Aerospace and Electronic Systems Magazine July 2017 Tutorial XI - 51

Reed, Lanterman, and Trostel
Table 2.

Table 3.

Key Vaisala WRK-200 Parameters

Rockwell Collins Airborne Weather Radar Parameters

Antenna Subsystem
Gain

Antenna Subsystem
45 dB

Gain

≤1°

Beamwidth

Transmitter Subsystem
Transmit frequency

34 dB
3.5°
Transmitter Subsystem

5.6-5.65 GHz

Transmit frequency

9.33 GHz

Peak power

450 kW

Peak power

150 W

Pulse width

0.5-2 μs

Pulse width

1-20 μs

PRF

200-2,400 Hz

PRF

In addition to these government radar systems, commercial
weather radars have been increasingly deployed since the late
1960s by local television stations to provide more local coverage than was available from the NWS NEXRAD systems. Most
of these commercial radars are Doppler and polarization capable.
These systems are often C band and use solid-state transmitters. A
typical commercial weather radar is the Vaisala WRK-200, which
is a dual-pol, klystron-based, C-band weather radar system. Some
key parameters for this system are given in Table 2.
Airborne weather radar systems are also used by both military
and commercial aviation to provide local avoidance of storm cells.
Because of airframe constraints, airborne systems tend to use Xband components. This shorter wavelength allows smaller transmit
and receive antennae but necessarily limits the operating range of
the airborne systems because of increased attenuation. The Rockwell Collins weather radar is an X-band airborne radar designed
for medium-sized aircraft. The radar has an approximate range
of 320 nautical miles for weather detection and a 5-nautical mile
range for wind shear detection. Some key parameters for this system are given in Table 3.

IV. WEATHER RADAR OPERATION
While the WSR-88D is not the only operational weather radar in
the world or United States, it is the most common. As a result,
much of the research in radar meteorology has focused, and currently focuses specifically, on the utility of the measurements
provided by the WSR-88D. Furthermore, much of the scattering
phenomenology that is discussed in Section V makes certain assumptions regarding the collected data, e.g., frequency band and
polarization. Thus, this section discusses some general aspects of
weather radar data collection and some more specific design details of the WSR-88D.
The WSR-88D was originally designed as a single-frequency
(S band), horizontally polarized radar. All of these radars have
been modified to support dual-pol capability [2], [6], transmitting
and receiving both horizontally and vertically polarized waves simultaneously. The simultaneous transmission is achieved by transmitting a linearly polarized wave at a 45° angle. The horizontally
and vertically polarized components of the wave are then simultaJULY 2017, Part II of II

180-9,000 Hz

neously received in the horizontal and vertical receive channels of
the radar, respectively. In addition, the network of WSR-88Ds has
been upgraded to include a super-resolution mode that yields data
points on a finer range-azimuth grid than the legacy resolution [2].

A. SIGNAL PROCESSING CHAIN
To collect data for a set of range resolution cells along a single
radial, the radar transmits a series of pulses in a CPI, receives the
reflected pulses through a coherent quadrature receiver, and applies a matched filter to the received signal. The result is a twodimensional Nr × Np range-pulse map of complex (i.e., I-Q) data.
These I-Q data are often called level I data. For each resolution
cell, the Np pulses are used to estimate the first three spectral moments of the data. The zero moment corresponds to the average
power reflected by scatterers in a given resolution cell. The first
and second moments provide estimates of the mean and standard
deviation of the underlying distribution of actual scatterer radial
velocities. Both the first and the second moments are estimated
using pulse-pair processing, as described in Sections V.B.2 and
V.C.2. These three products7 are referred to as level II data and
are the most rudimentary level of data recorded operationally [22].

B. COORDINATE SYSTEM AND PROPAGATION
A mechanically steered radar, such as the WSR-88D, collects data
from a three-dimensional volume by steering in both azimuth and
elevation. Each complete volume scan is composed of a series of
constant elevation scans, called cuts, with each cut consisting of a
full 360° in azimuth and ranges up to 460 km. The resulting data
are stored in a three-dimensional array, with the dimensions corresponding to the spherical coordinates of the radar. Azimuth is measured in degrees clockwise from due north, elevation is measured
in degrees from the altitude line of the radar, and range is defined
as the slant range in meters from the radar. Computing the ground
range and height of a particular data point is more complicated
7

In the weather radar literature, the term "product" can describe
both level II and other quantities derived from such level II data,
but it is rarely used to refer to level I data.

IEEE A&E SYSTEMS MAGAZINE

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine July 2017 Tutorial XI