Deep-Sea Rceeareh, 1968, Vol. 15, pp. 81 to 87. Pergamon Press. Printed in Great Britain.

Observations of coherent backscatter of 2 -10 M H z radio surface waves from the sea

D. D. CROMBIE* and J. M. WATTS*

(Received 10 October 1967)

Abstract--Radio waves propagating along the surface of the sea are known to be selectively scattered backwards by components of the sea waves which have lengths of half the radio wavelength, and which are traveling radially to the transmitter and receiver. The scattered signals have a Doppler shift which is characteristic of the radio frequency used.

An account is given of preliminary experiments designed to investigate some of the characteristics of ground wave signals scattered from the surface of the sea. The observations were made in the Gulf of Mexico using frequencies between ,~ 2 and 10 MHz. Thus the wavelengths of the sea waves observed were between 75 m and 15 m. The experiments verified that the Doppler shifts of the scattered signals were as expected from theory. Observational ranges of 200 km were obtained, but there is evidence that the range is smaller at night, probably because of tropospheric influences on the radio wave propagation. Two antennae having moderate directivity and oriented at right angles were used to demonstrate that waves traveling in different directions could be distinguished.

I N T R O D U C T I O N

THE COHERENT, or resonant, backscattering of radio frequency surface waves from the surface of the sea is now well known (WAIT, 1966). Unlike the scattered signals, or clutter, observed with microwave radars, which may have an extremely wide spectrum depending on the local sea state, the back scattered signal at medium and high fre- quencies has a rather narrow spectrum. The back scattered signal at medium and high frequencies has, moreover, a characteristic Doppler shift (CROMBIE 1955) which is proportional to the square root of the radio frequency used to make the observations. The Doppler shift is interpreted as being due to resonant scattering from the component of the sea wave spectrum which has a wavelength of half the radio wavelength and which is traveling radially to the transmitter and receiver.

The possible use of back scattered signals to examine the sea state at a distant point has been suggested by CROMBIE (1955), BARBER (1959), BRAUDE (1962) and others, although so far as is known this has not yet been done quantitatively. Because of this, we propose to describe some recent experimental observations of medium frequency and high frequency sea scatter and to discuss their possible relevance to mea- surement of sea wave spectra at a distance.

EQUIPMENT

Most of the observations discussed here were made using the existing ionosphere sounder at Eglin Air Force Base near Pensacola, Horida, on the Gulf of Mexico.

*Institute for Telecommunication Sciences and Aeronomy, Environmental Science Services Administration, Boulder, Colorado.

81

82 D.D. CROMBIE and J. M. WATTS

The ionosphere sounder is essentially a pulsed radar system whose operating frequency can be swept from ,~ 500 kHz to 25 MHz. Since it is used to examine the properties of the ionosphere, the antenna system is designed to radiate vertically. For studies of surface wave sea scatter, however, vertically polarized signals must be radiated in the horizontal direction. Thus the transmitting antenna was modified to act as a top-loaded vertical monopole. Vertically polarized waves are necessary because the3' suffer little attenuation along the surface of the sea. The receiving antenna was either a vertical monopole or one of two long-wire Beverage antennae which were used to study directional effects.

Because the back scattered signals were expected to show a known Doppler shift, the sounder was modified (FINDLAY, 1951) to show the relative phase of the received signals. Instead of displaying the rectified echoes on the oscilloscope time base, all signals were converted to a low frequency (75 kHz), whose individual cycles were resolved on the normal photographic film, range-time recording system of the sounder. The extra oscillator added to accomplish the conversion was stopped at the end of each time base sweep and restarted in coincidence with one of the cycles of the next trans- mitted pulse. The oscillator signal constitutes a stable phase reference. The 75 kHz signal obtained by mixing the reference signal and the echo from a stationary reflector thus has the same phase from pulse to pulse regardless of the r.f. phase of successive transmitter pulses. The received signal from a stationary reflector appears as a band of lines or fringes each separated by 1/75 msec, parallel to the time axis of the film record. The phase of the signals from a moving reflector, however, changes by 360 ° each time the reflector moves one wavelength radially. Thus the fringes in this case are inclined to the time axis. The slope of the lines in cycles per unit time, is equal to the Doppler shift imposed by the motion of the reflector. The direction of offset of the reference oscillator was chosen so that receding targets would show increasing range with time (positive slope on the range-time photographs). Thus positive slopes represent negative Doppler shifts.

