Edmund Fitzgerald

Date 9-10-06

Radar stands for radio detection and ranging. Radar was used first used on ships to detect icebergs and other obstacles. The most common radar transmits a train of narrow, rectangular-shape pulses modulating a sinewave carrier. The echo signal is returned and evaluated on a cathode ray tube, CRT. Distance is measured by the time it takes the pulse to travel to and from the target.

A continuous waveform, CW, radar can also be used. This radar shows the doppler frequency shift to measure the velocity of a target.

Most radars operate at frequencies from about 220 MHz to 35 GHz. However, some radars have operated as high as 5 MHz and others at 94 GHz. Knowing the frequency the wave length is determined by dividing the frequency in MHz into 300 to give the wave length in meters. Usually the higher the frequency, the sharper the return signal and the more accuracy can be obtained in determining distance and location.

• When a boat heels or rolls more than 10 to 15 degrees, a fixed radar system scans for submarines on one side and airplanes on the other.
• Clutter is what radar users call radar signals that do not come from actual targets. Rain, Snow, Waves, and the surface of the earth reflect energy, including radar waves. Such echoes can produce signals that the radar system may mistake for actual targets. Clutter makes it difficult to locate targets, especially when the system is searching for objects that are small and distant.
• A radar system uses four basic components: a transmitter, an antenna, a receiver, and a display. The transmitter generates radio signals. The antenna transmits these signals as electromagnetic radiation into the airspace. When a target, such as an ship, enters the radar airspace, it scatters some of these radio waves, which reach the receiving antenna. An electronic amplifier amplifies these returned signals and displays them on a cathode ray tube (CRT) display where a radar operator can examine them. The location of the object being detected is determined by measuring the time it takes for the radio wave to travel from the transmitter to the object and back to the receiver.
• Radar also helps ships navigate through dangerous waters and avoid collisions. Unlike air-traffic radar, with its centralized networks that monitor many craft, maritime radar depends almost entirely on radar systems installed on individual vessels. These radar systems search the surface of the water for landmasses; navigation aids, such as lighthouses and channel markers; and other vessels. For a ship’s navigator, echoes from landmasses and other stationary objects are just as important as those from moving objects. Consequently, marine radar systems do not include clutter removal circuits. Instead, ship-based radar depends on high-resolution distance and direction measurements to differentiate between land, ships, and unwanted signals.

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