Radar Basics - SailNet Community
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Radar Basics

If I could have only one instrument for coastal navigation, I would choose a radar unit. In the hands of a skilled operator, radar provides precise positioning and is an invaluable aid to boating safety. During periods of restricted visibility, radar not only provides navigational fixes but helps in collision avoidance and can even reveal uncharted hazards and buoys.

In a sense, radar is an extension of visual navigation, allowing you to see vessels, buoys and land areas at greater distances in any conditions. Color radar uses different hues to identify the intensity of a return. For example, by being able to read the stronger precipitation in a rainsquall or thunderstorm, you could choose to avoid these areas. Target plotting functions, on the other hand, allow the course and speed of other vessels to be measured for collision avoidance, such as in fog. On some radar units, the "Guard Zone" alarm generates audio and visual alarms when a target comes within a range, as preset by the user. This alarm system can also be switched to the anchor-watch mode for dragging. For those concerned about power consumption, most of the small-boat radar units come with a standby-sleep-mode conserving 12-volt usage.

Let's discuss now how marine radar works. The fundamental principle may be likened to an echo. A ship, for example, might determine the distance it is to a cliff-like shore by blowing its horn and timing the interval till the echo is received. Likewise, radar emits short pulses of microwave radiation, which, like sound, are reflected by objects in its path. Radar detects the reflected pulses and then calculates the distance to the object at that bearing. It uses the time delay between the transmission of the pulse and the arrival of its reflection. The reflected pulse is then amplified and converted for display on the screen. With the radar antenna scanning the horizon, this process is repeated in every direction. The resulting information is displayed on a screen as a 360-degree pictorial representation of the area surrounding the boat.

A typical radar system consists of three basic components: An R/T unit (transmitter and receiver also called a transceiver), an antenna (also called a scanner), and a display unit or scope. In most small radar units, the transceiver is located in the scanner. The transmitter portion of the R/T unit generates radio frequency pulses that are emitted as a beam as the scanner rotates. A switching device interrupts the transmission at regular intervals when it then uses the antenna and receiver unit to pick up radio frequency energy reflected off objects in the path of the transmitted beam.

The time it takes for the RF pulse to make the trip from the scanner to the object and for the echo to return is used to calculate the range. Radio frequency energy travels at the speed of light (162,000 nautical miles per second or 985 feet per microsecond). If the interval between the transmission of the RF pulse and return of the echo is 200 microseconds, the distance to the target is 985 times 200 divided by 2 which equals 98,500 feet or 16.2 nautical miles. The return is then displayed on the scope at that distance on the scanner's bearing.

The direction in which the scanner is pointing when it receives the echo represents the bearing of the object. (I won't get into the various types of bearings just yet.) Since the scanner is rotating at about 24 revolutions per minute, or less than three seconds per revolution, you can easily see that the scanner does not rotate very much in 200 microseconds (one microsecond is one millionth of a second.) However you must remember that a bearing measurement will always be less accurate than a distance measurement. This is why plotting multiple ranges will give a better fix than multiple bearings or a single range and bearing.

Marine radar operates on either the X-band (9000 MHz) or S-band (3000 MHz) frequencies. The higher the frequency, the shorter the wavelength. Because the shorter wavelength allows the use of a smaller antenna, most small-boat radar units operate on the X-band. Both bands will detect and display precipitation, but longer wavelengths do a better job of penetrating rain to spot targets. The shorter wavelength of X-band radar will weaken more when passing through precipitation, thereby reducing the amount of reflected energy from targets. If you don't have an Anti-Clutter Rain circuit, increasing the receiver gain for one sweep and then reducing it can sometimes allow you to burn through the precipitation and see a target.

The angular dimensions of the radar beam also affect the performance. The vertical angle of the beam width emitted from the scanner is typically 20 to 25 degrees with half of it above the horizontal plane of the scanner and half below. The horizontal beam width emitted from the scanner can be less than one degree and increases up to five degrees on the smaller radar sets. This large vertical beam width allows the radar to scan the water surface even during strong rolling and pitching movements of the vessel. The horizontal beam width depends upon the frequency or wavelength of the transmitted energy and the size of the scanner. For a given wavelength, narrower horizontal beam widths require larger scanners. The smaller the horizontal beam width, the better the target resolution of bearing will be. Whereas a small horizontal beam width can allow you to see separate target returns when they are close together, a larger horizontal beam width will blend them together as one large return. Three to five degrees is typical for small, less-expensive radar units, while larger more-expensive radar will have a horizontal beam width of one degree or less.

The radar horizon is not the same as the geometrical horizon of the scanner's height. Atmospheric density gradients bend radar rays as they travel to and from the target. This bending is called refraction. The radar horizon in nautical miles is 1.22 times the square root of the scanner height in feet, while the visible horizon in nautical miles is 1.169 times the square root of the height of eye in feet. The maximum radar range is determined by the height of the scanner and the power output of the transceiver. Objects beyond the radar horizon will not be detected unless a reflecting surface extends above the horizon. This is why tall inland hills, cliffs and mountains can be seen and displayed well before the lower shoreline. Nothing is depicted on the display beyond the first object detected unless it is taller. This is why you won't see a small low-lying island that is behind a larger one.

Most mariners use radar in addition to a GPS plotter or computerized navigation and electronic charting system. I firmly believe that the more aids to navigation you use, the safer you will be. I realize that not everyone has the required skill or training to operate radar and interpret the returns effectively in all conditions. Luckily there are many radar training aids available and with practice anyone can become proficient in using radar and skilled in interpreting the returns on the screen.

Jim Sexton is offline  
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