Microwave specular scattering response of soil texture at X-band

Available online at www.sciencedirect.com

www.elsevier.com/locate/asr

Advances in Space Research 44 (2009) 801–814

Microwave specular scattering response of soil texture at X-band

Rishi Prakash, Dharmendra Singh *, Nagendra P. Pathak

Department of Electronics & Computer Engineering, Indian Institute of Technology Roorkee, Roorkee 247 667, India

Received 10 December 2008; received in revised form 15 May 2009; accepted 18 May 2009

Abstract

Soil texture is an important soil parameter that is useful for meteorology, climatology, hydrology, ecology, etc. Therefore, it is impor-tant to classify soil based on soil texture (i.e., sand, silt and clay). A lot of studies with radar remote sensing have been carried out toestimate soil moisture and surface roughness, but less attention has been given to study the effect of individual soil texture on radar scat-tering, especially in specular direction. The main aim of this paper is to check the behavior of specular scattering with change in soiltexture. This effect has also been analyzed in presence of soil moisture and surface roughness. Scattering coefficient has been retrievedfor various soil texture fields with indigenously designed X-band bistatic scatterometer for a range of incidence angles (from 30� to 70� insteps of 10�) in both like polarizations, i.e., HH-polarization and VV-polarization. Observations were made at 10 GHz frequency. Fourdifferent fields were considered on the basis of soil texture variations; especially changes in sand percentage were made. Roughness(smooth soil to 1.4 cm rms surface height) and moisture (dry soil to 0.21 cm3 cm�3 volumetric soil moisture) conditions of these fieldswere varied for observations. Strong change in specular scattering coefficient is observed by changing the sand percentage in soil for HH-polarization, while in case of VV-polarization a lesser change is observed. Also a high change in specular scattering coefficient is noticedonce moisture is added to the soil. It is difficult to observe the change in specular scattering coefficient with change in soil texture whensurface is considered as rough. Therefore, it is important to minimize the roughness effect while observing the texture with specular scat-tering. For this purpose, polarization study was carried out to see how polarization can be helpful to minimize the roughness effect. Theeffect of soil texture on copolarization ratio is critically analyzed, and it is observed that for higher incidence angle (P50�), the distinctionin soil texture fields are clearly observable on the basis of copolarization ratio. This type of study will be helpful in near future to designthe bistatic radar system for soil parameter monitoring, especially for cartwheel satellite system.� 2009 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Microwave specular scattering; Soil texture; Scattering models

1. Introduction

It is generally known that soil is characterized basicallyupon it constituents, i.e., texture (i.e., sand, silt and clay).Further, study of soil texture is important as it exerts astrong influence on many hydrologic and biogeochemicalprocesses in agriculture and forest ecosystems by affectingthe ability of soils to retain water, and nutrient ions (Silveret al., 2000). Additionally, soil texture greatly affects waterholding capacity, soil erosion, soil drainage, soil tempera-ture as well as fertility and productivity (Zhai et al.,

0273-1177/$36.00 � 2009 COSPAR. Published by Elsevier Ltd. All rights rese

doi:10.1016/j.asr.2009.05.016

* Corresponding author.E-mail address: [email protected] (D. Singh).

2006). Soil texture is a term commonly used to designatethe proportionate distribution of the different sizes of par-ticles in a soil. According to their size, these particles aregrouped into ‘‘separates”. According to United StatesDepartment of Agriculture (USDA) system of nomencla-ture separates having the diameter between 2.00 and0.05 mm are categorized as sand, diameter between 0.05and 0.002 mm are categorized as silt and separates havingthe diameter less than 0.002 mm are categorized as clay(Brown, 2003). Microwave remote sensing as a tool toextract the soil parameters is becoming very popular nowa days. So it important to observe the effect of soil textureon microwave scattering. A lot of advantage can beachieved over conventionally used methods, such that

rved.

mailto:[email protected]

802 R. Prakash et al. / Advances in Space Research 44 (2009) 801–814

when a proper model is developed, it is less cumbersome toretrieve the soil parameters without going to the field. Sec-ondly, we may observe the field more frequently than anyother conventional methods that involve the field visit.

Till date most of the researches for the retrieval of soilparameters are focused on monostatic system. In contrast,very few works have been done to check the bistatic system,especially in specular direction. The need of present scenariois to look for other aspect of microwave remote sensing tocheck its response for soil parameters. Researches are goingon for system conceptualization in bistatic direction (Com-blet et al., 2006; Du et al., 2006; Krieger et al., 2003; Mocciaet al., 2000). Still, a limited work has been reported to char-acterize the soil parameters in bistatic direction (Ceraldiet al., 2005; Nashashibi and Ulaby, 2007; Singh and Dubey,2007). Hence, there is a need to check the microwaveresponse in specular direction on soil texture on one handand in other hand to see the effect of moisture and roughnesswith texture on specular scattering.

