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.

• Investigators

  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 (CCIW).

• Investigations
  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. Model function frequency dependence (R.S. Collyer, BYU)

     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.

  7. 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).

  8. 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 slope?

• 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 subsystems.