RADAR -Range and Bearing Discrimination and Accuracy

Radar – What is Radar Acronym – Radio Detection and Ranging An electronic device capable of detecting an object and determining its position in terms of range and bearing from the radar itself. With further analysis radar can deduce an objects speed and direction

Beam Width Beamwidth Beamwidth is the angular width, horizontal or vertical, of the path taken by the Radar pulse. Horizontal beamwidth ranges from 0.75 to 5 degrees, and vertical beamwidth from 20 to 25 degrees.

There are couple of terms related to radar that we need to cover before we proceed further. These are radar resolution and radar sensitivity. 1) Radar Resolution Radar resolution is the ability of the radar to show two targets which are close to each other as separate targets on radar. There are two types of radar resolution. Range resolution and bearing resolution

As the name suggests, better bearing resolution means radar can paint two near targets on same bearing as separate ones. Bearing resolution depends upon Horizontal beam width of the radio wave.

8 Same way, better range resolution means radar can paint two close targets (on different bearing) as separate ones. Range resolution depends upon pulse length.

Radar The basic principle of radar is to determine the range to an object or "target" by measuring the time required for an extremely short pulse of very high radio frequency, transmitted as a radio wave, to travel from a reference source (own ship) to a target and return as a reflected echo. The radar antenna (called the scanner) rotates to scan the entire surrounding area. Bearings to the target are determined by the orientation of the antenna at the moment when the reflected echo returns.

Radar Radar Resolution There are two important factors in radar resolution: bearing resolution and range resolution. Bearing Resolution Bearing resolution is the ability of the radar to display as separate pips the echoes received from two targets which are at the same range and close together. It is proportional to the antenna length and reciprocally proportional to the wavelength.

Radar Range Resolution Range resolution is the ability to display as separate pips the echoes received from two targets which are on the same bearing and close to each other. This is determined by pulselength only. Practically, a 0.08 microsecond pulse offers the discrimination better than 25 meters as do so with all Furuno radars. Test targets for determining the range and bearing resolution are radar reflectors having an echo area of 10 square meters

Radar Bearing Accuracy One of the most important features of the radar is how accurately the bearing of a target can be measured. The accuracy of bearing measurement basically depends on the narrowness of the radar beam. However, the bearing is usually taken relative to the ship’s heading, and thus, proper adjustment of the heading marker at installation is an important factor in ensuring bearing accuracy. To minimize error when measuring the bearing of a target, put the target echo at the extreme position on the screen by selecting a suitable range.

Radar. In piloting most radar bearings are accurate only to within 3 to 5 degrees. A welltuned radar gives ranges precise to within ±100 yards out to the radar horizon. Therefore radar range LOP's are preferred over radar bearings.

Radar. How Radar determines bearing Radar determines the range to a target by measuring the amount of time required for a reflected echo to return to the scanner. Bearing to a target is determined by the direction from which a reflected echo returns. The scanner rotates 360 degrees about its vertical axis, using a special gear. In order to achieve precise bearing resolution the antenna radiates RF (radio frequency) power in the form of a highly directional beam. “Super” beams having horizontal beamwidth on the order of one 1 degree or less provide highly precise bearing information. The sharper the beam, the more accurately the bearing of a target can be determined.

Radar. How Radar determines range The radio pulse makes a complete round trip, but only half the time of travel is needed to determine the range to the target. This equation shows how range is determined: D = 1/2 x cT c = Speed of Radio Pulse (3 x 108 m/sec) T = Time between transmission of radio pulse and reception of reflected echo D = Distance

Radar range Atmospheric conditions and target shape, material and aspect slightly affect Radar range. However, Radar range is generally calculated as follows: Figure 1 - Determining Radar range D is the distance from the scanner to the target horizon. Under normal atmospheric conditions, this distance is 6% greater than the optical horizonThis is because radio waves bend or refract slightly by atmospheric change. The higher the scanner or target is above the surface, the longer the detection range. For example, if the scanner is 9 meters above the sea surface and the height of the target is 16 meters, you should be able to see the target’s echo on the display when the target is 15 miles from the Radar.

Radar- Position fixing The Radar detects solid objects like other vessels and land. The display shows your relative position (in the centre of the display), and can give you a range and bearing to a distinctly identifiable point of land that is visible on the screen NB: Radar is very good at finding the range of a target but not so good at finding its bearing. This is because the beam sent out from the radar antenna has a width of a few degrees, this means that it can be difficult to identify the exact bearing of a target.

Radar So to position fix we are best using the ranges. With the range of a charted object you can draw an arc at that distance from that object to give you a position line. As with any position line, you need to cross it with other position lines to get an accurate fix. If the ranges to three targets are measured three curved position lines can be plotted to give a fix.