.c.East-North Atlantic Ocean


Titles of Investigations:

I.  An Investigation of the Imaging of Ocean Waves and Internal
Waves with SIR-C/X-SAR

II.  Optimization of SAR Parameters for Ocean Wave Spectra

III.  Comparison of SIR-C/X-SAR Simulated and Measured SIR-C/X-SAR
Image Spectra with Ocean   Wave Spectra Derived from Buoys
and Wave Prediction Models

Principal Investigators:

I.  Dr. Gordon Keyte
  Royal Aerospace Establishment

II.  Mr. Frank Monaldo
  Applied Physics Lab/Johns Hopkins University

III.  Dr. Werner Alpers
  Institut fur Meereskunde

Site Description:

 The supersite is defined by coordinates 45° to 54° north
and 10° to 20° west, lying west of the British Isles and
the Bay of Biscay and including a section of the European
continental shelf break.
 The site has been selected to be readily accessible from
the European coast and to combine good wave statistics with
a useful imaging geometry from ascending and descending SIR-C/X-SAR
passes.  From studies with the SEASAT radar altimeter and
from long-term weather station data, the region has been
shown to have a high probability of significant wave heights
in excess of two meters.  Although more northern sites have
even more reliable waves, at latitudes above 54°, we rapidly
lose the scientific benefit of swaths acquired on successive
ascending and descending orbits crossing at large angles.
 At latitudes below 45°, wave statistics become less dependable
for a relatively short duration Shuttle mission.  The supersite
is also within reception range of European ground stations
for "Image Mode" data fro the ERS-1 C-band SAR.  This will
offer a chance to obtain imagery at very much higher range-to-velocity
ratios than are possible with SIR-C.
 A successful wave imaging experiment was conducted close
to the chosen site as part of the SIR-B mission in 1984.
 As with that experiment, the in situ data collection will
be mobile and will use ship-deployed instrumentation at the
positions of SIR-C/X-SAR crossing swaths which will move
from day to day during the mission.  The selection of experiment
sites coincident with pairs of crossing swaths will allow
the relatively rapid re-imaging of similar wavefields from
different radar illumination directions.

Objectives:

I.  a) Improve our understanding of ocean-wave imaging by
synthetic-aperture radar (SAR).

  b) Test the assumptions of backscattering theory with regard
to short-wave properties.

  c) Develop new techniques for retrieving ocean-wave spectra
from multi-parameter SAR.

II.  a) Determine the relative contributions from proposed
mechanisms for the imaging of ocean surface waves by SARs.

  b) Establish the dependence upon geometry, radar frequency,
ocean wave height, and wind speed and direction of the loss
of azimuth resolution associated with SAR wave imaging.

  c) Select a set of SAR parameters (geometry, frequency,
polarization) that maximizes the fidelity of SAR derived,
two-dimensional ocean wave spectra in the context of azimuth
resolution limits.

III.  a) Apply a forecast and hindcast wave prediction model
to the wavefields in the North Atlantic from the measured
wind history during the SIR-C/X-SAR overflight.

Field Measurements:

 The core of the experiment in the East-North Atlantic will
be mobile, based around ship-deployed wave and meteorological
instrumentation, which will include directional wave buoys.
 The ship will travel to sites of crossing swaths on successive
days at which rapid re-imaging of similar wavefields from
different radar look directions will take place.  Supporting
wave data will come from hindcast modeling.

Crew Observations:

1) Crew Journal: Locate and sketch internal waves.  Document
ocean state and weather conditions at the site.

2) Cameras: Hercules and Hasselblad will be used to photograph
internal waves; low angle sun glint images are requested.

Coverage Requirements:

 The minimum requirements for the East-North Atlantic supersite
are four (4) crossing tracks.

Anticipated Results:

I.  a) An accurate empirical model of the dependence of wave-imaging
transfer function on radar frequency, polarization, and incidence
angle.

  b) A greater understanding of the roles of short-wave straining
and of scatterer motions in providing mechanisms for imaging
long waves.

  c) An improved knowledge of conditions under which non-linear
imaging limits the wave information recoverable from SAR
data.

  d) Optimum procedures for recovering wave information and
recommendations for SAR-system parameters for future missions.

II.  a) Determination of a composite SAR wave imaging model
including the effects of all relevant imaging mechanisms.

  b) Determination of whether azimuth resolution degradation
is alleviated at higher radar frequencies than L-band.

  c) Recommendations for SAR geometry, frequency, and polarization
to maximize the fidelity of SAR derived wave spectra and
alleviate the loss of azimuth resolution associated with
high sea states.

III.  a) Determination of the accuracy of the SAR imaging
theory for three frequencies through comparison of the measured
and predicted ocean wave spectra.