Further, researches have been performed to evaluate thesoil surface roughness and soil moisture content throughactive microwave remote sensing (Dubois et al., 1995;Hallikainen et al., 1985; Magagi and Kerr, 2001; Mattiaet al., 1997; Mironov et al., 2004; Oh, 2004; Oh et al.,1992; Singh, 2005; Singh et al., 2003; Wang and Schmugge,1980; Zribi and Dechambre, 2002; Zribi et al., 2005). Lessattention has been provided to check the response ofmicrowave scattering for soil texture. Efforts have beenmade to incorporate the soil texture effect in scatteringcoefficient while retrieving other soil parameters such assoil moisture and surface roughness (Srivastava et al.,2006). Studies have shown that dielectric constant of soilis dependent on soil texture (Hallikainen et al., 1985;Mironov et al., 2004; Wang and Schmugge, 1980). In addi-tion, the variation of the dielectric constant of soil withmoisture shows its dependency on the soil texture (Wangand Schmugge, 1980).

In case of active microwave remote sensing, scatteringcoefficient is a function of sensor parameters as well astarget parameters. Sensor parameters include incidenceangle, frequency, and polarization; while soil texture, soilsurface roughness and soil moisture are target parameters.Sensor parameters are known to us a priori, therefore,scattering coefficient can reveal the information of targetparameter being a function of it. The present challengelies in characterizing the response of each surfaces param-eter separately with one scattering coefficient value. Manytheoretical models, e.g., Kirchhoff model with scalarapproximation (SA), Kirchhoff model with stationaryphase approximation (SPA), and Small perturbationmodel (SPM) (Ulaby et al., 1982) as well as empiricalmodels, e.g., Oh model (Oh, 2004; Oh et al., 1992),Dubois model (Dubois et al., 1995) exist to characterizethe soil parameters. Shortcomings of the theoretical mod-els are that they are based on rigorous mathematical for-mulation and work for well defined surfaces and withintheir validity limits, hence failing to quantify the natural

surfaces. Further, empirical models work very efficientlyfor the condition in which these models are developedbut are less sensitive towards other atmospheric condi-tions. Surface roughness plays an important role whenscattering coefficient is considered. So, it is an importantpoint of research to minimize the surface roughness effectfor the retrieval of other soil parameters, e.g., soil textureand soil moisture. In case of natural surfaces it is difficultto characterize the surface roughness and when consider-ing remote sensing data this factor is unknown as there isa temporal change in soil surface roughness. Therefore,efforts have been made to minimize the effect of soil sur-face roughness by utilizing the multipolarization, multiin-cidence, and multifrequency data (Magagi and Kerr,2001; Mattia et al., 1997; Oh, 2004; Zribi and Dechambre,2002; Zribi et al, 2005).

In this paper, effort has been made to check the responseof specular scattering on soil texture. Further, changes insoil moisture and surface roughness were made to analyzeits impact on specular scattering for change in soil texturefield. Polarization effect was also studied and copolariza-tion ratio was considered to check its effect on surfaceroughness. Additionally, response of copolarization ratiofor different soil texture fields was analyzed.

2. Methodology

2.1. Specular scattering coefficient measurement

In order to analyze the response of microwave scatteringin specular direction for change in soil texture fields, an X-band bistatic scatterometer setup was developed indige-nously. It is capable of taking observations by changingincidence angle from 30� to 70� in both like polarizations.Measurements were made at 10 GHz frequency with dualpolarized pyramidal horn antenna for both like polariza-tions by changing incidence angle in steps of 10�.

Specular scattering coefficient was calculated at variousincidence angles for different soil texture fields. Moistureand surface roughness conditions were also considered.Following steps were involved in retrieval of specular scat-tering coefficient:

(1) Calibration: Calibration is required to provide accu-rate quantitative results from the data collected. For thepurpose, amplitude calibration was performed using abso-lute calibration technique (Curie and Well, 1989). Refer-ence target used for calibration was aluminum sheet.Radar cross section (RCS) of flat aluminum sheet is givenby Eq. (1) (Curie and Well, 1989):

rppAlðhÞ ¼ 4pA2

k2

sinðkb sin hÞkb sin h

� �2

cos2 h ð1Þ

where rppAlis radar cross section of aluminum sheet, A is

area of aluminum sheet, k is wavelength used, h is the inci-dence angle, b is the dimension of square aluminum sheet,pp employs for HH or VV-polarization and k ¼ 2p

k .

Table 1Soil texture information of fields used for observation.

Percentageof sand

Percentage of silt Percentageof clay

Percentageof gravels

Field-1 78.14 9.12 4.32 8.11Field-2 58.87 23.13 8.53 9.32Field-3 44.35 35.83 11.72 7.56Field-4 38.72 33.16 18.75 9.13

Table 3Validity limits for SPM, SPA and SA.