A typical record is shown in Fig. 1. In the first 40 km or so the fringes are parallel to the time axis since the signals causing them are due to the transmitter pulse, " r ing- i ng" in the receiver and by reflections from stationary reflectors in the vicinity of the transmitter. These will be referred to as the ground pulse. Between 40 and 60 km beats occur between the ground pulse and the sea echo. Between 60 and about 150 km is the Doppler shifted sea echo, which in this case has a positive Doppler shift. Between 220 and 250 km the echoes are reflections from the F region of the ionosphere. These also show a positive Doppler shift. Other records, depending on time of day and frequency, may show ionosphere echoes from the E region at 110-150 km as well as from the F region in which the Doppler shift can be positive, negative, or zero. These ionosphere echoes have no bearing on the sea echoes and will not be discussed further.

ANALYSIS AND INTERPRETATION OF THE OBSERVATIONS OBSERVATION OF DOPPLER SHIFT

The Doppler shift of the back scattered signal arises because the received signal results from the constructive interference of the component of the sea wave spectrum which has a wavelength of half the radio wavelength and which is traveling radially with respect to the transmitter and receiver (CROMBIE, 1955). The velocity v of small

Observations of coherent backscatter of 2-10 MI-Iz radio surface waves from the sea 83

amplitude sea waves in deep water depends on their length L in accordance (COULSON, 1947) with

v = ,V/[(gL/2~) tanh (2~d/L)], (1)

where g is the acceleration of gravity (9-81 m/sec z) and d is the depth of the water. Thus, the coherently reflected signal from sea waves in deep water (d/L > ¼) will be slightly altered in frequency by the Doppler shift,

A t ' = 20 = ~¢/(gl~r)t) = 1.02 × llYI%/(DHz, (2) A

where f is the transmitter frequency in Hz and ;t is the radio wavelength. The signals reflected from sea wave components of other lengths will not add

coherently at the receiver and will be of much smaller amplitude than those reflected from the sea waves of the correct length.

The sea surface is quasi-random in pattern and can thus be regarded as the super- position of waves of varying lengths and direction of travel. In fact, it appears that waves of all lengths up to a maximum are present. The maximum length depends on the fetch (distance exposed to wind), strength of wind, and length of time it has been blow- ing. The maximum length of the sea waves is probably not sharply defined, for reasons which include the presence of swell from other areas.

Thus, coherent reflections can be expected at all radio wavelengths up to a maxi- mum value. Furthermore, with an omnidirectional antenna (e.g., a vertical monopole) there will always be some sea waves traveling radially. It should then be possible to see Doppler-shifted sea scatter for all wavelengths less than the maximum value. If a directive antenna is used, however, the system will be most sensitive to those sea waves of the appropriate length which are traveling in the direction of the main lobe. Wave components having the correct length but traveling in some other direction will be discriminated against.

In the example shown in Fig. 1, the Doppler shift is positive and has a magnitude of 11.6 c/min. All the data, taken at various frequencies during the observation period, October 13-17, 1966, have been analyzed to find the magnitude of the Doppler shifts. The results are plotted in Fig. 2. Also plotted is the theoretical value obtained from (1) the water depth being greater than 30 meters. Despite the scatter, the general

5o I I I ~ t I I t I J

3 0

° . 2 0 - - _=

t s -

6 - -

5 I I 1 I I I I / 1 I 2 3 4 6 8 I0 15 20

Frequency, MHz

Fig. 2. O b s e r v e d a n d theore t i ca l r e l a t ion b e t w e e n D o p p l e r sh i f t o f s ea echo a n d r ad io f r equency .

84 D . D . CROMm~ and J. M. WATts

agreement between the observed and theoretical values is apparent. The major cause of the scatter is believed to be scaling errors since, it is quite difficult to scale the fringes accurately because of their small inclination. It should be remembered (KINSMAN, 1964) that waves of finite height have greater velocities than those given by (1) although the change is small for moderate heights. No information is available about the wave heights prevailing when the data in Fig. 2 were obtained.

However, other isolated observations of the Doppler shift of surface wave re- flections have been reported by ANDERSON (1956) at 1.85 MHz, SrUaT et al. (1956) at 18.39 MHz and 24.7 MHz, and by RANZI (1961) for frequencies of 4, 7, 10 and 28 MHz and are in agreement with (2).

