RADAR – Radio Detection and Ranging Performance Standards: IMO Resolution General Info: 2 main frequencies of Radar, 3 GHz (S Band), Band), 9GHz (X Band) Index Error: The difference between the true range between points on a chart chart and the detected range is termed radar index error. It can be checked when the vessel is abeam between two known points e.g. bridge pillars, canal banks, breakwater entrance. Parallax due to Rolling: A vessel which is rolling/ pitching heavily can affect the range of contacts detected by the radar. Glint: Beamwidth: As the radar beam travels further away from the vessel the beam of energy widens distorting objects somewhat. The further away the greater the error. Thus, radar bearings should be so far as possible avoided for position fixing, especially from objects at a range. Clutter: Radars are susceptible to clutter, particularly from rain and sea and hence the clutter controls provided. These should be used with caution however since they can suppress weak contacts which which may be navigating within the clutter area. For that reason vessels should actively search through the clutter to ensure that there are no contacts present. 9 GHz radars are more susceptible than 3 GHz radars which, because of there additional power can search beyond clutter or squalls. Vessels should be aware of the power limitations associated with 9 GHz band radars. Heading Misalignment: Misalignment: Heading markers can be manually set on a radar and therefore may be misaligned. To set the heading marker point the vessel directly at another vessel (at safe range) and take a bearing of the vessel using the compass. The heading marker should correspond to the bearing taken. Blind/ Shadow Sectors: Blind and Shadow sectors of radars are common to most types of ship, owing to structures on the ship which effectively block the radar such as the funnel, masts etc. etc. Shadow sectors should be known and displayed near the radar so that all OOWs are aware of the limits of such sectors. sectors. They can be checked in heavy clutter clutter by using the EBL. OOWs should be aware of the possibility possibility of vessels approaching from within a shadow sector and should therefore periodically alter the ships heading slightly to check them. Attenuation: This is the scattering and absorption of the energy in the radar beam as it passes through the atmosphere. It causes a decrease in echo strength. It is greater at higher frequencies and shorter shorter wave lengths. Double Echoes: Double echoes are caused by the radar pulse reflecting reflecting off part of the ship such such as the funnel and into the receiver. The display will show the contact detected in the direction that the reflected pulse was detected and not the true direction of the contact. Multiple Echoes: Multiple Echoes occur from the radar pulse reverberating from another ship and own ship several times. It can often give two or three objects detected on the screen. A characteristic trait of such an error is that the echoes further away will be equally spaced and will have speed vectors which are twice , three times etc that of the nearest (true) echo. False/ Second Trace Echoes: False Echoes can appear from a number number of sources. If a pulse emitted by the radar and returns returns after the PRF generator has emitted another pulse it may appear on the radar during another sweep and will be displayed as if it was being detected by the current sweep. Distorted Coastlines: if approaching a straight coastline it may appear curved on the radar or vice versa due to the distance it takes to reach and return from areas further away from the centreline heading of the vessel. Propagation Errors (Ducting, Sub/ Super Refraction): These errors occur due to certain atmospheric atmospheric conditions. conditions. Ducting is where the radar beam reflects off a layer within (e.g. temperature inversion) the sky and is deflecting back to ground and then off the sea surface. Input Limitations: Modern Radars and ARPAs are fed from a number of sources including GPSs, Gyro Compasses and Logs. Navigators should be aware of the limitations of these inputs and there affect on the radar. PI/ Coastline Limitations: Limitations : When using PIs, navigators should be aware of the possible distortion from beam width error of coastal points ahead of the vessel. PIs should be used, as far as possible from points which are abeam or nearly abeam of the vessel rather than ahead or astern. Because of the distortion, the vessel may seem to be off track and force the navigator to make an alteration. Coastlines detected by radar should be used with caution, especially if the coastal area is shallow shelving were drying out may occur. Indirect Wave Error: This is where the radar beam is deflected off the sea surface before it reaches the contact resulting in the beam travelling a greater distance than taking a direct path. Spoking and Interference Side Lobe
Echo Sounders Principle: The transmitter emits a pulse of energy which radiates out from the vessel, similar to radar. The pulse will be reflected and returned by the seabed and received by the receiver on board. The time taken for the pulse to return is converted into distance, much the same as radar except that sound travels at 1500 m/s in water. Errors Speed of Propagation: The speed of the pulse will vary with the temperatures and salinities of water and therefore may affect the pulse. Aeration: The presence of air in the water will affect the speed of the pulse since sound travels slower in air. The main sources of aeration are; sternway, turbulence caused from hard rudder, broken water over shoals, entering areas in which there has been bad weather/ turbulent waters, light ships pitching heavily. The ship’s bow wave would also constitute an area of aeration however most echo sounders are sighted forward of the position where the bow waves re enters the water. Bottom Reverberation: Caused by excess power being emitted and or shallow water beneath the vessel. The power is so great that the pulse actually rebounds off the seabed more than once, being sent back as it bounces of the vessel’s hull showing multiple seabeds. Attenuation due to Noise: Any ambient noise present in the water has the ability to degrade the echo sound signal. Such noise may be from marine mammals, turbulent water or other traffic operating in the vicinity. Absorption due to soft Seabeds: If the seabed is too soft then there is a possibility that the pulse will be absorbed or attenuated. False Echoes: May occur in deep water if the echo sounder is incorrectly set. The returning echo will be received after the stylus has completed one complete revolution showing a false bottom.
