Adv.SpaceRes.Vol. 12, No.7,pp. (7)73—(7)76,1992 0273—1177/92$15.00Printedin GreatBritain. All rightsreserved. Copyright© 1992 COSPAR

MONITORING OF SPINACH BY MICROWAVEREMOTE SENSING AT X-BAND

S. K. SharmaandK. P. Singh

DepartmentofElectronicsEngineering,Instituteof Technology,BanarasHindu University, Varanasi221005, India

ABSTRACT

Outdoor X-band scatterometer data acquired over a prepared test—site near the Department ofElectronics Engineering, Banaras Hindu University, Varanasi, were analyzed to determine thespinach field identification accuracies obtainable using bistatic multidate scatterometer

data. The collected data were used to compute normalized radar-cross section (eO), andbrightness temperature (TB) at frequency 9.79 GHz. The dependence of these parameters on

look angle, polarization and moisture content are computed. Observations were taken onseveral dates corresponding to growth stages of spinach, which seems to affect greatly theabove mentioned parameters. However, the nature of variation is consistent. Therefore, thelevel of accuracy in determining a crop-type is greatly enhanced.

INTRODUCTION

The radar remote sensing technique to agriculture land use can be understood from theknowledge of the dependence of the back as well as forward scattering coefficient (00) of avegetated scene. The back scatter from a vegetated target is influenced by its compositedielectric properties, geometry and background. The spaceborne and airborne radar, bothimaging and nonimaging, are in the process of developments /1/, undergoing feasibilitystudies of the remote sensing techniques. For the relationship of the sensor parameters isthe function of the signal scatter /2,3/ (frequency, polarization, angle of incidence) andthe target parameters (roughness and dielectric properties) it is therefore important tocollect the data and model them over a wide range of signal parameters which is practicallyfeasible with present day sensor.

In the present paper bistatic scattering measurements of scattering coefficient (a°) andbrightness temperature (TB) have been discussed for the several growth stages of the spinachfield identification for like (VV and HH) polarizations. However, due to in-houselimitations cross polarizations (VH and HV) could not be studied which could have lead tofurther revealations of crop signatures.

BISTATIC SCATTEROMETERSYSTEM

Figure 1 shows the schematic representation of the experimental set-up used in presentoutdoor measurements. Two pyramidal horn antennas, one connected to the transmitter sideand the other on the receiver side having half power beam width as 23.5° in H-plane and 24°in E-plane respectively with a gain of 20.74 dB and 20.27 dB, operating at 9.79 GHz, weremounted on wooden portable stand. The height and incidence angle of the antennas mounted onthe platform on this stand can be varied as well as read, so as to assure far fieldmeasurements to be consistent with remote sensing.

The polarization of a radiated signal is changed by using 900 E—H twist. For each degreevariation of look angle a particular height of the antenna, the separation from the centreof target was also calculated, for positioning of the transmitting and receiving antennas bychanging its look angle. This technique helps to calculate the near field and far fieldeffect. The angles of incidence of microwave signal are systematically changed and thecomponents of the signal in the direction of the specular reflection from the target underinvestigation are located for final positioning of the receiving antenna.

The overall system was calibrated by noting the signal returned from a flat aluminium plateplaced between the transmitting and receiving antennas. The details of calibration are not

(7)73

(7)74 S. K. ShannaandK. P.Singh

presented here -for the sake of brevity, however standard techniques are discussed in book byUlaby et al. /4/.

THEORETICAL DESCRIPTION

The reflectivity of the target has been computed from the following practical considera-tions. Let be the transmitted power, Gt and are transmitting and receiving antenna

gains, x - the wavelength, R1 and R2 the distances of transmitting and receiving antennas

from the centre of the illuminated area respectively. Then for a perfectly conducting flataluminium sheet as the reflecting surface, the power ‘P’ received at the receiving antennais given by /4/;

Al~r = [PtGtGr x2]/ [(4~)2 (R

1 + R2)2]. (1)

When the reflecting target is taken into consideration, the medium characteristics in theform of Fresnel’s reflection coefficient p

0 is to be taken into account. Therefore, for the

target as a reflecting surface the received power is written as;

= ~t Gt Gr x2 1p12] / [(4~)2 (R

1 + R2)2]. (2)

From equations (1) and (2) the reflectivity (y) of the target under observation is given by/5/;

