Range Folding


Figure 1. Folding of a radar echo by two consecutive pulses

Pulsed radar systems measure distance as a function of the amount of time that a pulse has traveled. It cannot however, distinguish which pulse the returned energy has come from – the most recent or a previous pulse. Radar will depict the target as though it had been returned from the most recent pulse. Range-folded echoes tend to occur when the atmosphere is relatively stable near the surface of the earth and/or strong convection is occurring at ranges typically greater than 275 km from the radar site.
 
Characteristics of Range Folded Echoes
  • Typically elongated along a radial
  • Indistinct in appearance, but often most intense on side nearest radar
  • Will disappear if the elevation raised slightly (most practical solution for real-time forecasting) because the radio signal will miss the far-away target when the antenna is pointing higher
  • Will move (change display range) if PRF is changed

When backscattered energy form a transmitted pulse is received after the next consecutive pulse has been transmitted, the corresponding echo is displayed at the proper azimuth (direction from the radar), but at a systematically incorrect range. The contemporary term for this is “range folding”, although traditionally the display of the reflecting target at a fictitious location is also called a “second-trip echo”.

The basic concept of range folding is illustrated in Figure 1. The fictitious (“second trip”) echo is indicated at a range from the radar equal to the distance of the true target beyond the unambiguous range of the radar. The unambiguous range of radars is generally large and second trip echoes are not a significant problem. When higher PRF’s are used however second trip echoes can become a serious problem. Doppler radars often operate with a PRF as high as 1200 hz. Range folding therefore becomes a significant problem. This will be discussed in more detail in the module on Doppler radar principles.