Helsinki Testbed > Profiler-info
Principles of the Wind Profiler
The wind profiler is an atmospheric remote sensing
instrument. An atmospheric remote sensing instrument gives information about a volume
of the atmosphere at a distance without being physically located in the volume.
Your eyes are a remote sensing device that can give you information about an
object some distance away without actually touching the object. An in-situ
instrument, such as an anemometer, gives information about a specific point in
the atmosphere. Because a remote sensing instrument generally measures a volume
and an in-situ instrument measures a single point, the data from these two
different kinds of instruments cannot be easily compared.
The profiler is a pulse Doppler RAdio Detection And Ranging instrument, commonly called a radar. It transmits a pulse of electromagnetic energy at a chosen frequency and in a chosen direction. When the pulse encounters a "target", electromagnetic energy is scattered. A small portion of this scattered energy, called backscatter, will return to the radar, which can then compute the distance to the target from the time delay between the transmission and reception of the return signal.
The generic name "profiler" comes from the radar's
ability to show data for many heights of the atmosphere at the same time, thus
giving a profile of the atmosphere. The profile from equally-spaced heights is
created by sampling backscatter at equally-spaced time intervals. The
sequential intervals during which backscatter is sampled are called range
gates. The atmosphere above the profiler is divided into range gates, and all
data from a chosen time span are placed into the same range gate.
The signal scattering targets or the wind profiling radar
are refractive irregularities in the atmosphere. A refractive irregularity is
anything that can change the course of a wave through a medium. In this case,
the medium is the atmosphere, and the wave is the radio frequency signal that
the profiler transmits. Maximum backscattering power occurs when irregularities
are about half the size of the radar wavelength (Bragg scatter).
Tracking refractive irregularities, which are carried by the
wind, reveals information about the wind itself. The profiler computes height
by using the time interval between transmission of the pulse and reception of
the return signal. However, wind speed and direction are determined by using
the Doppler principle. A wave will shift in frequency because of the motion of
the target relative to the observer. A frequency higher than the transmitted
frequency indicates that the wind is moving towards the profiler. A frequency
lower than the transmitted frequency indicates the wind is moving away from the
profiler. The profiler detects these small shifts in the frequency of the
backscatter and translates them into wind velocity data.
The profiler makes measurements in at least three directions
in order to compute wind measurements (speed and direction). The transmitted
pulse is directed to an antenna that has a beam width of less than ten degrees.
One beam is directed vertically. The four oblique beams are tilted from
vertical (23.5 degrees for the 4-panel antenna at 915/924 MHz or 15.5 degrees
for antennas at other frequencies or 9-panel antenna at 915/924 MHz) and
directed in the four orthogonal directions. The phased transmission of the
electromagnetic pulse to the radiating elements built into the antenna
subsystem, tilts the orthogonal beams.
The signal processing that the samples undergo is divided
into two parts, the time-domain stage and the frequency-domain stage. In the
time-domain stage, the samples are averaged or filtered. Time-domain averaging
is also called coherent averaging (integration) because the samples come from
targets that are more or less fixed in phase in relation to each other during
the time the sample was taken and because the sample is taken using a coherent
Overall, a longer averaging time improves signal
detectability. However, it is possible to choose an averaging time that is so
long that it reduces the highest unambiguous speed of radial wind measurements,
also called the full-scale velocity, detected by the wind profiler to an
unacceptable value. This results in a radial wind measurement that may be out
of the range required by the user. The number of samples taken is determined by
the desired velocity range measured, the inter-pulse period, and the frequency
of the radar. Operators must estimate the highest possible radial wind speed
that might occur and choose a number that will allow the wind profiler to
correctly detect that wind velocity if it does occur.
In the second stage of signal processing, the sample is
converted from the time domain to the frequency domain using an algorithm
called a Fast Fourier Transform (FFT). The operator may choose the number of
FFT points that are used in the FFT algorithm. The more FFT points used, the
finer the velocity resolution is. However, the number of FFT points also helps
determine the length of time necessary to acquire a spectral sample. A typical
number of FFT points chosen is 64 to 256 for wind data, and 2048 for RASS data.
The next processing step is spectral averaging. The operator may select the number of spectral averages that are used in the averaged spectrum, which is used in Moments selection. The inter-pulse period (IPP), the number of coherent averages, the number of FFT points, and the number of spectral averages determine the dwell time, also called the averaging time. Generally, operators should choose values that create a dwell time of 20 to 30 seconds. The radar profiler can save averaged spectral data in files with the suffix .SPC. The spectral data file also contains complete spectral moments data.
