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

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Range Folding

Range folding is the placement of an echo by the radar in a location whose azimuth is correct, but whose range is erroneous (but in a predictable manner). This phenomenon occurs when a target lies beyond the maximum unambiguous range of the radar (Rmax). That is, the target is located at an ambiguous range.

Figure 6 is an example of an accurately placed echo by the radar. The true target range is 200 nm while Rmax = 250 nm. This means that a pulse has time to travel a maximum of 500 nm (2 x Rmax, if no target is encountered, or out to a target at 250 nm and back) before the next pulse is transmitted.

Figure 6.  Target lies within Rmax.  Click for larger and animated view.

Figure 6: Please click for animated view in seperate window

When a pulse strikes the target at 200 nm, most of the energy in the pulse continues on its path, while some of the energy is reflected by the target (bottom portion). The reflected energy travels a total distance of 400 nm (2 x 200 nm), while the rest of the energy continues to travel away from the radar. The radar accurately places the target at 200 nm range, since a second pulse has not yet been transmitted. As a result, there is no ambiguity or error in the placement of the target at 200 nm.

In Figure 7, Rmax = 250 nm and a true target lies at 300 nm, 50 nm beyond Rmax. Pulse 1 strikes the target at 300 nm and some of its energy is reflected back to the radar, while the remainder of it continues outward. Each piece of energy has time to travel 500 nm or 2 x Rmax before another pulse is transmitted.

Figure 7.  Target located beyone Rmax.  Click for larger and animated view.

Figure 7: Please click for animated view in seperate window

The outward moving energy reaches 500 nm at the same time the reflected energy returns 200 nm back toward the radar (i.e., 100 nm range from the radar). However, another pulse (Pulse 2) is about ready to leave the radar. In the last frame of the animation, reflected energy from Pulse 1 has not returned to the radar so it is not considered by the radar as the radar transmits Pulse 2.

In Figure 8, at the time when Pulse 2 reaches a range of 100 nm where no target is located, backscattered energy from Pulse 1 reaches the radar. However, since Pulse 2 has already been transmitted, the radar now regards any returned power as coming from Pulse 2 and does not know that it is actually a 2nd trip echo from Pulse 1 and the target at 300 nm.

Figure 8.  Target located beyond Rmax with incorrectly-placed (range folded) echo.  Click for larger and animated view.

Figure 8: Please click for animated view in seperate window

The WSR-88D will regard the backscattered energy it received as coming from a target at 50 nm as seen by Pulse 2 and not at 300 nm as actually seen by Pulse 1. The target's apparent range is 50 nm even though its true range is 300 nm.

If a target lies a number of miles beyond Rmax (or multiples of Rmax), it will erroneously appear the same number of miles out from the radar. If Rmax is 250 nm, any target from 0 nm to 250 nm lies within the "first trip" and will be placed at the correct range. Ranges of 251 nm to 500 nm lie in the "second trip", etc. A target which is detected by the radar at an actual range of 550 nm would still be placed at 50 nm.

Another example would be the case where Rmax is 80 nm (a typical Doppler mode Rmax) and 3 targets lying at true ranges of 30 nm, 110 nm, and 190 nm. The result will be the radar receiving pulses from the 3 targets at the same time and these 3 targets occupying the same apparent range of 30 nm!

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Section 5: Radar Beam Characteristics