The purpose of the YSCAT-94 experiment is to acquire radar measurements
and simultaneous wind, wave, and other environmental parameters
to support scientific studies of air/sea interaction and ocean scattering.
The major investigations planned are listed below along with the
primary investigator for the investigation.
This experiment is being conducted under the direction of
Drs. D. Long and D. Arnold of Brigham Young University (BYU)
in collaboration with Dr. M. Freilich of Oregon State University
(OSU) and Dr. M. Donelan of the Canadian Centre for Inland Waters
- Bragg scattering regime (Dr. M. Freilich, OSU)
The primary purpose of investigation is to determine the
incidence angle range for which a two-scale-type Bragg scattering
model can be applied. There is little doubt that Bragg scattering
is dominate at high incidence angles or that Bragg scattering
is NOT applicable at nadir. The question is then: Over what
range of incidence angles is Bragg scattering applicable?
To address this question a given surface wavelength will
be observed as a function of incidence angle.
- Modulation transfer function (R. Reed, BYU)
The modulation transfer function (MTF) of the ocean's
surface has been studied intensively at X-band (9.375 GHz)
with some limited data reported for L, Ku, and Ka Bands.
These studies have revealed numerous questions that need
to be addressed regarding the MTF. Most important among
these are: What is the incidence angle dependence of the
MTF? Under what conditions is the MTF concept valid? What
is the MTF in the transition region between L and C bands?
Why are the measured MTFs larger than predicted from theoretical
models? To examine these issues, the MTF study will make
measurements at moderate incidence angles for several frequencies.
- Location of azimuth minima (D. Long, BYU)
Currently, the azimuthal response of the backscatter as
a function of the relative angle between the radar illumination
and the wind is assumed to be a double cosine with the minima
occurring at 90¡. Recent aircraft observations raise some
doubts regarding this assumption. The primary purpose of
the this observational investigation will be to observe
and measure the location of the azimuthal minima as a function
of incidence angle, wind speed and wave conditions. This
study will concentrate primarily on Ku-band (14 GHz) and
secondarily on C-band (5.3 GHz) measurements.
- Low-wind speed cutoff (D. Long, BYU)
The scattering response of the surface has been predicted
to roll-off steeply for low wind speeds. There is some aircraft-based
evidence that this does occur though it has not been observed
in space-based measurements. Nevertheless, if a low-wind
speed cut-off does exist, it could have important implications
on the measurement accuracy of spaceborne instruments at
low wind speeds. YSCAT is particularly well suited for addressing
this issue using a tower mounted instrument. This investigation
will examine the low-wind speed radar response of the surface
as a function of frequency, incidence angle, and azimuth
angle. While the study will concentrate on Ku-band (14 GHz)
and C-band (5.3 GHz) other frequencies will also be of interest.
The small spot size and ability to collect simultaneous
long and short-term averages are well suited for this study.
- Long-wave dependence (D. Long, BYU)
Previous studies have shown some sensitivity of the wind-dependent
radar response of the surface to the presence of long-waves.
A key limitation of previous studies has been the limited
data set. This investigation will examine the sensitivity
of the radar response to long waves, considering fetch and
off-axis swell. This study will concentrate primarily on
Ku-band (14 GHz) and secondarily on C-band (5.3 GHz) measurements.
- Model function frequency dependence (R.S. Collyer,
While operational model functions have been developed
at C-band (5.3 GHz) and Ku-band (14 GHz), a complete study
of the frequency dependence of the model function has not
be done. For moderate incidence angles, the Bragg scattering
model is generally felt applicable with surface waves of
from 1 to 15 cm being observed. Is there a particular ocean
wavelength in this range which is most sensitive to wind
speed or friction velocity? If so, this implies an optimum
scatterometer frequency exists. Unfortunately, this question
is made more complicated by the fact that measurements are
made over a range of incidence angles. The intent of this
study is to investigate the wind speed sensitivity of the
backcatter at different frequencies and incidence angles
to address this question.
- Near-nadir scattering (D. Arnold, BYU)
Recently, new models for nadir ocean scattering have been
successfully demonstrated. This investigation will focus
on extending these modeling ideas to off-nadir (but low
incidence) angles. To support this investigation, YSCAT
will be augmented by two fixed-pointing Doppler radars.
