Sunday, March 17, 2013

Marine radar

Marine radars are x-band or s-band radar to provide bearing and distance of ships and land targets in vicinity from own ship (radar scanner) for collision avoidance and navigation at sea.
Radar is a vital component for safety at sea and near the shore. Captains need to be able to maneuver theirs ships within feet in the worst of conditions and to be able to navigate "blind". This means inside a dark room with no visibility they need to safely navigate their way through waters in the worst of weather. Radars are rarely used alone in a marine setting. In commercial ships, they are integrated into a full system of marine instruments including chartplotterssonar, two-way radio communication devices, and emergency locators (EPIRB).
The integration of these devices is very important as it becomes quite distracting to look at several different screens. Therefore, displays can often overlay charting, radar, sonar into a single system. This gives the captain unprecedented instrumentation to maneuver the ship. With digital backbones, these devices have advanced greatly in the last years. For example, the newer ones have 3D displays that allow you to see above, below and all around the ship, including overlays of satellite imaging.
While private mariners are not subject to the same safety standards as commercial mariners, not having the correct electronics can lead to serious mishaps, including collisions with other vessels, running aground, running out of fuel and getting lost. It is very difficult to navigate waterways without navigation equipment and it is easy for a captain to get lost. You should have the correct equipment based on the size of your boat. This is not only for your safety but for the safety of others around you.
In port or in harbour, shore-based vessel traffic service radar systems are used to monitor and regulate ship movements in busy waters.

Collision avoidance
As required by COLREGS, all ships shall maintained a proper radar lookout if it is available on board to obtain early warning of risk of collision. Radar plotting or ARPA should be used to get the information of movement and the risk of collision (bearing, distance, CPA (closest point of approach), TCPA) of other ships in vicinity.

Marine radar systems can provide very useful radar navigation information for navigators onboard ships. Ship position could be fixed by the bearing and distance information of land target on radar screen.

Radar Controls
Marine radar has performance adjustment controls for brightness and contrast, gain, tuning, sea clutter and rain clutter suppression, and other interference reduction. Other common controls consist of range scale, bearing cursor, fix/variable range marker or bearing/distance cursor.

Radar Navigation
Marine and aviation radar systems can provide very useful navigation information in a variety of situations. When a vessel is within radar range of land or special radar aids to navigation, the navigator can take distances and angular bearings to charted objects and use these to establish arcs of position and lines of position on a chart. A fix consisting of only radar information is called a radar fix.
Some types of radar fixes include the relatively self-explanatory methods of "range and bearing to a single object," "two or more bearings," "tangent bearings," and "two or more ranges."
Parallel indexing is a technique defined by William Burger in the 1957 book The Radar Observer's Handbook. This technique involves creating a line on the screen that is parallel to the ship's course, but offset to the left or right by some distance. This parallel line allows the navigator to maintain a given distance away from hazards.
Some techniques have been developed for special situations. One, known as the "contour method," involves marking a transparent plastic template on the radar screen and moving it to the chart to fix a position.
Another special technique, known as the Franklin Continuous Radar Plot Technique, involves drawing the path a radar object should follow on the radar display if the ship stays on its planned course. During the transit, the navigator can check that the ship is on track by checking that the pip lies on the drawn line.
The Yeoman Plotter uses both radar, GPS and traditional charts to plot courses and is one of the most used plotters today.
After completing the plotting radar technique, the image from the radar can either be displayed, captured or recorded to a computer monitor using a frame grabber.

Navigation Processes
Day's work in navigation
The Day's work in navigation is a minimal set of tasks consistent with prudent navigation. The definition will vary on military and civilian vessels, and from ship to ship, but takes a form resembling:
  1. Maintain continuous dead reckoning plot.
  2. Take two or more star observations at morning twilight for a celestial fix. (prudent to observe 6 stars)
  3. Morning sun observation. Can be taken on or near prime vertical for longitude, or at any time for a line of position.
  4. Determine compass error by azimuth observation of the sun.
  5. Computation of the interval to noon, watch time of local apparent noon, and constants for meridian or ex-meridian sights.
  6. Noontime meridian or ex-meridian observation of the sun for noon latitude line. Running fix or cross with Venus line for noon fix.
  7. Noontime determination the day's run and day's set and drift.
  8. At least one afternoon sun line, in case the stars are not visible at twilight.
  9. Determine compass error by azimuth observation of the sun.
  10. Take two or more star observations at evening twilight for a celestial fix. (prudent to observe 6 stars)

Passage planning
Passage planning or voyage planning is a procedure to develop a complete description of vessel's voyage from start to finish. The plan includes leaving the dock and harbor area, the enroute portion of a voyage, approaching the destination, and mooring. According to international law, a vessel's captain is legally responsible for passage planning, however on larger vessels, the task will be delegated to the ship's navigator.
Studies show that human error is a factor in 80 percent of navigational accidents and that in many cases the human making the error had access to information that could have prevented the accident. The practice of voyage planning has evolved from penciling lines onnautical charts to a process of risk management.
Passage planning consists of four stages: appraisal, planning, execution, and monitoring, which are specified in International Maritime Organization Resolution A.893(21), Guidelines For Voyage Planning, and these guidelines are reflected in the local laws of IMO signatory countries (for example, Title 33 of the U.S. Code of Federal Regulations), and a number of professional books or publications. There are some fifty elements of a comprehensive passage plan depending on the size and type of vessel.
The appraisal stage deals with the collection of information relevant to the proposed voyage as well as ascertaining risks and assessing the key features of the voyage. In the next stage, the written plan is created. The third stage is the execution of the finalised voyage plan, taking into account any special circumstances which may arise such as changes in the weather, which may require the plan to be reviewed or altered. The final stage of passage planning consists of monitoring the vessel's progress in relation to the plan and responding to deviations and unforeseen circumstances.
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