Multifrequency radar measurements of soil parameters

Adv. Space Res. Vol.1, pp.l05—l09. © COSPAR, 1981. Printed in Great Britain.

02731177/81/04010105$05.OO/O

MULTIFREQUENCY RADAR MEASUREMENTS OF SOIL PARAMETERS G. Flouzat, T. Le Toan, A. Fluhr and M. Pausader Centre d’Etude Spatiale des Rayonnements, CNRS, Université Paul Sabatier, Toulouse, France ABSTRACT The radar response to soil surface was investigated in 1979 using a scatterometer operating at 1.5, 3, 4.5 and 9 GHz. Analysis of data in terms of the effect of frequency, polarization, incidence, soil moisture and surface roughness on the radar response is presented. Characterization of soil parameters suitable for data interpretation is developed. INTRODUCTION Since 1978, the Centre d’Etude Spatiale des Rayonnements has been involved in an experimental program of ground-based scatterometry on bare and vegetated test fields. The acquired data consists of radar backscattered signals recorded with the 4 frequency scatterometer RAMSES of the Centre National d’Etudes Spatiales (CNES) {1} associated with information collected in the field. This paper presents some of the results of the experiment performed in 1979 aimed at evaluating radar potential for acquiring data suitable for soil parameter detection purposes. DESCRIPTION OF THE EXPERIMENTS The specifications of the radar are suniiiarized in Table 1. Type Centre frequency F0 Relative frequency sweep ~F0/F0 Modulating frequency Transmitter power Transmitting antenna Receiving antenna diameter (Parabolic) Receiving antenna aperture (3 dB beam)

FM

Triangular wave forme 1.5 3 4.5 9 GHz 1.33 or 2.67 % 66 or 200 Hz 1 W 1 W 1 W 0.2 W 2 ridged horns for H and V polarization 1.10 m 0.47 0.47 0.47 dual polarized linear polarized 12° 6°—15° 10° 50

Polarization isolation

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TABLE 1 : Specifications of RAMSES scatterometer JASR 1,10

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H

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G. Flouzat et ci.

106

The scatterometer is mounted on a crane (construction elevator) at 15m height. For every angle of incidence, the signal is recorded continuously as the radar sweeps along the boom of the crane (16m). This motion of the radar contributed to increase the number of spatial independant samples. The displacement ~L of the antenna causes the change the difference in phase between two observed locations. A new independant samples is approximately 30 for 1.5 0Hz and 75 for 3, 4.5 and 9 0Hz. On the other hand, the contribution of the frequency sweep to the number of independant samples is respectively 3, 2, 7, 4 for 1.5, 3, 4.5, 9 0Hz at 600 (no contribution at incidence near vertical). Daily calibration was performed with a Luneberg lens and, in support of the microwave measurements, ground data were acquired, including soil surface roughness profile and soil moisture profile with depth. The surface roughness is evaluated by means of a rectangular metal system (3m) equipped with sliding vertical bars at every 1.5 cm interval. The soil moisture is determined by gravimetric method (5 samples at -1, -3, -5, -7, -10 cm). The standard deviation 3 for a moisture range of top tolayer fromthe0.01 0.24 moisture g/cm3. varied from 0.006 to 0.02 g/cm BACKSCATTERING COEFFICIENT ANALYSIS The data base included 51 sets of data, each consisted of the backscattering coefficient oo measured at 1 frequency, 3 polarizations (HH, HV, VV) and 9 angles of incidence 0, 5, 10, 15, 20, 30, 40, 50, 600. Effect of the angle of incidence The angular behaviour of 00 varies as a function of the surface roughness. Smooth surfaces correspond to specular behaviour and rough surfaces diffuse behaviour. Figure 1 compares two such angular behaviours for the smooth surface (rms height = 1.6 cm) and the rough surface (rms height = 5.6 cm). Furthermore, Figure 1 shows also that a° increases with soil moisture content. Spectral response Two trends are observed : 00 increases with the frequency for incidence higher than a transition value and decreases below. Figure 2 shows a ~epresentative case of rough surface at a medium scale moisture (0.14 to 0.16 g/cm ). F

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Fig. 2 Spectral response of the rough surface at different angles of incidence.