These X-band (10.02 GHz) radars will provide very long records
of the radar response of the ocean at two fixed incidence
angles (nadir and TBD 40 deg).
- Wave directional spectra, mean square slope, wind stress
(M. Donelan, CCIW)
The key environmental wave and stress parameters will
be obtained continuously throughout the experiment. This
will enable us to address such questions as: 1) Does the
radar respond to stress or wind speed? 2) Is the stress
vector turned by the long waves and swell and does the radar
track this? 3) How is backscatter affected by mean square
- YSCAT System Description
The BYU scatterometer system (YSCAT) consists of an integrated
system for simultaneous acquisition of radar measurements and
environmental measurements. Environmental measurements include:
dual wind vane/propeller anemometers, a rain gauge, an aspirated
temperature sensor, a humidity sensor, and a water temperature
sensor. The YSCAT radar has an operating band of 2-18 GHz with
dual antenna system which provides and essentially constant
5 deg beamwidth over this range (the beam increases to 6.5 deg
at 3 GHz and to nearly 10 deg at 2 GHz). The antenna positioning
system provides elevation angles from +30 deg to nadir and azimuth
angles of +/- 90 deg normal to the platform (the platform legs
are the limiting factor in the azimuth range).
For this experiment the YSCAT radar has been configured to
operate in a CW transmit mode. Either polarization may be transmitted.
Both receive polarizations are processed simultaneously. The
received signal is amplified then mixed with an offset RF LO
in the RF subsystem. The IF center frequency is 166 MHz. The
IF signal is beat down to baseband using quadrature mixing.
The baseband signal (which has a center frequency equal to the
Doppler shift) is amplified and filtered before digitization
at a 2kHz rate for each channel. A high pass filter (corner
frequency 1 Hz) is used to minimize the DC part of the signal
resulting from the antenna feedthrough. An 8th order low-pass
(corner at 900 Hz) filter is used to prevent aliasing. The quadrature
signals are processed in the computer into signal power measurements,
Doppler center frequencies, and Doppler bandwidth estimates.
The computer digitally applies a very narrow 60 Hz notch filter
to remove 60 Hz power ripple.
In order to provide a sampling of the backscatter as a function
of long waves, a continuous sampling of the echo for 1 minute
is obtained. This segmented into 100 ms windows for which the
echo power, center frequency and bandwidth are determined, giving
an effective 10 Hz sampling of these parameters for both polarization.
When switching frequencies, approximately 3 s of overhead
time must be allocated to switch the RF frequency and allow
the filters and other electronics to settle. To reposition the
antenna, 1 s must be allocated for mechanical settling. A change
in elevation (or azimuth) angle of requires approximately 2
s to accelerate, move, and stop the positioner for angles less
than 20 deg. Slightly longer (1-3 s) is required for much larger
angle changes. Taking all these into account, approximately
50 arbitrary one minute measurements can be made per hour. Once
a day, measurements must be interrupted for approximately 20
mins to transfer data from disk to tape.
For this experiment, this system will be augmented by two
additional fixed radars, six wire wave gauges and a 3-axis anemometer
supplied by CCIW, and an experimental 3-axis hot-wire anemometer
and a 3-wire wave gauge from BYU. To support these additional
sensors, an additional computer system will be used to digitally
sample and store data from the sensors. This sampling computer
will have the capability of simultaneously sampling all the
wire wavegauges and anemometers at a 10 Hz rate using a sample/hold
board while sampling the quadrature of the dual, fixed-pointing
Doppler radars at 1 kHz. Wavegauge and anemometer samples are
stored directly in the files while the Doppler radar data is
processed into power and Doppler center frequency measurements
at a 10 Hz rate. The fixed radars have 3 deg beamwidths. One
is directed at nadir, the other will be at a 40 deg incidence
angle at a fixed azimuth angle. The operating frequency is 10.020
GHz for both radars. The output of these radars is quadrature
baseband signal with a center frequency equal to the instantaneous
Doppler shift. Once a day, measurements must be interrupted
for approximately 20 mins to transfer data from disk to tape.
Data tapes will be changed approximately once per week by
the CCIW. However, sufficient storage capability is available
for as much as 10 days of unattended operation. The computers
are designed for unattended operation. However, a phone-modem
connection will be used to check on the operation of each computer
approximately once a day from BYU. A phone-based power control
system will enable remote shutdown and restart of the various