INTERFERENCE PATTERNS

A striking feature of the records is the periodic time variation in amplitude of the received signal at various ranges. A typical example is shown in Fig. 3. This is believed to be caused by beats between the Doppler-shifted sea scatter and the steady signal resulting either from ringing in the receiver circuits or coherent signals scattered from stationary irregularities on the ground. The beat frequency is the same as the Doppler shift expected from Fig. 2.

The shapes of the interference beat patterns may be explained by the sea wave patterns at different places not being completely correlated. Since the transmitted radio pulse was 50 tzsec long, reflections from about 15 km in range were not resolved, and the record at any range represents the resultant phase of signals integrated over a wide area. If we examine the pattern of phases at two different ranges we find that the two sets of ocean wave crests will tend to be independent of each other, and therefore we do not expect the new arrangement of crests to be a predictable function of the change in range. The smoothing effect of the pulse length and signal summation in the receiver limits the rate at which the resultant phase can change with range and results in smooth, but arbitrary shapes in the beat patterns.

Figure 4a shows another more complicated type of interference pattern. At the shorter ranges this is similar to the simple beat between ground wave and sea scatter from waves traveling toward the equipment discussed above. At the larger ranges, however, the beat frequency is doubled. A close examination in this region shows the presence of two sets of fringes which are equally but oppositely inclined to the time axis. These indicate the presence of waves both receding from the transmitter and approaching it.

Records showing both positive and negative Doppler shifts were observed only on the 13th and 14th of October. After 5:00 p.m. CST on the 14th, only positive Doppler shifts were observed. (The frequency and extent of the subsequent recording schedule is indicated in Fig. 6).

The interference or beats, between signals having positive, negative and zero Dop- pler shifts is believed to account for most of the observed beat patterns, including those shown in Figures 4b-d. The signals having no Doppler shift may arise by scattering from irregular features along the coast line, but this needs to be proved. These beats should also be observed on an appropriately situated conventional ionosonde which does not have the phase observing features (DOWDEN, 1957; WRIGHT, 1966).

Time Scale: One Minute i 3.5 MHZ 8:s o.m. 90. W.M.T.

Southeast Beverage Antenna October 15, 1966

Fig. 1. A typical range time record of sea scatter showing : (a) Ground pulse only, (b) Beat between sea echo and ground pulse, (c) Doppler-shifted sea echo, (d) Ionospheric F-region echo.

lc-------TTirne Scale: One Minute-

3.5 MHZ ?:46a.m. 90. W.M.T.

Southeast Beverage Antenna October 15, 1966

Fig. 3. (a) Ground pulse only, (b) Doppler-shifted sea echo and beat between sea echo and ground pulse, (c) Ionospheric E-region echo, (d) Ionospheric F-region echo.

k-Tim6 Scale: l/2 Minute4

316 p.m. $0. W.&T.

October 13, 1966 4.4 MHz, Vertical Troosmiifing ond RUCaiV-

ing Antenna?

ItsOl 0.m so= w.M.-c

October L3, IS66 4.3 MHz, Vefticol fronmnittii and Ret&t

ing Antennos

i4

fC_Time Seole~ 1/2 Minute-l

I:16 p.m. SO* W.M.T. October 13, 1966 4.4 MHz, Vertlcol Tronrmitting ond Receiv-

ing Ankmos

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Fig. 4. Other examples of sea echo beats. (a) Ground pulse only, (b) Both positive and negative shifted echoes beating with ground pulse and with each other, (c) Ionospheric F-region echoes, (d) Negative Doppler-shifted sea echo, (e) Beating between sea echo and ground pulse, (f ) Fixed range echo.

/-----SE SW --_-.+_.--sE -__--_--+--_ SJV _.__-c/ -Time Stole: One Minute --.I---

2.6 MHZ 4.19 p m 90” W.MT

SE and SW Beverage Antcnnos October 14, 1966

Fig. 5. Comparison of echoes from SE and SW Beverage Antennas.

i f, MHz

I I I I 75 50 30 25

Sea Wavelength, Meters

Fig. 7. Sea scatter as a function of frequency, f.

