Cross-polarized radar backscatter from moist soil

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C r o s s - p o l a r i z e d R a d a r B a c k s c a t t e r f r o m M o i s t Soil

HARUTO HIROSAWA, SETSU KOMIYAMA* AND YUKIHIKO MATSUZAKA lmt~tute of Space and Aeronautmal Sctence, Umvers~ty of Tokyo, 4-6-1, Kmnal~ Meguro-ku, Tokyo, lapan

Measurements of radar backscatter from bare sod at 9.0 GHz using a broad beam, with an effectave bearnwidth of product patterns and an incident angle of about 17 ° and 30", respectively, have shown that the sermtlvaty of the cross-polarized (HV) return to soil moisture content was much lugher than that of the like-pelanzod (HH) one Analysis of the data shows that the observed HV back scattering power has a dependency of nearly I~, where I" ~s the power refleehon coefficient at a plane soil-air interface Tim fact suggests that mullaple scattering on rough sod surface caused the sod morstttre sensllavaty of the cross-polartzed return to be lugh

Introduction Soil m o i s t u r e d i f f e r e n c e s c a u s e changes in radar cross section since the dielectric properties of soil are strongly affected by water content m the soil. A radar has a high potentiality for remote measurement of soil moisture of terrain surfaces. Many experimental studies have been made in recent years to examine the radar response to moisture content variations of natural soils. Moisture dependency was observed with the 0.4 and 13.3 GHz scatterometer carried in an aircraft (Dickey et al., 1974). Extensive systematic field investigations were made at the University of Kansas and the effects of frequency, look angle and surface roughness on the soil moisture detection sensitivity were examined (Ulaby, 1974; Ulaby et al., 1974). Optim u m radar parameters for mapping soil moisture were derived (Ulaby and Batlivala, 1976).

*Present address Shibuya-k-u. Tokyo

Nippon Hoso Kyokaa, Zmnan,

©Elsewer North-Holland Inc., 1978

Scattering on the surface or in the subsurface of terrain generally causes radar returns to depolarize, so we can expect further information in crosspolarized signals. In the foregoing studies, only experimental results on like-polarized radar returns were reported. In this paper, the measurement results on the response of a crosspolarized radar return to soil moisture content, along with that on a likepolarized one, are presented. Observations suggest the usefulness of crosspolarized radar return for sensitive remote detection of soil moisture. Experiment The radar system which is used in this experiment is a car-mounted X-band scatterometer, designed to measure both like- and cross-polarized radar backscatter simultaneously. Three horn-antennas are m o u n t e d on a platform aligned for maximum overlap of their main beams. The one for transmitting is horizontally polarized, and the other two for receiving are horizontally and verti0034-4257/78/0007-0211501.25

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cally polarized, respectively. Two polarizations, HH (horizontal transmit, horizontal receive) and H V (horizontal transmit, vertical receive), are obtained. The - 3 dB angular widths of the product gain, i.e., the product of a transmitting antenna gain and a receiving antenna gain, of the HH combination are 15 ° m elevation and 17 ° in azimuth, and that of the H V combination are 17 ° in elevation and 15 ° in azimuth. A Gunn diode oscillator of 40 m W power output is used as a transmitter. The frequency is 9.0 GHz. The modulation is AM and the modulating waveform is square, with a period of i msec. The receiving circuit consists of a square-law crystal detector, an amplifier and a phase sensitive detector. The detectmn circuit has an integration time of 300 msec. HH and H V signals are detected by two independent circmts. Calibrations of the ratio of received to transmitted power are made by supplying a part of the transmitting power attenuated by a calibrated attenuator to receiver circuits, and comparing the output from the attenuated transmitter s~gnal with that from backscattered ground signals. The external calibration of the system was made by trsmg a corner reflector Backscatter measurements were made on three bare fields. We designate the three fields as A1, A2 and B. A l and A2 were slightly rough fields and B was a rougher field. The root-mean-square (rms) height variations were 0.6 cm, 0.4 cm and 1.5 cm for fields A 1, A2 and B, respectwely. Surface roughness profiles were obtained by using a thin metal plate with a scale. The plate was inserted into the soil and was photographed. This is the same method as F. T. Ulaby et al. used (1976). The rms height of the field A1 was nearly the same as that of A2, but

