About NPN Profilers
Wind profilers are specifically designed to measure
vertical profiles
of horizontal wind speed and direction from near the
surface to above the tropopause. Data from this network are
distributed in real-time to government and university
atmospheric researchers, private meteorologists, the
National Centers for Environmental Prediction,
the Storm
Prediction Center, all
National Weather Service (NWS) Forecast Offices,
and foreign agencies responsible for weather prediction.
The NOAA Profiler Network (NPN) was first deployed in
1990-1992 and has operated continuously ever since. The
original network consisted of (31) 404 MHz profiler sites
located in the central United States and one site in Alaska.
Since January of 2000, there are (32) 404 MHz profilers in
the central United States and three 449 MHz profilers in
Alaska. See Site Location
Information (HTML), Site Location
Information (TEXT), or Site Map
for more information about NPN site locations.
Wind NPN profilers are designed to operate reliably and
unattended in nearly all weather conditions. To achieve this
reliability, they have a minimum number of moving parts;
therefor a fixed beam antenna is used. Obtaining wind
profiles consistently to the tropopause in nearly all weather
conditions requires the use of a relatively long wavelength
radar. Typical NWS weather radars that have been operational
for the past 30 years operate with wavelengths of 10 cm of
less and require cloud or precipitation particles to act as
reflectors. 404 MHz Wind profilers are relatively low-power,
highly sensitive clear-air radars, operating at a wavelength
of 74 centimeters. The radars detect fluctuations in the
atmospheric density, caused by turbulent mixing of volumes of
air with slightly different temperature and moisture content.
The resulting fluctuations of the index of refraction are
used as a tracer of the mean wind in the clear air. Although
referred to as clear-air radars, wind profilers are capable
of operating in the presence of clouds and moderate
precipitation.
The equipment shelter (shown below) houses the radar's
transmitter, receiver, data processing, and antenna beam
control electronics. The solid-state power amplifier provides
6 kilowatts of peak power, radiating 3.3µs (low mode)
or 20µs (high mode) pulses of 404 MHz RF energy. The
receiver digitizes atmospheric signal returns and passes the
digital data to the data processing system. The radar uses a
128-point fast-fourier transform (FFT) to perform spectral
analysis and extrapolate velocity estimates. The beam
steering electronics control the elevation angle of the
antenna by altering the phase progression of cables feeding
the antenna elements. The antenna beam azimuth is determined
by the physical orientation of two orthogonal antenna arrays.
For more information see the
Profiler System Block Diagram.
The antenna subsystem is formed from two coaxial colinear
arrays arranged orthogonal to each other to form a Vertical,
a North, and an East beam. The North and Vertical beams share
the same physical antenna array. The two arrays occupy an
area 40 feet by 40 feet. Each array is formed from 20 rows of
coaxial colinear array subassemblies fed by series of RF
power dividers (5:1, 4:1, 3:1 and 2:1 power dividers). The
cables connecting the power dividers and antenna sticks are
cut to specific lengths (multiples of 74 cm) which keeps the
phasing consistent throughout the antenna. These antenna
elements, with a row spacing of 0.711 wavelength enables the
generation of a scanned beam 16.3 degrees off vertical axis
with only five-pairs of discrete phase delay cables.
|
|
A ground plane formed by corrosion-resistant expanded steel
mesh is positioned a quarter wavelength below the arrays on
the 40 x 40 foot frame. The ground plane is located
approximately 3.5 feet above the ground to minimize the
effect of snow accumulation and to facilitate antenna
installation and repair.
Each array uses ten style-1 subassemblies consisting of two
14-element co-linear subsections fed by a 2:1 power divider
(as shown in top half of the figure below). Each array also
uses ten style-2 subassemblies consisting of two 12-element
and one 14-element co-linear subsection fed by 3:1 power
divider (as shown in bottom half of the figure below). The
completed assemblies are approximately 1-3/4 inches in
diameter and 40 feet long.
The complete antenna array assembly layout for a 404 MHz profiler is shown in the
figure below illustrating the orthogonal orientation, style-1
and style-2 subassembly configurations, and the row numbering
system of both arrays.
A profiler site equipped with the RASS option has the
capability to measure and produce vertical temperature
profiles. The speed of sound is affected by the
temperature of the atmosphere. Sound travels through the
atmosphere at slightly different speeds at different
temperatures. RASS uses this principal to track the speed of
the acoustic energy emitted from the RASS Transducers as the
sound waves propagate up through the atmosphere. The
temperatures of the atmosphere at specific heights are
extrapolated from the speed of the RASS wave propagation.
The frequency range of the RASS acoustic signal is approximately 850-900 Hz (~37cm) which is
1/2 of the wavelength of the radar's 404 MHz signal. Four
RASS transducers (as shown in figure to right)
are used to generate the acoustic energy. The sound originates
from a speaker (compression driver) and is fed through a horn
assembly towards the ground. The parabolic shaped dish
reflects the acoustic energy up into the atmosphere.
To limit the amount of noise pollution, the
interior of the RASS transducer enclosure is lined with a
special type of foam rubber designed to baffle noise. The
triangles at the top of the enclosure are called "Thnadner"
and are designed to baffle noise from emanating laterally from
the RASS transducer enclosure.
A dedicated RASS signal processor enables the radar to
simultaneously measure wind and temperature profiles during
the radar's vertical low mode. For more information about RASS
electronics refer to the
RASS System Block Diagram.
| 