Observations of coherent backscatter of 2-10 MHz radio surface waves from the sea 85

DIRECTION OF ARRIVAL

The directional spectrum of sea waves is very important for oceanographic studies. In an attempt to see whether the radio observations would be sensitive to direction, two long-wire receiving antennae (Beverage type) were installed, one pointing to the southeast and the other to the southwest. They were long enough (2 or 3 wavelengths) to have well-sharpened main lobes. Although the beam widths were not measured, calculations indicate that they should be ~ 90-120 °. In Fig. 5 the effect of switching between the two antennae while recording is shown. The southeast antenna favored the sea echoes, shown by their Doppler shift, while the other antenna showed only echoes without Doppler shift. This suggests that the angular spectrum of the sea waves was quite narrow and that they were traveling toward the northwest, rather than the northeast.

DIU R NAL VARIATIONS OF MAXIMUM RANGE OF ECHOES

DOWDmq (1957) and P.~NZI (1960) suggested that the maximum range at which sea echo can be observed varies diurnally, being greater by day than by night. In view of this, the maximum range of the sea echo during the current observation has been plotted in Fig. 6. The sea echo was assumed to be identified by those fringes having the correct slope. In this figure some diurnal dependence is indicated, with the maximum ranges being observed during daytime hours. Because of the wide range of frequencies employed during these observations and their short overall duration, no firm con- clusions can yet be reached.

DOWDEN (1957) suggested that the diurnal variation observed by him was due to reduction in sensitivity at night. He thought this was due to the effect of skywave interference on the automatic gain control system of his equipment. The observations described here are believed to be free of such an effect.

Itm .~50

300

250

200

150

I00

50

_ I J t t 1 i I L - - [ I I k I l t t__ M~ximum Ronge of Oopptet Shlfled Echoes

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18 24 05 I° 19 24 06 12 [8 24 06 ]2 12 13 I 14 l C.S,T. 15 f 16 I 17

Octobe¢, I~6G

Fig. 6. Maximum observed ranges of Doppler-shifted echoes.

DEPENDENCE OF ECHO STRENGTH ON FREQUENCY

Although some swept frequency records without the phase reference signals were taken at Eglin, the sea echo is mixed with echoes showing no Doppler shift which are believed to arise from a continuous array of fixed scatterers (such as trees) along the

86 D . D . CROMBIE and J. M. WATTS

coast line. Because of the absence of the Doppler indication it is not possible to separate the sea echo from the fixed echoes in these observations.

Swept frequency observations made by J. W. WRIGHt (1966) of this Laboratory using an ionosphere sounder on board an aircraft carrier in the Pacific Ocean are free from this defect. Although Doppler indication was not used, the non-ionospheric echoes he observed are almost certain to be due to sea scatter since there was no land within 300 km of the ship. One of his swept frequency records is shown in Fig. 7.

The speckled regions between 1.1 MHz and 6 MHz show coherent reflections from the sea extending to about 300 km at 2 MHz and about 200 km at 5 MHz. The solid lines are first- and second-hop reflections from the E and F regions of the ionosphere. There is evidence that near 4-5 MHz the sea reflections are also being observed after further reflection from the F region of the ionosphere.

From figures such as this we can infer certain probable features of the shape of the sea wave energy spectrum if we assume that the sea state is the same at all observed ranges and the system sensitivity is independent of frequency. First, from Fig. 7, because of the absence of signals at frequencies below 1.0 and above 6 MHz there was little energy in the sea wave spectrum at wavelengths greater than 150 m or shorter than 25 m. Then, because the scattered signal shows maxima at frequencies of 2 MHz and 5 MHz one can reasonably suggest that maxima occurred in the sea wave energy spectrum in the vicinity of sea wavelengths of 75 m and 30 m with a minimum at 50 m.

The true relation between the shape of the sea wave spectrum and the shape of the maximum range curve is at present unknown but is unlikely to change the general nature of the deductions made in the previous paragraph very much. If the quantita- tive relationship were known, however, it would be a major step forward in using radio scattering measurements for quantitative studies of the sea surface. A quantita- tive relation must include the variation of signal strength with distance, and wave- length, as discussed in the next section.