H HIROSAWA ET AL

the typical curvatures of the surfaces of A1 and A 2 were slightly different in the visual observations. Soil on the field is classified as loam, named Kanto loam, which is widely distributed m the Kanto district of Japan (see Appendix) Experiments were made on the days when the fields were m drv condition, and to examme the soil moisture dependency a part of each field, an area of about 10 m length and 2 m width, was irrigated, step by step, from a low moisture level to a high moisture content. Back scatter signals were recorded while the vehicle was being moved along the test fields. The scatterometer antennas were at a h o g h t of 2.3 m. The velocity of the vehicle was a b o ~ 40 cm/sec. Since the detecting circmt had a time constant of 300 msec, fluctuations of back scatter signals due to fading were greatly reduced in the detector output. Back scatter from rough surfaces is made up of many minor lobes and the along-track distance for the smallest backscatter lobe is approximately given by 19,1/2, where 19,, is the horizontal aperture of an antenna (Moore and Thomann, 1971). D~,/2 is approximately equal to 4 cm in our system, so about 3 lobes were crossed in 300 msec. Output signals were sampled m every 0.5 sec. This corresponded to taking about 5 samples along one illtunmated patch, since the along-track length of an illuminated patch was about 1 m. The vehicle was moved five times along the field of 10 m length, and thus the total number of samples taken was about 250 for each moisture level. These 250 samples were averaged and back scattering coefficients were calculated. The ratios of the standard deviation to the mean value of these data sets were about 0 . 2 ~ 0.3 in both HH and H V measurements

MOIST SOIL RADAR RETURNS

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zations are indicated in the form of subscripts. The main beam was directed to the surface with an incident angle of 30 ° . Since the beamwidths of the antennas were rather broad, measured back scatter

Experimental Results Figure 1 shows plots of the H H and H V back scattering coefficients as a function of soil moisture content. The polari-

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FIGURE 1. HH and H V back scattenng coef'hcients as a function of monmtre content. Frequency ts 9.0 GHz. Beam was directed to the surface with an incident angle of 30 ° Beamwidths of the product gain, in eleval~on, a r e 17 ° for HH and 15 ° for H V polarization. Moisture content ASa percent dry soil wenght, m ~ on a surface layer of about 3 cm depth.

214

signals were sums of returns from many surface elements distributed in a wide angle. Consldenng the angular pattern of the product gain and the variation of scattering coefficient with angle (Moore, 1970), the range of the actual incident angle that contributed to the back scattering signals is estimated to be about from 16 ° to 35 ° , and the angle at which a maximum contribution occured is estimated to be nearly 25 °. The back scattering coefficients given in Fig. 1 were derived using the assumption that scattering coefficients were constant over the illuminated area. Thus, they are to be considered as expressing averaged values around the effective incident angle of about 25 °. The moisture content is a percent of dry soil weight, that is, a percent ratio of liqtud weight to dry soil weight. Under natural conditions a soil moisture profile with depth is not constant, generally. Several approaches have been taken for defining an eqtnvalent moisture content of a soil with a variable moisture profile (Ulaby et al., 1974), and the equivalent incoherent reflectioncoefficient moisture content is proposed as the most appropriate definition (Ulaby and Batlivala, 1976). In this study we took the fixed depth moisture content (Ulaby et al., 1974), since the skin depth is rather thin at X-band. Soil samples were collected from a surface layer of about 3 cin depth. The volume of each sample was about 700 cm 3. Three samples were collected for each moisture level and average moisture content was measured. The moisture content in Fig. 1 is easily transformed to a moisture weight in unit volume by multiplying a dry soil density, that is the weight of solid in a unit volume of soil, and then dividing by 100. The dry soil density of soils in the test fields was 0 . 5 ~ 0 . 6 g / c m 3.

H HIROSAWA ET AL

Figure 1 shows the general trend of a hnear increase in scattering coefficient with increasing moisture content, but the remarkable fact is that the HV back scattering coefficient om, has a much higher dependency on moisture content, compared to o H . Two linear regression hnes, which were drawa~ using the data of fields A t and A:, clearly show the difference between two polarizations. The effect of the difference of the surface roughness is not significant in these results, as is seen m the figure. The correlation coefficients between back scattering coefficient and moisture content, calculated using the data of fields A l and A2, are 0 73 for HH polarization and 0.87 for HV polarization. Mso the moisture dependency in the field B appears to be given by nearly the same linear relations, thougla only two levels of moisture content were measured there. Radar soil moisture sensitivity is defined as the rate of change of radar back scattering coefficient w~th soil moisture content (Ulaby et al., 1974). Hence the slope of the linear regression line gives the soil moisture sensitivity. From Fig. 1, these are derived as 0.05 d B / % for HH polarization and 0.20 d B / % for H V polanzatlon. These values correspond to sensitivities of about 0.09 d B / 0 . 0 1 g / c m '3 and 0.36 d B / 0 . 0 1 g / c I n ~, respectively, where soil moisture is given by moisture weight in unit volume. We see that the sensitivity of the cross-polarized radar return is four times as large as that of the like-polarized one. The high sensitivity of the crosspolarized radar retun~ may be explained by considering a scattering mechanism. We reconstruct Fig. I by replacing the moisture content with the power reflection coefficient at a plane surface. The power reflection coefficients were calcu-