6

Radar Measurements of Soil Parameters

107

Effect of roughness The effect of roughness on radar response can be deduced from the angular behaviour of the signal {2} {3}. Backscattered signals at the nadir (°o) correspond to the specular component and decrease with increasing roughness. At angle of incidence o far from nadir, backscattering is affected mainly by the scattering phenomenon and increases with the roughness scale. When the roughness increases signals related to Oo and 0 tend towards the same limit corresponding to the Lambertian diffusion. The angular behaviour for intermediate angles may be deduced by interpolation and between 0°and 0, an incidence angle range relatively insensitive to roughness can be found. For a given frequency and polarization combination, this angle range may be determined. The data analysis yields the following ranges that minimize the effect of roughness : 5 to 20°relative to nadir, depending on the frequency. 20

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Figurescale 3 shows 00 as a function of rough— ness for 4.5 0Hz. These results confirm mainly conclusions of ULABY {4} optimum incidence range for soil moisture detection. Fig. 3 : Variation of a as a function of the roughness scale (RMS height) for different angles of incidence.

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Effect of soil moisture The scattering of the terrain is governed by the dielectric properties of the surface (or volume). To study the contribution of the surface and that of the sub-surface, a° is plotted as a function of soil moisture content in the upper layer and in deeper layers. Figure 4 shows o°versus moisture content of the 0—1 cm layer and figure 5, o° versus moisture content of 0—7 cm layer for 4.5 GHz. The volume contribution appears to have better correlation to o°than the surface contribution. 5

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CHARACTERIZATION OF SOIL PARAMETERS To interprete the relation between backscattered signals and soil parameters mainly moisture content profile and surface roughness - the first phase consists of determining intermediate parameters whose correlation with the physical signal can be approached. Soil moisture is dependent on the soil moisture content of an effective soil layer. The volume contributing to the scattering can be approched by the dumping calculations deduced from the dielectric properties of the medium. The soil is described as a succession of thin plane layers of homogeneous dielectric properties. I I I ~ Dielectric constants can be determined by 30. measurement or by theoretical modelling. Figure 6 shows result of laboratory measurement of the soil samples (clay 25 %, •~+REACPURT loam 20 %, sand 55 %) at frequency 3,26GHz The inflexion of the real part curve can be explained by the behaviour of the water I in the soil : at low moisture, water is 1 bound to soil particles and free water appears beyond a given moisture (12 % in this case). 0°

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Fig. 6 : Dielectric constants versus moisture Modelling calculation can provide dielectric constants for large ranges of frequency and moisture. The model used is based on a pedological description of the soil considered as a three parts medium (air, water and soil particles) {5} where significant volumetric proportions are known by matric potential measurements. Figures 7 show results of the modelling calculation compared to values given by the polynomial regression of Cihlar and Ulaby {6}. The transition zones are not observed in these cases. 30 1 I I I I I 1 I I I I I I I I II I I / / 11971.1 10GM, 27 CESR 19791 T70H5 27 CESR 119791 / 27 • CESR 119791 9.375GM, // cLy25s. ct~y~2S% -

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Figures 7 : Dielectric constants of the soil under study computed by modelling.

1.5

Radar Measurements of Soil Parameters

109

Surface roughness In the first approximation, surface roughness is represented by rms height which takes into account only the small scale roughness. In fact, many rough surfaces can be modeled as a superposition of two independent surface height distributions: small scale roughness or microroughness and large ondulations or macroroughness. These two components are obtained by Fourier transform, filtering and inverse Fourier transform. Then, prior to applying the two-scale models to describe the scattering mechanism {7} the surface roughness is decomposed in two surface height contributions. Figure 8 shows a result of such a decomposition attempt of the rough surface under study (rms height = 5.6 cm) by filtering method. Correlation function and spectral density calculated for each component will be used

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BIBLIOGRAPHY 1. tJ.M. Lopez :

The multifrequency ground based radar RAMSES Proceedings of the workshop “Microwave Remote Sensing on Bare Soil”. EARSeL publication Paris 1979. Theory of radar return from terrain - IRE Internat. Cony. Record 7 — Part 1 : 27 (1959) Reflection of electromagnetic wave by slightly rough surfaces Intersciences (1963). Microwave Backscatter Dependence on Surface Roughness, Soil Moisture and Soil Texture : Part I - Bare Soil IEEE Trans. Geosci. Electron. Vol GE 16, n° 4, pp 286—295 (1978). :A theory of the complex dielectric permittivity of soil containing water : the semi-disperse model. IEEE Trans. Geosci. Electron. Vol GE 15, n°1 (1977). F.T. Ulaby : Dielectric properties of soil as a function of moisture content. NASA CR 141868 RSL-TR-177-47 (40) (1975) Incoherent backscatter from rough surfaces : The two-scale model reexamined Radio Science Vol 13, n°3, pp 441-457 (1978) —

2. W.H. Peake : 3. S.O. Rice

:

4. F.T. Ulaby : 5. 0. Wobschall 6. J. Cihlar and

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7. J.C. Leader :

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