Sea Spikes at Moderate to NearGrazing Incidence Angles

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Sea Spikes at Moderate to Near-Grazing Incidence
Angles

• Mary R. Keller
• Bruce L. Gotwols
• Rick D. Chapman
• All at
• Johns Hopkins University
• Applied Physics Laboratory

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Talk Outline

• Review of data sources, past research
• Generalized Sea Spike Definition
• Ground Truth video data for calibration, wave
  properties at time of spike
• Summary of dependence of sea spike density on
  winds, incidence, polarization
• Conclusions Not all spikes are created equal

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Data Sources (675 GB Total)
Nordsee Tower 1990 (ONR)
RV FLIP 1995 (OSD)
Duck, NC Pier 1996 (OSD)
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Previous Work at Moderate Incidence Angles

From Jessup et al., 1991
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Generalized Spike Definition
1. Power must exceed mean over a time equal to
or greater than one second. 2. Bandwidth must
peak within one second of beginning of spike in
mean power.
Keller, Plant and Valenzuela, 1986
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Spike Time Duration HH pol
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Spike Time Duration VV pol
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Ground Truth with Video Data
VIDEO
RADAR
LADAS
Mooring Buoy
Power (arb. units)
LADAS
Mooring Buoy
Incidence in degrees
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Ground Truthing with Video Waves
Steep Wave
Dihedral
Crossing waves
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Wind Dependence of Spike Density
Wind 9.8 m/s from -3º (rel.)
HH ? solid
VV ? dashed
53.8º
64.3º
74.9º



Wind 6.5 m/s from 1º (rel.)



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Polarization Dependence of Spike Density
COPE
MISE
FPN



Wind 9.8 m/s from -3º (rel.)
Wind 5.6 m/s from 56.7º (avg.)
Wind 7.8 m/s from 30.2º (rel.)
HH ? solid
VV ? dashed
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Conclusions

• Have developed a definition of sea spike
  applicable to a broad range of microwave and
  ocean conditions.
• 80 to 85 of the spikes, as defined, are
  time-coincident with a surface of high curvature
  - can be from a sharp wave front or crest, or
  from the interaction of two or more steep waves.
• These spikes are more frequent as the wind speed
  increases, but also become more directional, i.e.
  are more likely to be seen when looking upwind at
  high winds as opposed to cross- and down-wind,
  but are more uniformly distributed with azimuth
  as the wind speed drops.
• The number of spikes is about comparable for VV
  and HH polarizations at mid-incidence angles, as
  Jessup et al. found.
• As the incidence angle increases, the VV-pol
  spikes (as defined) become more common than
  HH-pol spikes, with the reverse true as the
  incidence angle decreases.
• These are not all the spikes known to occur!
  Averaging wipes out the spikes due to Rayleigh
  fading, while only 1/3 of spikes of one second or
  longer in time have accompanying bandwidth peaks.
  Thus, different classes of spikes have different
  generating mechanisms these spikes are probably
  caused by surfaces of steep curvature focusing
  the energy back into the antenna.

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Future Work

• Extend incidence angle range through full
  analysis of MISE data.
• Work toward developing a model relating wave
  crests and feature-generated backscatter
• Apply spiking definition to upcoming field data.