MOIST SOIL RADAR RETURNS

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lated for a normally incident wave using the dielectric constants measured as a function of moisture content on Kanto loam (given in the appendix along with a brief description of the measurement method). The regression lines in Fig. 1 I

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H HIROSAWA ET AL

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reflection coefficient at normal incidence. Nearly the same results were obtained when we took the power reflection coefficient at the incidence angle of 30 °. It should be noted that the F dependency of cross-polarized backscatter is larger than F l, actually near to F 2. Mechanisms that are known to be able to produce cross-polarized radar returns are (Janza, et al., 1975): (1) the difference between the Fresnel reflection coefficients for horizontal and vertical polarizatmns in quasi-specular reflection; (2) volume scattering due to nonhomogeneltles; (3) multiple scattenng on rough surfaces; and (4) anisotropic surface stnlctures Of these mechanisms, (1) is applicable only to smoothly undulating

surfaces and can contribute only at larger incident angles. F dependence of the remained three mechanisms are as follows: (4) can predict, at most, F 1 dependence. When the second mechamsm is dominant, the effect of increasing F should rather be to reduce cross-polarized returns. In contrast, the third meehamsm can predict F 2 dependence, since scattermg twice should be the most important process. Thus the observed F 1.5-21 dependence suggests that the multiple scattenng at soil-a~r interfaces is a mam mechanism m producing cross-polarized radar returns. We can attribute the high moisture content sensitwlty of H V backscatter to this nearly [-2 dependency.

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MOISTSOILRADARRETURNS Conclusions The preliminary experiments have shown that the cross-polarized radar return had high sod moisture sensitivity, which was about four times, in decibel, as large as that of like-polarized radar return, at 9.0 GHz. The cross-polarized signals seem to have a promising capability of sensitive, remote detection of soil moisture. Improved understanding of its usefillness needs filrther experimental studies of the radar response over a wide range of the parameters, such as frequency, look angle and surface roughness. Acknowledgments The authors wish to thank 7". Yarnagami for his help in this work, H. Saito for the dielectric constants measurement, and M. Fulii for his comments on the manuscript. APPENDIX Kanto loam is a soil widely distributed in Kanto district of Japan. It is classified as a loam. Figure A(1) shows the dielectric constants of Kanto loam measured as a function of moisture content at 9.2 GHz. Standard standing-wave measurem e n t t e c h n i q u e s w e r e used. T h e measurements were made for the moisture content from 0 percent to about 100 percent of dry soil weight, and for the

217 dry soil densities of 0.5 g / c m 3 and 0.6 g / c m 3.

References Dickey, F M, King, C., Holtzman, J C., and Moore, R. K. (1974), Moisture dependency of radar backscatter from irrigated and non-irrigated fields at 400 MHz and 13.3 GHz, IEEE Trans. on Geoscience Electronics GEl2, 19-22. Janza, F J (1975), in Manual of Remote Sensing (R. G. Reeves, Ed.), Vol 1, American Society of Photogrammetry, Virginia, pp. 75-179 Moore, R K (1970), in Radar Handbook (M. I Skolnlk, Ed.), McGraw-Hill, New York, Chap 25. Moore, R. K., and Thomann, G. C. (1971), Imaging radars for geoscience use, IEEE Trans. on Geosc~ence Electronics GE9, 155-164. Ulaby, Fawwaz T (1974), Radar measurement of soil moisture content, IEEE Trans on Antennas and Propagation AP22, 257-265. Ulaby, Fawwaz T., and Bathvala, Percy P. (1976), Optimum radar parameters for mapping soil moisture, IEEE Trans. on Geoscience Electronics GEl4, 81-93. Ulaby, F T., Cihlar, J., and Moore, R. K. (1974), Active microwave measurement of soil water content, Remote Sens. of Environ. 3, 185-203. RecewedAtml, 1977, retnsedNovember1977