Validity limit for SPM(Gaussian surface)

Validity limit for Kirchhoff model(Gaussian surface)

Stationary phaseapproximation

Scalarapproximation

ks < 0.3 and rmsslope < 0.3 ks > 2 ks < 1 andrmsslope < 0.25

R. Prakash et al. / Advances in Space Research 44 (2009) 801–814 803

In decibel (dB), RCS of aluminum sheet can be writtenas Eq. (2).

rppAlðhÞ ðdBÞ ¼ 10log10rppAl

ðhÞ ð2Þ

(2) Radar cross section for soil can now be computed asgiven in Eq. (3).

rppsoilðhÞ ¼ P ppsoil

ðhÞP ppAlðhÞ

� �rppAlðhÞ ð3Þ

where rppsoilðhÞ is radar cross section for soil, P ppsoil

ðhÞ ispower received for soil at various incidence angles andP ppAlðhÞ is the corresponding power received for aluminum

sheet at various incidence angles.Scattering coefficient is defined as the radar cross section

per unit area. In present case the illumination cell size waskept 1 m2. Therefore, the division of radar cross section forsoil by illumination area provides scattering coefficient(Curie and Well, 1989). Scattering coefficient for soil is rep-resented by r�ppsoil

ðhÞ. Scattering coefficient of soil in decibel(dB) can be written as Eq. (4).

r�ppsoilðhÞ ðdBÞ ¼ 10log10r

�ppsoilðhÞ ð4Þ

2.2. Field data measurement

Four fields having different percentage of sand, silt andclay were considered for study of scattering behavior inspecular direction. Soil texture information of these fieldsis given in Table 1. Sieve analysis and Hydrometric testwere carried out to find out the percentage of sand, siltand clay in different soil texture fields. Sieve analysis usingvarious sieves of different mesh opening (4.75–0.075 mm)was used to calculate the percentage of sand. The percent-age of soil retained on each sieve is calculated on the basisof the total mass of the soil sample. In addition, soil frac-tion finer than 0.075 mm were separated out for furtherhydrometric test. Hydrometric test was carried out for par-

Table 2Moisture and roughness values used for the study of each field.

Moisture (cm3 cm�3) RMS surface height (cm)

s1 s2 s3

Dry Smooth 0.9 1.40.1 Smooth 0.9 1.40.15 Smooth 0.9 1.40.21 Smooth 0.9 1.4

ticle size lesser than 0.075 mm to determine the percentageof silt and clay in soil (Apparao and Rao, 1995).

For each field, one dry and three moisture values weretaken into account. Soil moisture measurements were madeby taking five soil samples for each moisture condition.Weights of moist soil samples were measured. Further,samples were kept at 110 �C for 24 h for drying (Holahet al., 2005) and the gravimetric soil moisture was calcu-lated as given in Eq. (5).

mg ¼wmoist � wdry

wdry

ð5Þ

where mg is gravimetric soil moisture, wmoist is weight ofsoil with moisture, wdry is weight of dry soil.

Fig. 1. (a) Scattering coefficient vs. incidence angle for dry smooth soil inHH-polarization. (b) Scattering coefficient vs. incidence angle for drysmooth soil in VV-polarization.


Soil bulk density was measured and then gravimetric soilmoisture was multiplied with soil bulk density to make itvolumetric soil moisture (mv). Mean values of soil moisturewere taken which range from 0.10 cm3 cm�3 to0.21 cm3 cm�3 with standard deviation of 0.016. For eachsoil texture field with one dry and three moisture condi-tions, one smooth (s1) and two roughness values (s2 and

Fig. 2. (a) Scattering coefficient vs. incidence angle for smooth soil with moistusmooth soil with moisture 0.15 in HH-polarization. (c) Scattering coefficient vs.Scattering coefficient vs. incidence angle for smooth soil with moisture 0.10 in Vwith moisture 0.15 in VV-polarization. (f) Scattering coefficient vs. incidence a

s3) were considered as given in Table 2. Soil surface rough-ness was measured with the help of pin profile meter (Zribiet al., 2005). Vertical roughness of soil surface is generallyrepresented by root-mean-square (rms) height and definedas the standard deviation of the measured vertical heights.For each field, three measurements were made and averagevalue of rms height has been reported.

re 0.10 in HH-polarization. (b) Scattering coefficient vs. incidence angle forincidence angle for smooth soil with moisture 0.21 in HH-polarization. (d)V-polarization. (e) Scattering coefficient vs. incidence angle for smooth soilngle for smooth soil with moisture 0.21 in VV-polarization.

