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DLOC Principles of Meteorological Doppler Radar:
WSR-88D Fundamentals

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WSR-88D Range Resolution

Best Range Resolution

As the radar beam sweeps along its path, approximately 40 to 200 pulses (depending on the VCP invoked) move through a particular space in the atmosphere sampling the reflectors in that space. For each pulse, the information from these reflectors makes up individual range resolution cells. The information from the resolution cells is then combined and processed by the radar to produce Base Data. Each individual pulse defines a pulse volume and has dimensions of ~ 500 meters (~ 1500 meters) in length by ~ 1° or ~ 0.5° wide in short pulse (long pulse) mode. This means that two targets along a radial must be at least 250 (750) meters apart for the radar to be able to distinguish and display them as two separate targets (i.e., more than H/2 range separation distance).

VCPs 11,12,21,121, 211, 212, 221, and 32 operate in short pulse and have a range resolution of 250 m. VCP 31 operates in long pulse with a range resolution of 750 m. Section 6 will describe how VCP 31 long pulses are converted to a display resolution of 250 nm.

Figure 12 depicts why two targets must be separated by at least one-half a pulse length along a radial. The figure shows a pulse of radar energy hitting target A. As the pulse hits target A some of the energy (lower green bar) is scattered back toward the radar. The pulse continues until it has just reached target B. Energy continues to reflect back from target A.

Figure 12.  Illustration of separation of two targets along a radial.  Click for animated view.

Figure 12: Click for animated view in seperate window

The pulse striking target B starts to scatter energy (lower orange bar) back from that target. Reflected energy (lower green bar) from target A continues to scatter toward the radar as the pulse propagates. The reflected energy (lower orange bar) from target B is now one-half pulse length long. The front end of the reflected energy from storm B is nearly coincident with the back end of that from storm A. If the targets were any closer than one-half pulse length, their backscattered energy would overlap. The radar could not distinguish between the two and would combine the two pieces of information into one long echo.

Range Bin

The returned power, velocity, and spectrum width are assigned at 250 m (0.25 km, 0.13 nm) range resolution, which is the best possible. The base data are assigned a memory address, or "bin" in the computer that corresponds to a 250 m (0.25 km, 0.13 nm) by 1° (or 0.5° with super-res base data) volume in the atmosphere.

Range Gate

When dealing with WSR-88D Products, the display pixels are referred to as range gates and can be comprised of one or more 250 m range bins depending on the product display resolution. For example, if the display resolution of a product is 250 m, then the range gate of data on the display is the same as a range bin since the gate consists of only one bin of data.

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