SECTION 68 Oceanography

NANOSECOND RADAR OBSERVATIONS OF THE OCEAN SURFACE

FROM A STABLE PLATFORM

BY

B. S. Yaplee, A. Shapiro, B. D. AuD. L. Hammond and E. A. Uliana

E. 0. Hulburt Center for Space ResearchNaval Research Laboratory

Washington, D. C.

INTRODUCTION

Radar as a remote sensor of ocean surface conditionshas been in use for sometime. These radars, using eitherCW or long pulse modulation require that the measurementsbe taken at various look angles. The ocean surface condi-tions are inferred from the steepness of the curves ofreflected energy versus angle. However, if one were toreduce the pulse width of the radars until the pulse widthresolves the vertical water wave structure and looks onlyin the direction normal to the ocean surface, an entirelydifferent set of radar returns are available to describethe ocean surface characteristics. This paper will describesome preliminary results obtained by measuring the oceansurface with a nanosecond radar pulse from a fixed platformat vertical incidence.

RADAR SYSTEM DESCRIPTION

A simplified block diagram of the radar system isshown in Figure 1. The transmitted one nanosecond pulseis generated by rapid pulsing of a step recovery diodewhich is mounted in an x-band waveguide (WR-90). The diodeoutput is amplified by a TWT amplifier and fed to a trans-mitting antenna. Duplexing is achieved using separatetransmitting and receiving antennas which provide adequateisolation. The received pulses are amplified at rf untilsufficient signal is available to drive diode detectorswith extremely fast response time. The detected signal is

https://ntrs.nasa.gov/search.jsp?R=19720004668 2020-05-20T21:13:09+00:00Z

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displayed on a sampling scope. The sampling scope storesthe information in a manner permitting A-D conversion forrecording on magnetic tape. All the synchronization of therf signals and control signals are programmed by the controlunit. The radar system parameters are:

Pt = 0.1 WPRF = 70 KHz

T = 1 nsAntenna Diameter D = 76 cmAntenna Beamwidth = 2°Receiver Bandwidth = 109 MHz

Antenna Gain = 35 db

OBSERVATIONAL PLATFORM

The radar system was installed on the Chesapeake LightTower (Figure 2) which is located about 15 miles east ofVirginia Beach, Virginia. The antennas were mounted under-neath the catwalk as shown in Figure 3. The antennas areabout 70 feet above mean sea level and the radar illuminatesa spot on the ocean about one meter in diameter. In orderto calibrate the radar system, both in range and amplitude,a corner reflector held on a rigid pole was placed in thecenter of the antenna beam and mounted about 60 feet fromthe transmitting antenna (Figure 4). To provide the primaryground truth concerning the waves, three wave poles wereplaced in a delta configuration surrounding the radarilluminated spot. The electrical output of the three wavepoles, using 0.1 second time constant, were recorded simul-taneously with the radar data on magnetic tape.

DATA

Figures 5 and 6 are computer plots of the radar dataas recorded. These two runs will be used to i l lustratetwo extreme sea conditions that were recorded. Figure 5represents a calm sea condition with moderate swells.Figure 6 corresponds to a 20 knot wind with wind driven5-foot waves. Since the wave pole measurements arerecorded simultaneously, their output is arbi t rar i lyplaced to the left or right of the radar data and theira l ignment in time is automatic . The plots, as shown, are

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an attempt to illustrate, as much as possible with onephotograph, a sequence of the time history of the wavespassing the radar illuminated spot. Each division of thevertical scale corresponds to one second in time. Thehorizontal scale measures the time delay of the radarpulse in nanoseconds, where each division corresponds to5 ns (75 cm) .

Figures 7 and 8 are plots of the power spectrum forthe data of Figures 5 and 6 respectively. In addition,the wave pole power spectrum is superimposed on the radarspectrum. The agreement is so close that one can trulyinterchange the two results without serious error.

As mentioned earlier, a corner reflector was placedin the beam so that calibrations may be possible in powerand range. The radar returns from the sea of Figures 5and 6 were calibrated with respect to the radar return ofthe corner reflector. The power return was averaged foreach increment of range resolution so that a plot ofreflected power versus depth into the wave height ispossible. Figures 9 and 10 show the reflected power versuswave depth for the two examples. The crest is to the leftand the trough is to the right. Note that the reflectivityin these two cases is not uniform but increases toward thetrough.

If one were to plot the effective amplitude distri-bution of the returned signals over the vertical water wavestructure and normalize the area under the curve, one thenobtains the equivalent impulse response for the sea. Thereturns were plotted in this manner in Figures 11 and 12with an accompanying photograph of the sea at the time thedata were taken. Superimposed is the wave pole amplitudedistribution. It is seen that the effective differencebetween the radar and wave pole distribution is minor, andthat the error introduced by the electromagnetic distortionon the derived mean height and wave heights should be small.

SUMMARY

Nanosecond pulse radar systems show promise for measuringthe sea and provide information comparable with wave polemeasurements. Until further analysis with additional dataand thorough comparison with wave pole data, this report isintended as a preliminary discussion of progress to date.

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TRANSMITTING ANTENNA

STEPRECOVERY

DIODE

1

CONTROL UNITA/D

MULTIPLEXER

SAMPLING_SCOPE_ _j

RECEIVING ANTENNA

Fig. 1 Radar System

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Fig. 2 Chesapeake Light Tover,

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Fig. k Wave Pole and Corner Reflector

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^_o^-- aiGwmcAicr WAVE HEIGHT 3.1 rr

Fig. 5 Computer Plot of Radar Data

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Fig. 6 Computer Plot of Radar Data

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