Fig. 3. (a) Scattering coefficient vs. incidence angle for dry soil having rms height 0.9 cm in HH-polarization. (b) Scattering coefficient vs. incidence anglefor dry soil having rms height 1.4 cm in HH-polarization. (c) Scattering coefficient vs. incidence angle for dry soil having rms height 0.9 cm in VV-polarization. (d) Scattering coefficient vs. incidence angle for dry soil having rms height 1.4 cm in VV-polarization. (e) Scattering coefficient vs. incidenceangle for different soil texture field with moisture 0.10 and rms height 0.9 cm in HH-polarization. (f) Scattering coefficient vs. incidence angle for differentsoil texture field with moisture 0.10 and rms height 1.4 cm in HH-polarization. (g) Scattering coefficient vs. incidence angle for different soil texture fieldwith moisture 0.10 and rms height 0.9 cm in VV-polarization. (h) Scattering coefficient vs. incidence angle for different soil texture field with moisture 0.10and rms height 1.4 cm in VV-polarization. (i) Scattering coefficient vs. incidence angle for different soil texture field with moisture 0.15 and rms height0.9 cm in HH-polarization. (j) Scattering coefficient vs. incidence angle for different soil texture field with moisture 0.15 and rms height 1.4 cm in HH-polarization. (k) Scattering coefficient vs. incidence angle for different soil texture field with moisture 0.15 and rms height 0.9 cm in VV-polarization. (l)Scattering coefficient vs. incidence angle for different soil texture field with moisture 0.15 and rms height 1.4 cm in VV-polarization. (m) Scatteringcoefficient vs. incidence angle for different soil texture field with moisture 0.21 and rms height 0.9 cm in HH-polarization. (n) Scattering coefficient vs.incidence angle for different soil texture field with moisture 0.21 and rms height 1.4 cm in HH-polarization. (o) Scattering coefficient vs. incidence angle fordifferent soil texture field with moisture 0.21 and rms height 0.9 cm in VV-polarization. (p) Scattering coefficient vs. incidence angle for different soil texturefield with moisture 0.21 and rms height 1.4 cm in VV-polarization.



2.3. Copolarization ratio for scattering in specular direction

Theoretically copolarization ratio was obtained byKirchhoff model with scalar approximation (SA), Kirch-hoff model with stationary phase approximation (SPA),and Small perturbation model (SPM). In case of specularscattering, copolarization ratio for SPM, SA, and SPA pro-vides the same mathematical expression and given in Eq.(6) (Ceraldi et al., 2005).

f ðhÞ ¼ r�HH

r�VV

¼ jRhðhÞj2

jRvðhÞj2ð6Þ

Fig. 3 (con

where

RhðhÞ ¼cos h�

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffie� sin2 h

pcos hþ


p

RvðhÞ ¼ �e cos h�


pe cos hþ


pwhere h is the incidence angle and e is the dielectricconstant.

Validity range for SPM, SA, and SPA is provided inTable 3, where s denotes surface rms height, k is wave num-ber and rmsslope defines the slope of rough surface (Ceraldi

tinued)


et al., 2005). Since all these three methods provide the samemathematical expression for copolarization ratio, there-fore, copolarization ratio calculated from Eq. (6) includesthe validity range of all three theoretical models and isvalid for a wide range of surface roughness values.

Dielectric constant of soil is a function of soil texture aswell as soil moisture and this value can be retrieved withempirical relation given by Hallikainen et al. (1985) andwritten in Eq. (7).

e ¼ ða0 þ a1S þ a2CÞ þ ðb0 þ b1S þ b2CÞmv

þ ðc0 þ c1S þ c2CÞm2v ð7Þ

where ai, bi, and ci (i = 0–2) are empirical constants. S and C,respectively, are sand and clay texture component of soil inpercent by weight and mv is volumetric soil moisture content.

3. Results and discussion

3.1. Response of microwave scattering in specular direction

for soil texture

3.1.1. Dry and smooth soil surface

In order to analyze the response of microwave scatteringfor different soil texture fields, specular scattering coeffi-cient were plotted with varying incidence angles for bothlike polarizations. Good dynamic range of approximately

Fig. 3 (con

9 dB is obtained with varying incidence angle in HH-polar-ization for all four different soil texture fields as shown inFig. 1a. As the proportion of sand is decreased from78.14% to 58.87%, from 58.87% to 44.35% and furtherfrom 44.35% to 38.72% a noticeable change in specularscattering coefficient has been observed for all incidenceangle for HH-polarization (Fig. 1a).

In Fig. 1b the behavior of scattering coefficient with inci-dence angle is shown for four different soil texture fields inVV-polarization. Good dynamic range of 20 dB withchange in incidence angle is obtained. Further, smallchanges are observed in specular scattering coefficient forchange in soil texture fields (i.e., by changing the sand per-centage from 78.14% to 58.87%, from 58.87% to 44.35%and further from 44.35% to 38.72%).