Four quantities are calculated for each set of spectrally
averaged data during the frequency-domain processing:
The moments and spectra are considered "raw" data. The profiler can also produce wind and/or (with optional RASS) virtual temperature data. The data in this file have been processed with a consensus averaging algorithm. The algorithm uses two evaluation values for each radial direction to determine whether data are valid. One value is a velocity range in which the samples must fit. The second value is a percentage number of samples taken during the consensus period that must fit within that range for the mean consensus value is accepted as valid. Both values are chosen by the operator.
The Radio Acoustic Sounding System (RASS) provides profiles
of virtual temperature data. Virtual temperature is a temperature measurement
that is uncompensated for humidity or pressure. The RASS system, usually
composed of four acoustic sources (one on each side of the profiler antenna)
transmits an acoustic wave directed vertically. The profiler uses the acoustic
wave as a target, receiving and processing the resulting backscatter and
effectively measuring the speed of sound propagation. The profiler can compute
virtual temperature profiles because the speed of sound is easily related to
The minimum and maximum height coverage of the profiler
depend on a combination of several factors. Some of these factors are built
into the profiler and, therefore, beyond the control of the operator. These
The wavelength a radar uses depends on the kind of measurements you want to make with the profiler. The radar profiler was designed to provide atmospheric data from the lower atmosphere. A radar profiler that uses a 915-MHz frequency has a 32.8-cm wavelength and the 1290-MHz frequency has a 23.2-cm wavelength. The relatively short wavelength allows a relatively small antenna size.
The aperture of the antenna, which is controlled largely by practical considerations, determines two other characteristics of the antenna, the beam width and the antenna gain. The beam width, which is inversely proportional to the aperture, is less than 10 degrees in the four-panel Vaisala Windprofiler LAP®-3000 configuration. The antenna gain is directly proportional to the aperture. The small aperture of the Vaisala Windprofiler LAP®-3000 4-panel wind profiler antenna produces an array gain of about 24 dBi.
The amount of power in the transmit pulse affects the height
coverage of the profiler directly. The higher the peak transmit power, the
greater the height coverage. The amplifier used in the Vaisala Windprofiler LAP®-3000
has a peak output power of about 600 W. The profiler has a duty-cycle
capability of 0 to 10%, determined by the parameters chosen by the operators.
The minimum height coverage is limited by the recovery rate of the profiler electronics and nearby ground clutter. Some of the backscatter energy returns to the profiler antenna during and immediately after the transmit pulse from hard clutter targets. Unfortunately, the early backscatter is often quite strong and overwhelms the receiver's ability to ignore it and find the desirable data. Together, the limits of the receiver's recovery rate, which is fixed by the system configuration, and the nearby ground clutter determine the minimum height coverage of the profiler.
There are, however, two factors that the operator can
control that affect the height coverage of the profiler. The first factor that
affects the height coverage is the pulse duration, also called the pulse length
or the pulse width. Lengthening the pulse raises both the minimum and maximum
height coverage. Operators may choose the longest pulse possible to gather data
from the highest range gates.
The other operator-controlled factor affecting range/height
coverage, is the inter-pulse period, called the IPP for short. The IPP is the
time interval between any two pulses. First glance at the IPP suggests making
the IPP as long as possible to give the backscatter from the higher reaches of
the atmosphere time to return to the profiler. But in reality, lengthening the
IPP decreases the profiler's duty cycle, that is, the amount of time the
profiler is transmitting compared to the time the profiler is receiving
(listening). Decreasing the duty cycle decreases the average power the profiler
is producing, and decreasing the average power reduces the available height
range of the profiler.
A solution that resolves some of these limitations is pulse
coding. Instead of transmitting one simple pulse, the profiler transmits
complementary pairs of pulses composed of phase-coded pulse cells connected to
each other to form one long pulse. The pulse cells are transmitted in a known
sequence and with a known phase relationship to each other. A higher maximum
height range is obtained using the longer pulse duration composed of multiple
pulse cells rather than an uncoded pulse. A finer range resolution is obtained
from the individual phase-coded pulse cells instead of the entire pulse. The
operator can choose a pulse duration that has 2, 4, 8, 10, 16, or 32 pulse
cells, increasing the pulse duration transmitted by factors of 2, 4, 8, 10, 16,
or 32. However, the total pulse duration must not exceed 12 microseconds. The
longer pulse duration allows the user to increase the wind profiler's duty
cycle, which boosts the maximum height coverage.
