282
P. Joe
• The incident and backscattered waves are linearly polarized.
• The main lobe of the antenna radiation pattern is of Gaussian shape.
• The antenna is a parabolic reflector type of circular cross section.
• The gain of the antenna is known or can be calculated with sufficient accuracy.
• The contribution of the side lobes to the received power is negligible.
• Absorption of the transmitted signal by ground clutter in the beam is negligible.
• The peak power transmitted (Pt ) is the actual power transmitted at the antenna, that is,
all waveguide losses, etc. are considered.
• The average power measured (Pr ) is averaged over a sufficient number of pulses or independent samples to be representative of the target pulse volume.
This simplified expression relates the echo power CPr) measured by the radar to the radar
reflectance factor, Z, which in turn is related to the rainfall rate, R. These factors and their
relationship are important in interpreting the intensity of the target and in estimating precipitation amounts from radar measurements. In spite of the many assumptions of varying
importance the expression provides a reasonable estimate of the target mass. This estimate
can be improved by further consideration of factors in the assumptions.
12.2.3 Non-coherent radar
The power backscattered from weather targets is of the order of 10- 8 to 10- 15 watts, covering a
range of about 70 dB from the strongest to weakest targets detectable. To adequately cover this
range of signal, a logarithmic receiver is normally used. Linear receivers with 90 dB dynamic
range are just being introduced in research radars (Keeler et aI., 1995). Many pulses must be
averaged in the processor to provide a significant measurement; the pulses can be integrated
in different ways but usually in a digital form, and must account for the logarithmic scale. In
practice, for a typical system, the signal at the antenna is received, amplified logarithmically,
averaged over many pulses, corrected for (log) averaging and converted to a reflectance factor
using the radar range equation.
The reflectance factor is the most important parameter for radar interpretation. The factor
derives from the Rayleigh scattering model and theoretically is defined as the sum of particle
(or drop) diameters to the sixth power per unit volume.
(12.3)
In many cases, the numbers of particles, composition and shape are not known and an equivalent
or effective reflectance factor Ze is defined. Snow and ice particles must refer to an equivalent
Z and represents the Z assuming the backscattering particles were all spherical drops.
A common practice is to work in a logarithmic scale or dBZ units which are numerically defined
as:
(12.4)
P. Joe
• The incident and backscattered waves are linearly polarized.
• The main lobe of the antenna radiation pattern is of Gaussian shape.
• The antenna is a parabolic reflector type of circular cross section.
• The gain of the antenna is known or can be calculated with sufficient accuracy.
• The contribution of the side lobes to the received power is negligible.
• Absorption of the transmitted signal by ground clutter in the beam is negligible.
• The peak power transmitted (Pt ) is the actual power transmitted at the antenna, that is,
all waveguide losses, etc. are considered.
• The average power measured (Pr ) is averaged over a sufficient number of pulses or independent samples to be representative of the target pulse volume.
This simplified expression relates the echo power CPr) measured by the radar to the radar
reflectance factor, Z, which in turn is related to the rainfall rate, R. These factors and their
relationship are important in interpreting the intensity of the target and in estimating precipitation amounts from radar measurements. In spite of the many assumptions of varying
importance the expression provides a reasonable estimate of the target mass. This estimate
can be improved by further consideration of factors in the assumptions.
12.2.3 Non-coherent radar
The power backscattered from weather targets is of the order of 10- 8 to 10- 15 watts, covering a
range of about 70 dB from the strongest to weakest targets detectable. To adequately cover this
range of signal, a logarithmic receiver is normally used. Linear receivers with 90 dB dynamic
range are just being introduced in research radars (Keeler et aI., 1995). Many pulses must be
averaged in the processor to provide a significant measurement; the pulses can be integrated
in different ways but usually in a digital form, and must account for the logarithmic scale. In
practice, for a typical system, the signal at the antenna is received, amplified logarithmically,
averaged over many pulses, corrected for (log) averaging and converted to a reflectance factor
using the radar range equation.
The reflectance factor is the most important parameter for radar interpretation. The factor
derives from the Rayleigh scattering model and theoretically is defined as the sum of particle
(or drop) diameters to the sixth power per unit volume.
(12.3)
In many cases, the numbers of particles, composition and shape are not known and an equivalent
or effective reflectance factor Ze is defined. Snow and ice particles must refer to an equivalent
Z and represents the Z assuming the backscattering particles were all spherical drops.
A common practice is to work in a logarithmic scale or dBZ units which are numerically defined
as:
(12.4)