It is observed from the study of dry and smooth soil thatby changing soil texture (major change in sand% value);there is a change in specular scattering coefficient for bothlike polarizations. The effect is more prominent in HH-polarization than in VV-polarization and in case of HH-polarization at higher incidence angles (i.e., P50�) betterdifferentiation in specular scattering coefficient is observed.It may be due to the reason that HH-polarized wave inter-acts with different soil texture fields more prominently thanVV-polarized wave in specular direction. Similar resultswere also obtained by Nashashibi and Ulaby (2007) andDe Roo and Ulaby (1994).

tinued)


3.1.2. Moisture effect on response of specular scattering from

different soil texture fields

Fig. 2a–c shows the behavior of specular scattering forfour different soil texture fields with moisture0.10 cm3 cm�3, 0.15 cm3 cm�3, and 0.21 cm3 cm�3, respec-tively, in HH-polarization. With different soil texture fields,change in specular scattering coefficient are difficult toobserve at 30� incidence angle, but as the incidence angle

Table 4Copolarization ratio for different surface roughness height at various incidenc

Copolarization ratio

Incidence angle (�) 30Rms height (cm)

Dry soil Smooth 3.040.9 3.51.4 3.1

Moisture = 0.10 cm3 cm�3 Smooth 1.670.9 1.481.4 2.01



Fig. 4. (a) Theoretical result of copolarization ratio of dry soil. (b) Theoreticalof copolarization ratio with soil moisture 0.15. (d) Theoretical result of copol

is increased, change in scattering coefficient are observed.At higher incidence angles better differentiation in specularscattering coefficient are obtained, i.e., changing sand per-centage from 78.14% to 58.87%, from 58.87% to 44.35%and further from 44.35% to 38.72%, change in specularscattering coefficient is obtained at 50�, 60� and 70� inci-dence angles for moisture variation of 0.10–0.21 cm3 cm�3.At soil moisture 0.15 cm3 cm�3 and 0.21 cm3 cm�3, a con-

e angles with change in soil moisture value.

40 50 60 70

6.51 14.81 15.77 10.596.2 13.5 14.79 9.497 13.81 15.07 9.6

4.26 6.96 13.84 20.754.19 6.82 12.3 19.594.5 7 13.09 20.02

4.54 6.23 10.04 24.434.54 6.1 10.44 22.74.52 6.2 10.5 22.54

2.02 5.69 8.38 15.182.16 5.78 8 15.52.21 5.52 8.21 15.7

result of copolarization ratio with soil moisture 0.10. (c) Theoretical resultarization ratio with soil moisture 0.21.


tinuous decrease in specular scattering coefficient isobserved from 30� to 50� incidence angles for all four fields,but in contrast an increase is observed at 60� incidenceangle. Increase in specular scattering coefficient at 60� inci-dence angle was also observed in Nashashibi and Ulaby(2003, 2007). Effect of soil texture on specular scattering,in the presence of soil moisture, is noticed from the changein specular scattering coefficient that is valid for a range ofsoil moisture (0.10–0.21 cm3 cm�3).

Fig. 2d–f shows the scattering behavior of four differentsoil texture fields in specular direction for VV-polarizationhaving moisture value 0.10 cm3 cm�3, 0.15 cm3 cm�3, and0.21 cm3 cm�3, respectively. For VV-polarization, at lowerincidence angle (i.e., <50�), less changes are observed in spec-ular scattering coefficient with the change in soil texture fieldfor all moisture conditions. But, at 50� and 60� incidenceangle, better differentiation in specular scattering coefficientis observed. With the study of moisture effect it is observedthat the change in specular scattering coefficient is betterobserved with moisture than at dry soil for both like polar-izations. Further, at higher incidence angles (i.e., P50�), dif-ferentiation in scattering coefficient values are better thanany other incidence angle for all moisture values considered.

Fig. 5. (a) Experimental result of copolarization ratio of dry smooth soil. (b) Ex(c) Experimental result of copolarization ratio of smooth soil with moisturemoisture 0.21.

While comparing to HH- and VV-polarization, it isfound that specular scattering coefficient is decreasing withincidence angle at low soil moisture value (volumetric soilmoisture 0.10) for both like polarizations (Fig. 2a and d),whereas, due to increase in moisture content from 0.15 to0.21, an increment of specular scattering coefficient isobserved at higher incidence angles (i.e., P50�) for HH-polarization (Fig. 2b and c). While at VV-polarization nosuch trend is observed (Fig. 2e and f) at higher moisturevalue.

3.1.3. Surface roughness effect on response of specular

scattering from different soil texture fields

Fig. 3a–p shows the scattering behavior of soil withincreased roughness values (rms surface height of 0.9 cmand 1.4 cm) for dry soil as well as moisture variations. Withthe increased surface roughness, it is difficult to observe aclear behavior for specular scattering with the change insoil texture. In case of HH-polarization, when the rms sur-face height is 0.9 cm, with the increase in soil moisturevalue, differentiation in specular scattering coefficient stillexist, but due to surface roughness effect change in specularscattering coefficient with change in soil texture field is not

perimental result of copolarization ratio of smooth soil with moisture 0.10.0.15. (d) Experimental result of copolarization ratio of smooth soil with


uniform, i.e., decreasing the sand percentage in soil, specu-lar scattering coefficient is not decreasing uniformly(Fig. 3a, e, i and m) as observed for smooth soil. Similaris the case when rms height is 1.5 cm. For VV-polarization,we observe the same behavior as with HH-polarization(Fig. 3c, d, g, h, k, l, o and p). But with VV-polarization,the differentiation in specular scattering coefficient is lessthan that of HH-polarization at all incidence angles. Aclear behavior in specular scattering coefficient is notobserved due to increased surface roughness.

