Icom MR-1220 Instruction Manual - Page 21

Basic operation 2 Basic radar theory DDMinimum range The minimum range of the radar is primarily determined by transmitter pulse length, vertical beam width, and the scanner unit height. The quicker transmission, the faster the echo returns so that the distance can be measured faster. If the scanner is placed too high from the surface of the water, the scanner's vertical beam overshoots the targets that are closer to your vessel. Vertical beam width Out of the beam cannot be detected. Pulse length Shorter target than the pulse length cannot be detected. DDBlind and shadow sectors A blind or shadow sector may appear because of obstructions such as masts or other metal objects. An obstruction may throw either a complete or partial shadow as shown below. If a target is in a shadow sector, target echoes may not appear on the screen. Mast or similar obstruction Shadow sector When tall and massive targets such as a large island are located at close range also shadowed without producing any echoes. This phenomenon is called blind sector. It is very important to know the bearings and widths of all shadow sectors caused by your own vessel's obstructions. DDTarget resolution 1 Target resolution is determined, by the horizontal beam width and transmit pulse width. It can be difficult 2 to detect two targets that are separated by a short distance or in the same direction. 3 Distance resolution 4 When the distance between two targets is more than the pulse width, they are displayed as two echoes. 5 Target 1 Target 2 6 7 Transmit Echoes pulse Target 1 8 Target 2 9 However, if the distance between them is less than the pulse width, they are displayed as one echo. 10 Target 1 Target 2 11 12 Transmit Echo pulse Overlapped 13 targets 1 and 2 14 15 Direction resolution When two targets are separated by more than the 16 horizontal beam width, they are displayed as two echoes. 17 Target 1 18 Target 2 19 Scanner Target 1 Target 2 20 21 However, if they are closer by less than the horizontal beam width, they are displayed as one echo. Target 1 Your vessel Large island Small island Shadow sector Scanner Target 2 Overlapped target 1 and 2 16