Compasses and Care of Compasses Gyro Compass
Limitations
Polar Navigation Speed/ Latitude Precession Gimballing Electrical Power
Magnetic Compass Errors/ Limitations Variation Deviation Construction Heavy Steel or Iron Objects Electrically Induced Magnetic Fields Correctors
ECDIS
GPS 24 Operational satellites, 6 orbital planes, 55° inclination to the Equator at altitudes of 11000 miles. Configuration ensures a minimum of 4 satellites with suitable elevations are visible to a receiver, anywhere on the Earths surface, however there is poorer coverage in the Polar regions. A GPS Fix is obtained from measuring the ranges from a series of selected satellites to a receiver and the ranges are determined by measuring the propagation times of satellite data transmissions. The ranges measured are not ‘true’ ranges but are termed ‘pseudo ranges’ and contain a receiver clock offset error. For a 2-D fix on the Earths surface at least 3 pseudo ranges are required.
Errors with GPS Ionospheric and Atmospheric Error Caused by; Signal passing through Ionosphere and Troposphere. Ionosphere – high Iondensity - Signal path delays can be predicted using complex mathematical models. Troposhpere is the area in which the Earth’s weather is produced and because of its volatility and constantly changing parameters, errors cannot be effectively predicted in this region. Errors in Position; Ionosphere: Up to 5m Troposphere: Up to 1 metre Multipath Errors Caused by: Reflections of the signal off surfaces near the receiver. Instead of following a straight path to the receiver’s antenna it is bounced off various local obstructions which increase the length of the signal. These signals confuse the receiver’s calculations and cause a wave interference with the direct signal which produces a confused or ghosting effect. Errors in Position; Undetermined but good quality receivers can minimize the problem. Satellite Clock Bias Errors Caused by: Minute discrepancies in accuracy of the atomic clocks. Very small errors can translate into signal time measurement errors. Errors in Position; Degradation by about 1.5 metres. Ephemeris/ Position Errors Caused by; Small errors in the exact positioning of the satellites. Known as ephemeris errors, they occur between the specific monitoring periods resulting in small inaccuracies with the calculated position. Errors in Position: approximately 2.5 metres. Geometric Dilution of Precision (GDOP/ VDOP/ HDOP) Caused by: Receivers tracking satellites in poor geometric positions. If satellites are being tracked at low elevation they will produce a poor vertical position (VDOP). If the receiver selects satellites that are widely spaced the result would be good crossing at nearly right angles. Errors in Position: Undetermined. Solar Activity Caused by: Solar activity, eruptions and storms etc can have a dramatic effect on the transmission of the GPS signal. The disturbances are known as Coronal Mass Ejections (CMEs). Sun’s activity varies on 11 year cycle – next maximum cycle expected 2012. Errors in Position: Anything from negligible distances to total loss of signal. Datum Errors Caused by: GPS system being designed to work on one datum only, WGS84. Although many GPS receivers have the ability to switch between the range of datum’s available, it is recommended that WGS84 is maintained. This is further compounded by nautical charts since some charts are not constructed with reference to WGS84 datum and thus plotting a WGS84 GPS position onto the chart could prove detrimental. Navigators should read chart notes and identify what datum the chart is constructed on before plotting positions. Selective Availability Caused by: Intentional degradation of GPS signal by adding random noise into the clock data thereby degrading the timing signals. Errors in position: Up to 100m Source ALRS Vol 2
Improvement of Accuracy DGPS DGPS compares the position of a fixed point (reference station) with a position obtained from the GPS receiver at that point. It calculates a 2-D or 3-D geographical co-ordinate offset (position differential) or series of corrections to the satellite range data (pseudorange differential). The data is calculated and corrections are broadcast to GPS receivers by radio; thus an MSK radiobeacon receiver and GPS receiver capable of incorporating DGPS correction data is required by the user. SBAS (Satellite Based Augmentation Systems) Overlay systems for GPS/ GLONASS which use a suite of geostationary satellites and networks of ground relay stations that offer more reliability to the user through improved accuracy, availability, integrity and continuity. There are 3 major components; ● EGNOS (European Navigation Geostationary Overlay Service) ● WAAS (American Wide Area Augmentation System) ● MSAS (Japanese Multi-functional Transport Satellite based AS) Signals broadcast by SBAS are totally compatible and do not interfere with GPS/ GLONASS reception. No set up or subscription fees and standard SBAS receivers do not require any additional equipment. Positions reduced to between 1 – 5 metres.
