Measurements collected in the Shuttle Imaging Radar (SIR-B) Extreme Waves Experiment confirm the ability of Synthetic Aperture Radar (SAR) to yield useful estimates of wave directional energy spectra over global scales, at least for shuttle altitudes. However, azimuth fall-off effects tend to become severe for wavelengths shorter than about 100 m in most sea states. Moreover, the azimuth fall-off problem becomes increasingly severe as the platform altitude increases beyond 300 km. The most viable solution to the global wave measurements problem may be a low altitude spacecraft containing a combination of both the SAR and the Radar Ocean Wave Spectrometry (ROWS). Such a combination could have a synergy which yield global spectral estimates superior to those of either instrument singly employed

Beal, R. C.

English

NASA Technical Reports Server

N87- 17149
OPERATIONAL WAVE FORECASTING WITH SPACEBORNE SAR:
PROSPECTS AND PITFALLS
R. C. Beal
Applied Physics Laboratory
The Johns Hopkins University
Laurel, Maryland
I. INTRODUCTION
In April 1965, now more than 20 years ago, a small number of radio
scientists and oceanographers congregated at the Woods Hole Oceanographic
Institute to consider how the new tools of remote sensing might enhance our
understanding of the global ocean environment. Among the techniques
considered were radar altimetry, radar scatterometry, and synthetic aperture
radar. Of course, at that time, good data sets were scarce, and many of the
recommendations were by necessity somewhat speculative in nature.
Nevertheless, the complement of active microwave instruments that was
identified was very similar to the suite later flown on SEASAT in 1978.
Of particular interest to the topic of ocean wave forecasting is
the following quotation from Gifford Ewing, the conference editor. With
respect to the value of SAR in monitoring the global wave spectrum, he said:
"What is needed is the directional energy spectrum of the
waves on a two dimensional surface, and for this, the
vantage point offered by a _atellite is ideal .... It is
within the capability of present day radar technology to
give a complete description of the sea surface."
That statement was made over 21 years ago. Since then, we have had
SEASAT, SIR-A, and SIR-B, all containing high resolution synthetic aperture
radars, and all collecting varying "descriptions of the sea surface." The
descriptions, however, are only more or less complete, and we are just
recently beginning to accumulate the evidence necessary to assess the true
value of SAR for obtaining useful estimates of the global directional energy
spectrum.
II. THE PROBLEM OF WAVE PREDICTION
Ocean wave prediction over global scales has been a fond hope of
oceanographers for several decades. Significant progress was made during and
soon after World War II, particularly when the concept of a "wave spectrum"
was advanced to describe the statistical properties of the wave field. The
idea of an "equilibrium spectrum" gained support from both theoretical and
experimental perspectives in the late 1950's, and has served as a framework
for departure even up to the present. Moreover, theories of wave growth are
now well developed in terms of the source wind field properties, such as fetch
and duration, and to some extent even in terms of the "background" wave field.
And yet, in the perspective of the global wave prediction problem, there are
still several fundamental unanswered questions. These questions center on both
the models and on the means for verifying them. For example:
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https://ntrs.nasa.gov/search.jsp?R=19870007716 2020-03-20T13:01:40+00:00Z
Howoften must we measure the wave field in order to predict
it?
Does the background wave field modify wave growth? Are its
directional characteristics important?
Is the directional wave spectrum a sufficiently complete
descriptor? How important are wave groups, which may give
rise to non-Gaussian statistics?
Howgood are the models? How efficiently can they assimilate
the data?
How well can we remotely measure the wave field?
can we measure it from orbital altitude?
How well
o Canwe learn any new physics from the measurements?
What is the optimum strategy for global wave monitoring? Is
it practical? Is its implementation likely to improve wave
forecasting?
Wedo not yet know the answers to these questions, but we are
beginning to accumulate a data base which is offering somemajor contributions
toward the solutions.
III. THESIR-B EXTREMEWAVESEXPERIMENTOFFCHILE
From October 8 to 12, 1984, as part of the SIR-B experiment,
simultaneous and coincident measurementsof the ocean wave directional energy
spectrum were made in a NASAP-3 aircraft flying off the southwest coast of
Chile. Sea states during the experiment ranged from 1.5 m to 4.5 m, and
included fresh seas, decaying swells, and both unimodal and bimodal wave
fields. The two primary aircraft instruments were the Surface Contour Radar
(SCR; Walsh, 1985) and the Radar Ocean Wave Spectrometer (ROWS;Jackson,
1985), each of which was independently capable of measuring the directional
energy spectrum.
Figure 1 illustrates the geometry of the Chile experiment, and
Table I summarizesthe various means of estimating (or predicting) properties
of the wave field. The SIR-B was operated through a variety of off-nadir
incidence angles, from 40° to 18 ° . Independent spectral estimates also were
made with the airborne optical laser (AOL) and the "AAFE" altimeter, a nadir-
looking altimeter yielding estimates of significant wave height. In addition
to the aircraft and spacecraft estimates, the U.S. Navy's Global Spectral Ocean
Wave Model (GSOWM), which only recently began operating in the Southern
