If the wind is moving precipitation away, the frequency of reflected energy decreases. If wind is moving precipitation toward the radar, the frequency of reflected energy picked up by NEXRAD increases. The Doppler effect is the alteration of the frequency of the radar-transmitted pulse of energy at the moment it is reflected off of a moving target (precipitation). NEXRAD determines the movement of wind-driven precipitation using a principle known as the "Doppler effect". Forecasters use other information at their disposal to make this determination. In addition, radar alone cannot determine if precipitation is rain or snow. The radar is so sensitive that it can detect clouds, dust and aerosol particles, insects, and birds, not to mention airplanes and ground based objects near the RDA itself. Unfortunately, the radar can display "echoes" that may or may not represent meteorological targets. The most common is a reflectivity image, which shows the areal extent of precipitation. The Advanced Weather Interactive Processing System (AWIPS) workstation, allows forecasters to display radar data(top picture).
DOPPLER RADAR GENERATOR
The processing computer, known as the Radar Product Generator (RPG), performs various data quality checks of the raw radar data and creates radar images and products. The returned energy, called base data, is sent from the RDA site to the processing computer located here in the office(bottom picture). Part of the energy will be reflected back to the antenna, where the dish focuses the returned energy. When this energy hits an object, like a raindrop or snowflake, it is reflected in all directions. The transmitter at the antenna within this protective dome sends out a pulse of energy. The radar dish (pictured above) is protected by a covering that resembles a giant soccer ball. It all starts at the Radar Data Acquisition unit (RDA), which is the tower (pictured above) located adjacent to the forecast office. Radar requires three integral parts to work: (1) an antenna/receiver, (2) computers that process the raw radar data, and (3) an interactive workstation that the forecasters can use to display the processed radar data. The term WSR-88D is simple to explain: WSR stands for Weather Surveillance Radar 88 represents the year the first NEXRAD was commissioned for use (1988) and the D means it is a Doppler radar. Within the National Weather Service, NEXRAD is officially called the WSR-88D. In this way, the radar images can be shared with all interested groups. News agencies, television stations, military installations and other sites have access to the radar images through cooperation with the National Weather Service. Therefore, the value of Doppler frequency, $f_d$ is $926HZ$ for the given specifications.The National Weather Service's Doppler radar, known as NEXRAD (for the NEXt generation of RADar), is the only Doppler weather radar on Long Island. So, the total angle of excursion made by the electromagnetic wave during the two-way communication path between the Radar and target will be equal to $4\pi R/\lambda$ radians.įollowing is the mathematical formula for angular frequency, $\omega$ −įollowing equation shows the mathematical relationship between the angular frequency $\omega$ and phase angle $\phi$ − We know that one wave length $\lambda$ corresponds to an angular excursion of $2\pi$ radians. If $\lambda$ is one wave length, then the number of wave lengths N that are present in a two-way communication path between the Radar and target will be equal to $2R/\lambda$. Therefore, the total distance between the Radar and target in a two-way communication path will be 2R, since Radar transmits a signal to the target and accordingly the target sends an echo signal to the Radar. The distance between Radar and target is nothing but the Range of the target or simply range, R. Now, let us derive the formula for Doppler frequency. The frequency of the received signal will decrease, when the target moves away from the Radar. The frequency of the received signal will increase, when the target moves towards the direction of the Radar. This effect is known as the Doppler effect.Īccording to the Doppler effect, we will get the following two possible cases − If the target is not stationary, then there will be a change in the frequency of the signal that is transmitted from the Radar and that is received by the Radar. In this chapter, we will learn about the Doppler Effect in Radar Systems.
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