It is found that the frequency domain target signatures are distinct for different aircraft types and at different aspects. As a result, target identification in the frequency domain is just as viable as using conventional high-range resolution profiles in the range domain. In addition, there are many advantages of working in the frequency domain; many of the problems encountered in the range domain can be avoided.
This leads to a much simpler task in performing target recognition.
Introduction to radar target recognition. - Free Online Library
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The X-Band radar which makes use of a small light antenna operates on the 9, MHz band and has a wavelength of 3cm. The short wavelength allows better directivity but signal loss in rain is higher than on S-Band which operates 3, MHz band and has a wavelength 10 cm.
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S-Band is used for longer range detection and is less affected by poor weather. Radar was first used some 70 years ago on merchant ships in and initially mostly by ferries allowing them to continue to operate safely in fog and at night. In recommendations for the use of radar were formulated for inclusion in the collision regulations.
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Later in with the new SOLAS convention, radar was made mandatory on ships in a rollout programme starting in and completing as surprisingly recently as All passenger ships and other ships above gt are obliged to carry at least one radar system operating on the X-band and ships over 3,gt are required also to carry a second radar operating in the S-band.
The radar systems installed can be stand-alone systems or they can be systems that connect to other navigation systems in an integrated system. Considering the length of time that radar has been used on merchant ships and that use of radar forms part of the competencies required of navigators under STCW, its improper use has been cited in many official investigations as the root cause of collisions and groundings.
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Most criticisms centre on mistakes in plotting and identification of targets which carry no identifying features. It was to help with this that AIS was made mandatory and as part of the introduction of AIS and the move to increasing use of electronic navigation, electronic plotting was also mandated. Ships built before 1 July are subject to slightly different regulations with regard to how the movement of targets on the radar display can be plotted. For system fitted the since 1 July radars must be equipped with plotting aids, the type of which depends upon the size of ship On ships of 3,gt and over the second radar must also be equipped with an ATA, the two ATAs must be functionally independent of each other.
Detection, Range and Doppler Estimation
Automatic Radar Plotting Aid ARPA ARPA equipment provides for manual or automatic acquisition of targets and the automatic tracking and display of all relevant target information for at least 20 targets for anti-collision decision making. It also enables trial manoeuvre to be executed Ships of 10,gt and over. To estimate risk of collision with another vessel the closest point of approach CPA must be established. The apparent precision of digital read-outs should be treated with caution.
If two radars are fitted mandatory for ships of 3,gt and over it is good practice, especially in restricted visibility or in congested waters, for one to be designated for anti-collision work, while the other is used to assist navigation.
If only one of the radars is fitted with ARPA then this should be the one used for anti-collision work and the other for navigation. Because the radar display is produced electronically from the signals generated by the system, it was always going to be an essential component of e-Navigation and integrated systems. No one would argue that the migration to wide screen displays as used in integrated systems has taken radar to a new level compared with the original cathode ray tubes but future improvements and enhancements promise to add more value and functionality.
Introduction to Radar Target Recognition
Moving away from the monochrome cathode ray tube display to modern screen displays also allows the use of a wider colour palette to differentiate between systems and targets. ETD enhances the display of slow-moving or stationary targets without interfering with the normal radar appearance or controls by treating static returns in a different way from moving returns and displays the moving ones in a different colour. Other added value systems include oil spill radar and small target radar that can identify very small targets in rough seas and have radar that can accurately determine dominant wave length, direction and period, significant wave height and superficial currents and send the data to systems for use in navigation support and safety measures.
Oil spill radar is not a navigational requirement but its use in offshore oil production and anti-pollution activities means it is carried on many ships working in those sectors.
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All passenger vessels and other ships above gt are required to be fitted with an echo sounder which uses sonar to measure the depth beneath the ship. The IMO performance standards require a range from 2m — m and the ability to use two different scales; one for shallow waters to 20m and a second for deeper waters. The echo sounder must be capable of giving an alarm when a pre-determined minimum depth is encountered.
Data must be recorded and at least the previous 12 hours of information recorded and be available. Older echo sounders recorded the information on paper rolls and this is still a permitted method.