3.2. Response of copolarization ratio for soil texture

With above discussion, it may be noticed that it is diffi-cult to observe soil texture in presence of roughness fromspecular scattering coefficient at X-band. To make the soiltexture observable on specular scattering, copolarizationratio analysis has been carried out. Table 4 shows the copo-larization ratio for different surface roughness height val-ues at various incidence angles with change in soilmoisture values for field 1. It may be noticed from Table4 that with change in surface roughness height for constantsoil moisture and at a particular incidence angle, copolar-

Fig. 6. (a) Experimental result of copolarization ratio of dry rough soil havinmoisture 0.10 and rms height 0.9 cm. (c) Experimental result of copolarizationcopolarization ratio with moisture 0.21 and rms height 0.9 cm.

ization ratio remains approximately same. Results fromthree other soil texture fields show the same behavior. Sim-ulation results, obtained by Ceraldi et al. (2005) also showthe similar trend. Theoretically, when the scattering is inspecular direction, copolarization ratio is given by Eq.(6). As Eq. (6) satisfies SPM, SA and SPA, the main featureof this copolarization ratio is that it does not depend on thesurface roughness values. In addition, surfaces satisfyingvalidity limits of SPM, SA or SPA, copolarization valuesare in good agreement (Ceraldi et al., 2005). Theoreticallycalculated copolarization ratio (Eq. (6)) for different soiltexture fields for varying soil moisture have been plottedin Fig. 4. Further, Figs. 5–7 show the copolarization ratioretrieved from experimental data. Plots comparing theexperimentally observed copolarization ratio and theoreti-cally calculated copolarization ratio are shown in Fig. 8a–d, which represent good agreement between observed andcalculated values of copolarization ratio.

With the above study, it is observed that the copolariza-tion ratio is approximately same with the change in rms sur-face height, i.e., copolarization ratio is approximatelyindependent to rms surface height. These results are equallyvalid for all soil moisture values at all incidence angles.

g rms height 0.9 cm. (b) Experimental result of copolarization ratio withratio with moisture 0.15 and rms height 0.9 cm. (d) Experimental result of


3.2.1. Statistical analysis for copolarization ratio

It is found that the response of copolarization ratio withchange in soil texture is better observed at higher incidenceangles (i.e., P50�) than at lower incidence angles (i.e.,<50�). When the percentage of sand is changed from78.14% to 38.72% and observations are made at incidenceangle less than 50�, the distinction among copolarizationratio for all four different soil texture fields is difficult toobserve (Figs. 5–7). But in contrast at higher incidenceangles, a change in copolarization ratio is clearly observed(Figs. 5–7). Above finding can be validated by applyingregression analysis in which copolarization ratio was takenas dependent variable and soil texture and soil moisture asindependent variables. The regression analysis was per-formed for fields by changing moisture conditions. Resultsof regression are shown in Table 5. It may be noticed fromTable 5 that R2 values are quite low and SE are high forlower incidence angles (i.e., <50�) at all soil moisture con-ditions and in most of the cases, R2 is less than 0.50. Lesservalues of R2 give rise to the assumption that at lower inci-dence angles, specular scattering coefficient poorly corre-late with soil texture and soil moisture. For higherincidence angles (i.e., P50�), R2 values are always greater

Fig. 7. (a) Experimental result of copolarization ratio for dry soil having rms h0.10 and rms height 1.4 cm. (c) Experimental result of copolarization ratiocopolarization ratio with moisture 0.21 and rms height 1.4 cm.

than 0.83 and SE are low. Therefore, it may be inferredthat higher incidence angles better correlate the specularscattering coefficient with soil texture and soil moisture.Higher incidence angle is also considered by Ceraldi et al.(2005), Nashashibi and Ulaby (2007), Singh and Dubey(2007) for scattering in specular direction. Further, it maybe observed from Table 5 that at 60� incidence angle, R2

have the maximum value (i.e., >0.95) and SE have the min-imum value (i.e., <0.40) for all moisture conditions whichled to the conclusion that 60� is best incidence angle formonitoring soil parameters when working in speculardirection.

3.3. Response of copolarization ratio for change in sandpercentage in soil

It was discussed in the previous section that to check theresponse of change in soil texture on specular scattering inpresence of soil surface roughness and soil moisture, copo-larization ratio at higher incidence angle (i.e., P50�) isappropriate. Further, it was observed that 60� is best suitedincidence angle. df/dS (where f is copolarization ratio andS is sand percentage in soil) is shown in Fig. 9 for dry soil

eight 1.4 cm. (b) Experimental result of copolarization ratio with moisturewith moisture 0.15 and rms height 1.4 cm. (d) Experimental result of

Fig. 8. (a) Experimental copolarization ratio vs. theoretical copolarization ratio for dry soil. (b) Experimental copolarization ratio vs. theoreticalcopolarization ratio for different soil texture field with moisture 0.10. (c) Experimental copolarization ratio vs. theoretical copolarization ratio for differentsoil texture field with moisture 0.15. (d) Experimental copolarization ratio vs. theoretical copolarization ratio for different soil texture field with moisture0.21.


