are the methods [3]:
(1) Grain, Sea and Rain.
Gain is used to increase the receiving sensitivity of the radar. we use Sea control to reduce the clutter echo caused due the surface of the sea. and we use Rain control to reduce the clutter echo caused by rain. We must use Sea control only when we have moderate rough sea or when we think that interference on the radar is because of sea state. Otherwise, we should not use the sea control. In the calm sea if we clutters, it is better to reduce the gain than increase the Sea control. Moreover, we should not try to remove the interference at long range with Sea control. This is because the interference from the sea will be at close range only. The correct way to set these controls is to increase the gain maximum. reduce the sea and Rain controls to a minimum, reduce the gain to a level where most of the clutters are just removed and the targets can be distinguished, and if required increase the sea control to reduce the sea clutters near the centre of screen.
(2) Pulse Length
Short pulse length: better range resolution, less sensitivity ((targets are smaller in diameter)).
Long pulse length: less range resolution (may plot two closer targets as one), better sensitivity (targets are bigger in diameter)
Ideally, short pulse is used in short range (up to 3NM) and long pulse for long ranges (more than 12NM). But we can use the short pulse in long range and vice versa.
3.1 IMO Performance Standards
Radar minimum performance standards are set by the International Maritime Organisation. The full list of requirements and standards is contained within IMO Resolution msc.192(79) Annex 34 ‘Adoption of The Revised Performance Standards for Radar Equipment’. A copy of this resolutions annex is available to download below. The main performance standards that you should be aware of for your oral examination are:
Range Discrimination
The radar system should be capable of displaying two targets on the same bearing, separated by 40 m in range, as two distinct objects.
Range Accuracy
The accuracy of ranges should be within 1% of the range of the range scale in use or 30 m, whichever is the greater distance.
Bearing Accuracy
The system accuracy of bearings should be within 1%.
Bearing Discrimination
The radar system should be capable of displaying two targets at the same range, separated by 2.5° in bearing, as two distinct objects.
Roll and Pitch
The target detection performance of the radar equipment should not be substantially impaired when own ship is rolling or pitching up to +/-10°.
Recommendation
References
- Merrill I. Skolnik. Factors affecting radar performance. https://www.britannica.com/technology/radar/Doppler-weather-radar
- Academy. “Limitations of marine radar”. https://academy.marineinsight.com/topics/limitations-of-marine-radar-3/
- Capt Rajeev Jassal (May 23, 2016), “Marine radar: How best to set up to have it plot perfect targets on screen”. MySeaTime. #https://www.myseatime.com/blog/detail/marine-radar-how-best-to-set-up-to-have-a-perfect-targets-on-screen