"; function search(frm) { win = window.open("","scrollbars"); win.document.write(page); txt = frm.srchval.value.split(" "); fnd = new Array(); total=0; for (i = 0; i < item.length; i++) { fnd[i] = 0; order = new Array(0, 4, 2, 3); for (j = 0; j < order.length; j++) for (k = 0; k < txt.length; k++) if (item[i][order[j]].toLowerCase().indexOf(txt[k]) > -1 && txt[k] != "") fnd[i] += (j+1); } for (i = 0; i < fnd.length; i++) { n = 0; w = -1; for (j = 0;j < fnd.length; j++) if (fnd[j] > n) { n = fnd[j]; w = j; }; if (w > -1) total += show(w, win, n); fnd[w] = 0; } win.document.write("

Total found: "+total+"
"); win.document.close(); } function show(which,wind,num) { link = item[which][1] + item[which][0]; line = ""+item[which][2]+" Score: "+num+"
"; line += item[which][4] + "
"+link+""; wind.document.write(line); return 1; } Wireless Technology

Home  | About Us | Site Map |
  
 
.......................................................................................................................................................................................................
Wireless Systems Wireless Technology Protocols & Languages Professional Bodies Glossary
Wireless Technology                                                                RADAR
  Blue Tooth
  Radar
  Satellite Communication
  WiFi
  WiMAX
  WLAN
  Zigbee

Radar is actually an acronym that stands for Radio Detection And Ranging. It was developed in the early 1940s. Radar uses the echo principle. Radar equipment emits a high energy radio signal from an antenna. The signal travels out from the source until it is reflected back by contact with an object. The radar antenna relays this signal to a scope where the image is displayed. Using the time it takes for the emitted signal to reach the object and reflect back to its source, the distance to the object can be computed. The radar signal is moving at the speed of light and can make such a trip in microseconds.

Let's take a typical radar set designed to detect airplanes in flight. The radar set turns on its transmitter and shoots out a short, high-intensity burst of high-frequency radio waves. The burst might last a microsecond. The radar set then turns off its transmitter, turns on its receiver and listens for an echo. The radar set measures the time it takes for the echo to arrive, as well as the Doppler shift of the echo. Radio waves travel at the speed of light, roughly 1,000 feet per microsecond; so if the radar set has a good high-speed clock, it can measure the distance of the airplane very accurately. Using special signal processing equipment, the radar set can also measure the Doppler shift very accurately and determine the speed of the airplane.

The radar antenna sends out a short, high-power pulse of radio waves at a known frequency. When the waves hit an object, they echo off of it and the speed of the object Doppler-shifts the echo. The same antenna is used to receive the much-weaker signals that return.

In ground-based radar, there's a lot more potential interference than in air-based radar. When a police radar shoots out a pulse, it echoes off of all sorts of objects -- fences, bridges , mountains, buildings. The easiest way to remove all of this sort of clutter is to filter it out by recognizing that it is not Doppler-shifted. A police radar looks only for Doppler-shifted signals, and because the radar beam is tightly focused it hits only one car.

Police are now using a laser technique to measure the speed of cars. This technique is called lidar , and it uses light instead of radio waves .

 

BACK