as well as soil with varying moisture conditions at 60� inci-dence angle. It may be observed from Fig. 9 that with thechange in percentage of sand in soil there is a change incopolarization ratio. An increasing trend in copolarizationin obtained with the decrease in sand percentage. Thistrend can be observed for dry soil as well as soil with differ-ent moisture conditions. Further, it may be noted that thedynamic range for copolarization ratio with the change insand percentage is approximately 4 dB for dry soil, whichis maximum and approximately 2 dB for soil having volu-metric soil moisture 0.21. For volumetric soil moisture 0.10and 0.15 the dynamic range for copolarization is approxi-mately 3.5 dB and 3 dB, respectively. Therefore, it maybe stated that the different soil texture fields (when themajor variation is made in sand percentage) is betterobserved at dry soil and at lower moisture values. In addi-tion, it may also be observed from Fig. 9 that for a fixedvalue of sand percentage in soil, copolarization ratio ismaximum for dry soil and decreases with the increase insoil moisture.

4. Conclusions

With the present work it has been demonstrated that thesoil texture is an important feature when scattering is inspecular direction, specifically at higher incidence angle.Study shows the potential of discriminating soil texturebased on copolarization ratio in specular direction. Specu-lar scattering coefficient show a clear distinction betweenthe different type of soil texture fields when fields whereconsidered as smooth. This distinction is also observedfor wide range of soil moisture values. But, as the surfaceroughness is considered and its standard deviation of sur-face height is increased, distinctions between scatteringcoefficient of different type of soil texture fields are difficultto observe. Still, differentiation between different soil tex-ture fields can be achieved on the basis of copolarizationratio for higher incidence angle (i.e., P50�). Differentiationbetween different soil texture fields based on copolarizationratio is valid for a wide range of surface roughness and soilmoisture conditions.

Table 5Results of regression analysis.

Incidence angle (�) R2 SE

Dry soil 30 0.5293 0.378140 0.5714 0.332450 0.8515 0.318760 0.9502 0.297470 0.891 0.3137

Moisture = 0.10 cm3 cm�3 30 0.2213 0.465140 0.4345 0.405450 0.8794 0.412760 0.9827 0.325370 0.9189 0.3994

Moisture = 0.15 cm3 cm�3 30 0.23 0.578140 0.3835 0.511550 0.839 0.41260 0.9734 0.38270 0.8963 0.4326

Moisture = 0.21 cm3 cm�3 30 0.395 0.49140 0.4496 0.438750 0.8569 0.386460 0.9657 0.350870 0.914 0.4112

Fig. 9. Variation of copolarization ratio with change in sand percentage.


It is an important feature of this study that soil textureconfirms its effect on scattering in specular direction for awide range of soil moisture. Further, independency ofcopolarization ratio on surface roughness value also addsto its feature to have a clear look on different soil texturefields in presence of roughness. Response of microwavespecular scattering for soil texture at X-band will providea better insight for the characterization of natural surfaces.

Acknowledgments

Authors are thankful to Ministry of Human Resourceand Development (MHRD) of India for providing fundsto support this work.

References

Apparao, K.V.S., Rao, V.C.S. Soil Testing: Laboratory Manual andQuestion Bank. Laxmi Publication, New Delhi, 1995.

Brown, R.B. Soil Texture. University of Florida, IFAS Extension, http://edis.ifas.ufl.edu/SS169, 2003.

Ceraldi, E., Franceschetti, G., Iodice, A., Riccio, D. Estimating the soildielectric constant via scattering measurements along the speculardirection. IEEE Trans. Geosci. Remote Sens. 43 (2), 295–305, 2005.

Comblet, F., Khenchaf, A., Baussard, A., Pellen, F. Bistatic syntheticaperture radar imaging: theory, simulations, and validations. IEEETrans. Geosci. Remote Sens. 54 (11), 3529–3540, 2006.

Curie, N.C., Well, G.W. Radar cross section measurement concepts, in:Curie, N.C. (Ed.), Radar Reflectivity Measurement: Technique andApplications. Artech House, Norwood, MA, pp. 61–91, 1989.

De Roo, R.D., Ulaby, F.T. Bistatic specular scattering from roughdielectric surfaces. IEEE Trans. Anten. Propag. 42 (2), 220–231, 1994.

Du, Y., Qi, Y., Chen, H., Kong, J.A. Bistatic scattering model for roughsurfaces. In: Proceedings of the Geoscience and Remote SensingSymposium, 2006, IGARSS’06, pp. 2949–2952, 2006.

Dubois, P.C., Zyl, J.V., Engam, T. Measuring soil moisture with imagingradars. IEEE Trans. Geosci. Remote Sens. 33 (4), 915–926, 1995.

