Page 106 - Fundamentals of Radar Signal Processing

rather than the RCS η. The value of Z can then be related to the amount of water

in the air or the precipitation rate. A number of models are used to relate the
observed values of Z to rain rates. These models depend on the type of
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precipitation, e.g., snow versus thunderstorm rain versus orographic rain. A
common model is that of Table 2.2, which shows the six-level equivalence
between observed Z values (in dBZ) and rainfall rates used in the U.S.

NEXRAD national weather radar system. Very similar scales are used in the
commercial “Doppler weather radar” systems familiar to every watcher of
television weather forecasts.

TABLE 2.2 Correspondence between dBZ Reflectivity and Rain Rate

It is important to note that the dBZ values in Table 2.2 are 10 times the
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3
3
6
base 10 logarithm of Z in mm /m . When Z is given in m /m = m , it must be
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multiplied by 10 to convert it to units of mm /m before converting to a decibel
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3
6
scale and using Table 2.2.
2.2.5 Statistical Description of Radar Cross Section
The radar cross section of real targets cannot be effectively modeled as a
simple constant. In general, RCS is a complex function of aspect angle,
frequency, and polarization, even for relatively simple scatterers. For example,
the conducting trihedral corner reflector of Fig. 2.4 is often used as a calibration
target in field measurements. Its RCS when viewed along its axis of symmetry

(looking “into the corner”) can be determined theoretically; it is (Knott et al.,
1985)