Principios Basicos De Radar

CHAPTER 1 — BASIC RADAR PRINCIPLES AND GENERAL CHARACTERISTICS
INTRODUCTION The word radar is an acronym derived from the phrase RAdio Detection And Ranging and applies to electronic equipment designed for detecting and tracking objects (targets) at considerable distances. The basic principle behind radar is simple - extremely short bursts of radio energy (traveling at the speed of light) aretransmitted, reflected off a target and then returned as an echo. Radar makes use of a phenomenon we have all observed, that of the ECHO PRINCIPLE. To illustrate this principle, if a ship’s whistle were sounded in the middle of the ocean, the sound waves would dissipate their energy as they traveled outward and at some point would disappear entirely. If, however the whistle sounded near an objectsuch as a cliff some of the radiated sound waves would be reflected back to the ship as an echo. The form of electromagnetic signal radiated by the radar depends upon the type of information needed about the target. Radar, as designed for marine navigation applications, is pulse modulated. Pulse-modulated radar can determine the distance to a target by measuring the time required for an extremelyshort burst of radio-frequency (r-f) energy to travel to the target and return to its source as a reflected echo. Directional antennas are used for transmitting the pulse and receiving the reflected echo, thereby allowing determination of the direction or bearing of the target echo. Once time and bearing are measured, these targets or echoes are calculated and displayed on the radar display. Theradar display provides the operator a birds eye view of where other targets are relative to own ship. Radar is an active device. It utilizes its own radio energy to detect and track the target. It does not depend on energy radiated by the target itself. The ability to detect a target at great distances and to locate its position with high accuracy are two of the chief attributes of radar. There aretwo groups of radio frequencies allocated by international standards for use by civil marine radar systems. The first group lies in the Xband which corresponds to a wavelength of 3 cm. and has a frequency range between 9300 and 9500 MHz. The second group lies in the S-band with a wavelength of 10 cm. and has a frequency range of 2900 to 3100 MHz. It is sometimes more convenient to speak in termsof wavelength rather than frequency because of the high values associated with the latter. A fundamental requirement of marine radar is that of directional transmission and reception, which is achieved by producing a narrow horizontal beam. In order to focus the radio energy into a narrow beam the laws of physics prevail and the wavelength must be within the few centimeters range. Theradio-frequency energy transmitted by pulse-modulated radars consists of a series of equally spaced pulses, frequently having durations of about 1 microsecond or less, separated by very short but relatively long periods during which no energy is transmitted. The terms PULSEMODULATED RADAR and PULSE MODULATION are derived from this method of transmission of radio-frequency energy. If the distance to a targetis to be determined by measuring the time required for one pulse to travel to the target and return as a reflected echo, it is necessary that this cycle be completed before the pulse immediately following is transmitted. This is the reason why the transmitted pulses must be separated by relatively long nontransmitting time periods. Otherwise, transmission would occur during reception of thereflected echo of the preceding pulse. Using the same antenna for both transmitting and receiving, the relatively weak reflected echo would be blocked by the relatively strong transmitted pulse.

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A BRIEF HISTORY Radar, the device which is used for detection and ranging of contacts, independent of time and weather conditions, was one of the most important scientific discoveries and technological...