After operators have chosen the IPP, the number of pulse code cells, and the pulse duration, then the next set of decisions the operators make will affect how the altitude samples are taken.
The sampling delay determines the first height sampled by
the profiler. The sampling delay is the time interval between the end of the
transmit pulse and the first range gate. This value is corrected in the
calculation of height for the instrumental delay inherent in the profiler. The
value chosen should not normally result in a first sample height of less than
Gate spacing is the altitude difference between the range gates. Gate spacing is typically the same interval as the transmit pulse. If pulse coding is used, then gate spacing MUST be the same interval as the transmit pulse. Sometimes operators, using an advanced or text editor, may choose a gate spacing that is less than the transmit pulse. This results in a backscatter-weighted average over the volume corresponding to the pulse duration.
The number of range gates represents the number of range samples collected for each pulse. This value determines the maximum altitude that measurements are collected. Operators should choose a reasonable number that reflects achievable expectations for the atmospheric conditions at the site.
The choice of sampling parameters has many effects on
profiler operation. Pulse duration determines range resolution. The IPP and
number of coherent averages determines the maximum radial wind velocity that
can be unambiguously resolved.
In normal operation, the profiler gathers wind velocity data
for the largest portion of an averaging period and virtual temperature data for
the remaining portion. Operating for fifty minutes in wind mode and 10 minutes
in the operational RASS mode is a common arrangement. However, the averaging
periods may be shorter.
The range performance of all profilers depends on atmospheric conditions, which can change dramatically and rapidly. An inexperienced operator, when confronted with a reduced height range or large "holes" in the data, may assume that the profiler is not working properly, when in fact the profiler is working correctly, but the atmospheric conditions have changed. The conditions that most affect profiler performance are: humidity, turbulence, precipitation, high winds, and temperature.
The amount of moisture in the atmosphere affects the height range
performance of the profiler. Generally, the more moisture there is, the better
the profiler works for winds because the air has larger refractive index
variations. Often, wind data will show a band of range gates for which the
profiler will give no values, even though the profiler is showing data in range
gates above and below. This "hole" in the data is often caused by a
layer of dry air, which can be verified by examination of corresponding
radiosonde data. Because of the dryness of the air, data from profilers located
in extremely cold places such as the high latitude regions often have such
holes. Marine environments make good profiler sites because of the moisture
usually prevalent in those regions.
The amount of turbulence in the atmosphere also affects the range performance of the profiler. The more turbulence there is in the atmosphere, particularly turbulence with a scale of one-half the profiler wavelength, the better the profiler works. The profiler data has a low signal-to-noise ratio when the air is stable with laminar airflows, a condition commonly found at night. This is because there is little thermally-created turbulence available to reflect the profiler's signals. On the other hand, regions around hills and mountains make good sites because the topography creates turbulence. Convective conditions can produce strong turbulence and correspondingly good profiler data. However, highly localized convective conditions can produce erroneous data. Turbulence is also beneficial to RASS operation. Turbulence helps distribute the acoustic wavefront, helping increase the range in the presence of winds. The wavelength of the acoustic signal must be half that of the radar signal in order to measure the velocity of propagation of the acoustic signal.
Most types of precipitation such as rain, snow, and hail can
affect the performance of the profiler. When precipitation moves in a direction
that's different from the air around it, the vertical beam measures the
movement of the hydrometeors rather than the vertical component of the wind.
This is because precipitation returns stronger signals than clear air. The
precipitation occurring during a thunderstorm can overwhelm the data collected
from the regions above the storm, creating a "shadow" in the data.
However, if the precipitation is carried with the wind, then the horizontal
winds might still be measured because the particle velocity in the off-zenith
beam can be corrected with the vertical beam measurement, assuming spatial
Ground clutter most often affects the quality of data in the
lower range gates. High winds can cause clutter signals from objects such as
trees and power lines to exhibit sufficient Doppler velocity width that the
profiler's ability to screen out this clutter is overwhelmed. Choosing sites with
minimal ground clutter will improve the range and data quality of the profiler.
Temperature has more of an effect on RASS than on the
profiler's wind measurement. Acoustic attenuation varies as a function of
temperature, humidity, and pressure. Cold dry air exhibits highest attenuation,
which can exceed -40 dB per kilometer. Very moist or warm air propagates
acoustic signals better, resulting in improved range for virtual temperature