Hemisphere, yielded separate estimates of the directional wave spectrum.
108
IV. EXPERIMENTALRESULTS
Although a complete set of spectral comparisons is available for
each of the last four days of the experiment, only one set will be shown here,
that of October ii, 1984. Preliminary comparisons are also reported in Beal
et al, 1986a, and more complete comparisons of the entire data set will be
forthcoming.
Figure 2 shows the four-way comparison of wave number spectra for
each of the aforementioned methods. The spectra from the SCR,ROWS,and GSOWM
are all displayed in relative spectral energy density units of m4. The SAR
spectrum is displayed in a similar format simply by assuming that the image
Fourier transform, after appropriate spectral smoothing and correcting for a
stationary point source response function, Beal et al, 1983; 1986b, is closely
related to the ocean wave slope spectrum. With this assumption, the displayed
spectrum is simply the slope spectrum times i/k 2, where k = wavenumber.
All the spectral estimates are similar in their gross features,
with both the ROWSand SARexhibiting 180° ambiguity. There are, however,
slight differences, particularly in the high wavenumber tails of the spectrum,
which are not yet satisfactorily resolved. Nevertheless, all instruments
agree well in wavenumber and direction, as well as in spectral and angular
width of the dominant system. On the other hand, the GSOWM estimate appears
rotated clockwise by nearly 30 ° This angular bias of GSOWM persists
throughout the entire data set, and clearly shows the potential value of
actual wave measurements as an update and correction for global models.
On other days, the agreement is not always so impressive. In
particular, azimuth traveling waves in low sea states (i.e., wave systems
having large dominant wavenumber) are severely attenuated and sometimes
obliterated. This azimuth wavenumber distortion results from the Doppler
nature of the SAR, and is a particular problem for high altitude (>300 km)
orbiting platforms. For this reason, it now appears that global spectral
estimates may best be obtained from a low altitude satellite employing both a
SAR and a ROWS. Such a design has been described at a recent symposium on
"Measuring Ocean Waves from Space," held at the Applied Physics Laboratory. A
more complete description of the concept, dubbed "Spectrasat," can be found in
the Symposium Proceedings, to appear in the first half of 1987.
SUMMARY
Measurements collected in the SIR-B Extreme Waves Experiment
confirm the ability of SAR to yield useful estimates of wave directional
energy spectra over global scales, at least for shuttle altitudes. However,
azimuth fall-off effects tend to become severe for wavelengths shorter than
about i00 m in most sea states. Moreover, the azimuth fall-off problem
becomes increasingly severe as the platform altitude increases beyond 300 km.
The most viable solution to the global wave measurements problem may be a low
altitude spacecraft containing a combination of both the SAR and the ROWS.
Such a combination could have a synergy which would yield global spectral
estimates superior to those of either instrument singly employed.
109
ACKNOWLEDGMENTS
This work is a portion of the SIR-B Extreme Waves Team effort.
Members of the team are: F. M. Monaldo, D. G. Tilley, D. E. Irvine, E. J.
Walsh, F. C. Jackson, D. W. Hancock III, D. E. Hines, R. N. Swift, F. I.
Gonzalez, D. R. Lyzenga, and L. F. Zambresky. Many others also contributed.
This work was supported by NASA and the Office of Naval Research.
REFERENCES
Beal, R. C., F. M. Monaldo, D. G. Tilley, D. E. Irvine, E. J. Walsh, F.C.
Jackson, D.W. Hancock III, D. E. Hines, R. N. Swift, F. I. Gonzalez,
D. R. Lyzenga, and L. F. Zambresky, "A Comparison of SIR-B Directional
Ocean Wave Spectra with Aircraft Scanning Radar Spectra and Global
Spectral Ocean Wave Model Predictions," Science (in press), 1986a.
Beal, R. C., F. M. Monaldo, and D. G. Tilley, "Large and Small Scale Spatial
Evolution of Digitally Processed Ocean Wave Spectra from the SEASAT
Synthetic Aperture Radar," J. Geophys. Res., 88, 1761-1778, 1983.
Beal, R. C., T. W. Gerling, D. E. Irvine, F. M. Monaldo, and D. G. Tilley,
"Spatial Variations of Ocean Wave Directional Spectra from the SEASAT
Synthetic Aperture Radar," J. Geophys. Res., Vol. 91, No. C2, 2433-2449,
February 15, 1986.
Jackson, F. C., W. T. Walton, and C. Y. Peng, "A Comparison of In Situ and Air-
borne Radar Observations of Ocean Wave Directionality," J. Geophys. Res.,
90, 1005-1918, 1985.
Walsh, E. J., D. W. Hancock III, D. E. Hines, R. N. Swift, and J. F. Scott,
"Directional Wave Spectra Measured with the Surface Contour Radar," J.
Phys. Oceanogr., 15, 566-592, 1985.
ii0
I_HUTTLE
P-3 _ "°
_AIR_
SAR SWATH
J
NASA SCR NASA ROWS
Figure I. SIR-B Chile experiment
iii
SCR
33oo/_- N _o °
0155Z_ I ,o_--,_
300°/
C"_ 1,, E
Loo_ __-__
I 21oo_ S'_--_ _ lsoo
ROWS 1/5
' 330°_ N _--.._o0
3ooo / \ I 7'_\ \_oc
! 2_6o__ s / _goo
" 300o_---- N _-_ 3,0o
o_o3zZ_ _ _,,
ioo_ ____;_ )o_
120°'<
[ _,o_"---_ s 1_oo
SAR ($24, 5548-55)
33og_/-- N _-..3o °
00z--_J _ _ ?o_--.._
210 o _ _ _ 150 °
GSOWM (55S, 80W)
Figure 2. SIR-B spectral comparisons, Chilean coast, October ii, 1984;
data take 91.50 (flight #4)
112
Table i. Chile data set summary
Wave estimates
Date SIR-B NASA P-3 aircraft
FLT # SCR ROWS AOL AAFE
GSOWM
8 oct 40° 1 x x X/ X/
9 Oct 35 ° 2 %/ X/ X/ %/ X/
i0 Oct 30 ° 3 %/ x/ X 6v/) v_
n Oct zs° 4 _/ _/ V _/
12 Oct 18 ° 5
_/ _/ X/ X/ ,,/
Data quality: x/good --(X/) marginal X absent
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Operational wave forecasting with spaceborne SAR: Prospects and pitfalls

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