DEPENDENCE OF THE AMPLITUDE OF COHERENT SCATTER ON RANGE

During the experiments at Eglin, observations of the way in which the amplitude of coherent scatter depends on the range were precluded by the use of the intensity modulated display. However, other workers have reported observations of this type. ANDERSON (1956) working at 1.85 MHz found that the amplitude at distances R out to 250 km fell off somewhat more rapidly than the R -z dependence which is expected from the radar equation for point targets.

DOWDEN (1957) found that at a frequency of 7 MHz the echo amplitude decreased approximately as R -aI2. On the other hand STotr et aL (1956), working at 18.39 MHz, found that the range factor was about R-L Their experiments did not however, permit the difference from an R -3t2 law to be identified.

Because of the varying amplitude of the echoes with time, and because the sea state most certainly varies with range, near a coast at least, it is difficult to determine the actual variation with range. The problem is further complicated if tropospheric refraction has to be included, as would be suggested by the existence of a regular diurnal variation.

WAIT (1966) has pointed out, however, that with a uniform sea the groundwave

Observations of coherent backscatter of 2-10 MHz radio surface waves from the sea 87

echo amplitude should vary as R -a/2 when the groundwave attenuation factor can be ignored, as it can be for frequencies less than about 5 MHz and for ranges to 100 km or so. WAIT'S calculations are in accordance with the experimental observations of DOWDEN (1957) and apply specificially to a sinusoidal surface of infinite width. The reason for the departure from the R -z law expected from the radar equation for point targets is that although the wavecrests are infinitely long, essentially only the signal returned from the first Fresnel zone contributes to the amplitude. The width of a Fresnel zone is oc A/(R;~). Thus as the range R increases, the width of the area return- ing signal also increases as %/R.

CONCLUSIONS

The foregoing discussion of experimental data demonstrates that certain properties of the sea surface at a distance could be determined at least qualitatively, with appro- priate radio observations. These properties are

(a) the approximate shape of the sea wave spectrum, averaged over a large area; (b) how this varies with direction; (c) how it varies with distance (provided that propagation effects can be accounted

for).

These interesting properties would be much more valuable if they could be put on a quantitative basis. For this to be done it would be necessary to derive a quantitative relationship between the scattering cross section of the sea and its directional energy spectrum.

REFERENCES

ANDERSON E. P. (1956) Characteristics of sea reflections at 1-85 Mc. Stanford Electronics Lab., Report Prepared under Contract AF 19 (604)-795 ASTIO Doe AD 110185. (Unpublished manuscript).

BARBER N. F. (1959) A proposed method of surveying the wave state of the open ocean. N. Z. Jl. Sci., 2, 99-108.

BRAUDE S. Ya (1962) The radio oceanographic investigation of sea swells. Akademiya Nauk Ukrainskoi SSR, Kiev, Institut Radiofizika i Electroniki. U.S. Office of Naval Intelligence Translation, No. 2067, 158 pp.

COULSON C. A. (1947) Waves. Oliver &Boyd, London, 75, 159 pp. CROMBIE D. D. (1955) Doppler spectrum of sea echo at 13.56 Mc/s. Nature, Lond., 175, 681. DOWD~N R. L. (1957) Short range echoes observed on ionospheric recorders, J. Atmos.

Terr. Phys., 11, 111. FrNDLAY J. W. (1951) The phase and group paths of radio waves returned from region-E

of the ionosphere. J. Atmos. Terr. Phys., 1 (5/6), 353. KINSMAN B. (1964) Wind waves. Prentice Hall, 640 pp. RANZt I. (1960) Tropospheric influence on HF backscatter near the sea. Scientific Note,

Contract AF 61 (052) 139, Centro Radioelectrico Sperimentale " G. Marconi" Rome, Italy. No. 2 (Unpublished manuscript).

RANZI I. (1961) Doppler frequency shift of HF to SHF radio waves returned from the sea. Tech. Note, Contract AF 61 (052) 139, Centro Radioelectrico Sperimentale "G. Marconi," Rome, Italy, No. 6 (Unpublished manuscript).

STUTT C. A., S. J. FRICKER, R. P. INGALLS and M. L. STONE (1956) Preliminary report on ground-wave-radar sea clutter. M.I.T., Lincoln Lab., Rep. No. 134. (Unpublished manuscript).

WAIT J. R. (1966) Theory of HF ground wave backscatter from sea waves, Jr. geophys. Res., 71 (20), 4839-4842.

WRIGHT J. W. (1966) Private communication.