Hallikainen, M.T., Ulaby, F.T., Dobson, M.C., El-Rayes, M.A., Wu, L.Microwave dielectric behavior of wet soil. Part I: empirical models andexperimental observations. IEEE Trans. Geosci. Remote Sens. 23 (1),25–34, 1985.

Holah, N., Baghdadi, N., Zribi, M., Bruand, A., King, C. Potential ofASAR/ENVISAT for the characterization of soil surface parametersover bare agricultural fields. Remote Sens. Environ. 96, 78–86, 2005.

Krieger, G., Fiedler, H., Hounam, D., Moreira, A. Analysis of systemconcepts for bi- and multi-static SAR missions. In: Proceedings of theGeoscience and Remote Sensing Symposium, 2003, IGARSS’03, pp.770–772, 2003.

Magagi, R.D., Kerr, Y.H. Estimating surface soil moisture and soilroughness over semiarid areas from the use of the copolarization ratio.Remote Sens. Environ. 75, 432–445, 2001.

Mattia, F., Toan, T.L., Souyris, J.C., Carolis, G.D., Floury, N., Posa, F.,Pasquariello, G. The effect of surface roughness on multifrequencypolarimetric SAR data. IEEE Trans. Geosci. Remote Sens. 35 (4),954–966, 1997.

Mironov, V.L., Dobson, M.C., Kaupp, V.H., Komarov, S.A., Kles-hchenko, V.N. Generalized refractive mixing dielectric model for moistsoils. IEEE Trans. Geosci. Remote Sens. 42 (4), 773–785, 2004.

Moccia, A., Vetrella, S., Bertoni, R. Mission analysis and design of abistatic synthetic aperture radar on board a small satellite. ActaAstronaut. 47 (11), 819–829, 2000.

Nashashibi, A.Y., Ulaby, F.T. Millimeter-wave polarimetric bistatic radarscattering from rough soil surfaces. In: Proceedings of the Geoscienceand Remote Sensing Symposium, 2003, IGARSS’03, pp. 788–790, 2003.

Nashashibi, A.Y., Ulaby, F.T. MMW polarimetric radar bistaticscattering from a random surface. IEEE Trans. Geosci. Remote Sens.45 (6), 1743–1755, 2007.

Oh, Y. Quantitative retrieval of soil moisture content and surfaceroughness from multipolarized radar observation of bare soil surface.IEEE Trans. Geosci. Remote Sens. 42 (3), 596–601, 2004.

Oh, Y., Sarabandi, K., Ulaby, F.T. An empirical model and an inversiontechnique for radar scattering from bare soil surfaces. IEEE Trans.Geosci. Remote Sens. 30 (2), 370–381, 1992.

Silver, W.L., Neff, J., McGroddy, M., Veldkamp, E., Keller, M., Cosme, R.Effects of soil texture on belowground carbon and nutrient storage in alowland Amazonian forest ecosystem. Ecosystems 3, 193–209, 2000.

Singh, D. A simplistic incidence angle approach to retrieve the soilmoisture and surface roughness at X-band. IEEE Trans. Geosci.Remote Sens. 43 (11), 2606–2611, 2005.

Singh, D., Dubey, V. Microwave bistatic polarization measurements forretrieval of soil moisture using an incidence angle approach. J.Geophys. Eng. 4, 75–82, 2007.

http://edis.ifas.ufl.edu/SS169

http://edis.ifas.ufl.edu/SS169


Singh, D., Singh, K.P., Herlin, I., Sharma, S.K. Ground basedscatterometer measurement of periodic surface roughness andcorrelation length for remote sensing. Adv. Space Res. 32 (11),2281–2286, 2003.

Srivastava, H.S., Patel, P., Navalgund, R.R. Incorporating soil texture insoil moisture estimation from extended low-1 beam mode RADAR-SAT-1 SAR data. Int. J. Remote Sens. 27 (12), 2587–2598, 2006.

Ulaby, F.T., Moore, R.K., Fung, A.K.. Microwave Remote Sensing –Active and Passive, vol. 2. Artech House, Norwood, MA, 1982.

Wang, J.R., Schmugge, T.J. An empirical model for the complex dielectricpermittivity of soil as a function of water content. IEEE Trans. Geosci.Remote Sens. 18 (4), 288–295, 1980.

Zhai, Y., Thomasson, J.A., Boggess, J.E., Sui, R. Soil texture classificationwith artificial neural networks operating on remote sensing data.Comput. Electron. Agric. 54, 53–68, 2006.

Zribi, M., Dechambre, M.A. A new empirical model to retrieve soilmoisture and roughness from C-band radar data. Remote Sens.Environ. 84, 42–52, 2002.

Zribi, M., Baghdadi, N., Holah, N., Fafin, O. New methodology forsoil surface moisture estimation and its application to ENVISAT-ASAR multi-incident data inversion. Remote Sens. Environ. 96,485–496, 2005.

Microwave specular scattering response of soil texture at X-band

Documents

Transcript of Microwave specular scattering response of soil texture at X-band