![]() ![]() The maximum radar range is one of the most important concerns for engineers, when constructing radar systems. There are three known factors that can limit the maximum range of a radar system. First is line of sight which depends on the radar antenna's height above ground. If the antenna cannot directly "see" the object to be detected, the radio path is blocked and must be cleared. Second is a parameter called "maximum non-ambiguous range". This is the distance the pulse is reflected and received before the next pulse is transmitted. Finally, radar sensitivity brought on by environmental conditions (rain, snow, etc) and radar cross section also limit the maximum range of the system.Precipitation intensity is measured by a ground-based radar that bounces radar waves off of precipitation. The Local Radar base reflectivity product is a display of echo intensity (reflectivity) measured in dBZ (decibels). "Reflectivity" is the amount of transmitted power returned to the radar receiver after hitting precipitation, compared to a reference power density at a distance of 1 meter from the radar antenna. Base reflectivity images are available at several different elevation angles (tilts) of the antenna the base reflectivity image currently available on this website is from the lowest "tilt" angle (0.5°). The maximum range of the base reflectivity product is 143 miles (230 km) from the radar location. This image will not show echoes that are more distant than 143 miles, even though precipitation may be occurring at these greater distances. To determine if precipitation is occurring at greater distances, link to an adjacent radar. In addition, the radar image will not show echos from precipitation that lies outside the radar's beam, either because the precipitation is too high above the radar, or because it is so close to the Earth's surface that it lies beneath the radar's beam. NEXRAD ( Next Generation Radar) can measure both precipitation and wind. ![]() The radar emits a short pulse of energy, and if the pulse strike an object (raindrop, snowflake, bug, bird, etc), the radar waves are scattered in all directions. A small portion of that scattered energy is directed back toward the radar. This reflected signal is then received by the radar during its listening period. Computers analyze the strength of the returned radar waves, time it took to travel to the object and back, and frequency shift of the pulse. ![]() The ability to detect the "shift in the frequency" of the pulse of energy makes NEXRAD a Doppler radar. The frequency of the returning signal typically changes based upon the motion of the raindrops (or bugs, dust, etc.). This Doppler effect was named after the Austrian physicist, Christian Doppler, who discovered it. You have most likely experienced the "Doppler effect" around trains.Īs a train passes your location, you may have noticed the pitch in the train's whistle changing from high to low. As the train approaches, the sound waves that make up the whistle are compressed making the pitch higher than if the train was stationary. ![]() Likewise, as the train moves away from you, the sound waves are stretched, lowering the pitch of the whistle. The faster the train moves, the greater the change in the whistle's pitch as it passes your location. The same effect takes place in the atmosphere as a pulse of energy from NEXRAD strikes an object and is reflected back toward the radar. ![]()
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