Basic Radar, Sonar & Doppler Theory

1645 words - 7 pages

BASIC RADAR, SONAR, & DOPPLER THEORYRADARThe term RADAR is an acronym made up of the words radio detection and ranging. In today's society radar plays an important part of our every day lives. Thousands of lives rely on the accuracy and speed of radar to guide their plane through the skies unscathed. Radar is also used to provide us our daily weather report and forecast. While radar is taken for granted in our daily lives, it is a relatively new concept. Early versions of radar systems were first put into use shortly after the beginning of World War II.The principle on which radar works is relatively simple and very similar to reflections of sound waves, echo, a phenomenon many of us have experienced. In an echo, a sound is sent out in all directions. When the sound waves find an object, such as a cliff face or bottom of a well, they will bounce back to the source of the echo.One of the primary commercial and military uses of radar is ranging. Technique of calculating range based on radar data is quite simple. We know electromechanical (EM) energy radiated from a radar system travels at approximately 984 feet per microsecond, and we know that a nautical mile is 6,080 feet. Therefore, we can calculate using the formula 6,080 feet divided by 984 feet per micro-second equals the time it takes for EM energy to travel one nautical mile, 6.18 microseconds, or, 6,080/984 = 6.18 ms. Keep in mind, however, that in order for the time to be calculated, the energy also has to return that same distance back to the radar receiver. So we can say- the time required for energy to travel one mile and return is 12.36 ms. This 12.36 microsecond interval is referred to as a radar mile (NAVEDTRA 1-3). Remember I said calculating range using radar data is quite simple? Here it is: range = total elapsed time/12.36. So if elapsed time is 62 microseconds, then the distance would be five nautical miles. Of course since most stop watches don't measure micro-seconds, range is calculated in the radar system's processor.Modern radar systems are much more complex and provide much more data than just range. In fact, today's radar systems are capable of telling us direction the echo comes from, direction of movement, and using Doppler effect, speed. High tech radar systems also have a high degree of resolution. In other words, they are capable of determining when there are two or more contacts very close together. This is can be critical to the military who needs to know if there is one airplane flying inbound or several.DOPPLERThe technical gist of Doppler Theory as related to sound waves or EM waves is the change in the apparent time interval between two waves which results from the relative motion of a listener or radar receiver together with the known velocity of transmission of the waves. A simpler way to word it would be: a motion-induced change in the observed (heard or received) wavelength (frequency) of a wave. The examples I most often use when teaching radar...


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