It was learned during World War II that electromagnetic radiation could be sent out, bounced off an object and returned to a listening device. By timing the time it took for the energy to travel to the target and back (hence the word echo), one could give a location of that object. Radar is an acronym for Radio Detection and Ranging. It was soon learned that raindrops made excellent targets for S-Band radar (~10cm) and thus weather radar units were set up across the United States in the mid 1960's. Recent advances in technology has resulted in a new system of Doppler radars installed across the US in a program called NEXRAD, for Next Generation Weather Radar. The new radars would be co-located with modernized weather service offices to help usher in the new millennium with unprecedented radar coverage. The new radars were WSR-88D - the D standing for Doppler. These Doppler radars would replace the conventional non-coherent radars from 1957.
Benefits of the WSR-88D over the WSR-57
The following is a breakdown of reflectivity (in dBZ's) corresponding to various Video Integrator and Processor (VIP) levels of the WSR-57. The WSR-88D's will have fifteen different levels. The minimum detectable reflectivity for the WSR-57 was 18 dBZ whereas the minimum detectable return for the WSR-88D is -28 dBZ.
The above radar report shows a lake-enhanced snowfall. Imagine what the image would be like if nothing under 18 dBZ's showed up as echoes! The new WRS-88D's are a great improvement for snowfall observation. Because of the sensitivity of the radar for slight echoes, the non-precipitation mode is often used for snowfall. Heavier rainfall, and especially hail show up well in severe weather mode.
The maximum unambiguous range, Rmax, corresponds to half the distance electromagnetic energy can travel between pulses (since the energy needs to travel to the target and back). The pulse repetition frequency (PRF) is a measure of how frequently the pulses are transmitted. If c is the speed of light (taken as 3x108 ms-1), then
In words, the maximum unambiguous range is inversely proportional to the PRF.
If a second pulse is transmitted before energy from the first pulse has been received, the echo from a target beyond the maximum unambiguous range is displayed at the proper azimuth but at a distance one Rmax closer to the radar. This "second-trip echo" exemplifies range folding. Echoes may be folded from third and fourth trips if the Rmax is small enough to allow for targets to be observed at multiple intervals of Rmax.
Average Power Return
A radar is able to detect a small amount of power returned from a target. Obviously, the more sensitive the receiver, the more a radar is able to detect smaller or less efficient scatterers. The radar needs to average returned power from many successive pulses since the power returned from pulse to pulses is highly variable. The WSR-88D will typically average power from 25 pulses to determine a representative value. A radar can increase its ability to detect a target by not only changing its listening ability, but by maximizing the power it transmits. The greater the output, the greater the return.
Maximum Transmitted Power
Whereas the power output of the WSR-57 is 410,000 watts, the output of the WSR-88D is 750,000 watts. The return power is directly proportional to the transmitted power.
Gain is a measure of the antenna's ability to focus the radiated energy. The antenna's gain indicates the relative amount that the energy is focused compared to what it would be if it were an isotropic radiator (giving off radiation equally in all directions). The gain of the WSR-88D is 35,481 (compared to the WSR-57 with a gain of 6,460), meaning a target will be struck with over 35,000 times more energy than it would without a dish. Power received from a given target is directly related to the square of antenna gain.
Angular Beam Width
Power returned to a radar is directly related to the square of the angular beamwidth (assuming a beam narrower than 2° and at a range less than 125nmi). A narrower beam will improve sensitivity by focusing the outgoing power and increase the resolution by decreasing the size of the beam's cross-sectional area.
|Range||2.0 degree beam||0.95 degree beam|
The radar transmits energy for a given period of time known as the transmission time. When that is multiplied by the speed of light, a train of energy, or pulse, extends outward from the antenna with a pulse length, H. Long pulses are used to determine target locations, while short pulse lengths help determine target intensity and motion characteristics. The power received from a target is directly related to pulse length. The longer the pulse length, the more energy is being transmitted. This will improve the sensitivity of the radar.
NEXRAD Information Dissemination Service
See here for examples.
ES115 Notes Radar Notes