= = ~r’Al~r

The emissivity of the medium is defined under thermal equilibrium to be equal to theabsorptivity of the medium. When the target is lossy and extended into half space, there isno transmission through the target and therefore, by /5/;

Emissivity (e) = 1—Reflectivity (y) (4)

Brightness temperature (TB) is the black body equivalent radiometric temperature of the

surface and is calculated from the emissivity and the surface temperature of the target.Therefore, the brightness temperature is written as /5/;

TB = eT5,

where, Ts is the surface temperature for irregular target whose standard deviation of the

surface roughness is appreciable compared with the incident wavelength. The average powermeasurement provided a good estimate of the scattering coefficient for the incident anglesoff vertical. In the Fraunhofer zone computation when range ‘R’ can be taken large enoughso that ‘R’ could be considered constant over A0 (the surface), the radar equation reducesto /4/;

= ~t ~2 ~ Gt G0 ~0 dS] / [(4~)3 R4], (6)

where, R1 = R2 = R.

For the average measurement of the scattering coefficient (~0) of the target, the scattero-meter system is calibrated for the targets of known radar cross section. For perfectly flatand smooth aluminium plate which is taken as a standard target, the power received is;

P — P G G 2 / (4~)2 (2R)2 (7)

Stdr - t tO rOX

where, Gto and are the maximum gain of the transmitting and receiving antennas

respectively. From equations (6) and (7) we get;

/ Std~r = I/sR (8)

where, I = 5A0 a~Gtn

6rn dS

and = Gt/Gto, Gm = Gr/Gro~

If o~ is taken constant over 3—dB band width of the antenna beam then

= sR2 ~r / Std~r 11A0 Gtn Gm dS. (9)

From equations (8) and (9) we rewrite;

= irR2 I~

0I2/I~ (10)

where, I0 is the illuminated area of the target.

MonitoringofSpinachby Microwave RemoteSensing (7)75

From Figure 2, the value of 10 is evaluated, which after substitution in equation (10)yields ~0 in dB as;

~o (dB) = 10 log10 [2 I ~oI cot $az/2 cosec el/2] /

[sec (° el/2) + sec (O+4)el/2fl (11)

RRUNSJ4ITIERSiDE RECEIVER SIRE

TUNER TRERU

WET

sciis,~ncOTAGRAM OF ~STA5CSCRTTRROMETERSYSTEM

Fig.l. Schematic diagram of bistatic Fig.2. Foot print geometry of the

scatterometer system. madam antenna illumination.

RESULTS AND DISCUSSIONS

Using the bistatic scatterometer system, as discussed above and shown in Figure 1, thescattering coefficient ~0 from spinach are obtained for e = 200 to e = 700. The look anglevariation mange is limited due to system which is always in human control whereas the targetvariables are nature governed. ~0 measurements are helpful in designing spaceborne activesensors and decide accurately system parameters to minimize the system cost and maximize theutility of the system. It leads, therefore, to a basic requirement of finding out bestcompromise with an aim to gain maximum. With this objective, knowledge pool is essentialand hence our effort is one of the few steps in achieving the same. Active microwavesensors are basically meant for ~0 measurements from various targets only. Figure 3 showsa

0variation with angle of incidence e~ for both HH— and VV-polarizations. Effort has been madeto keep spinach morphology same by cutting and allowing it to grow to a height around 11 ±1cms, and then conduct scattering measurements. It is noted that young crop at the age of 46days show 6-7 dB lower scattering than older crop of 76 days and 80 days. Worth noting hereis the moisture content of the background soil. 46 days old crop has background soilmoisture almost 50% to that of the 76 days and 80 days one. It indicates, therefore, thatmoisture content of the soil is more dominating than any other factor. The confidence inthis statement can, however be questioned because of the undefined composite dielectricconstant behaviour of the crop due to density—geometry and morphology. One thing can beconcluded for definite that near nadir a is always higher than those of higher e valuesfOr VV-polarization. This is an indicator of using near—nadir measurements withVV—polarization from spaceborne sensors. However, in case HH—polarization is to be used,the converse is true: higher the a , more is a°. Even the lowest moisture content backgroundis showing nearly 5 dB higher a~ between a = 500 to 600. An overall picture emerges,therefore, that HH-polarization will give better confidence in monitoring the spinach fromactive airborne sensors.

For the sake of space conservation emissivity and reflectivity results are not beingpresented here. However, brightness temperature TB variation with look angle a variationis an important aspect of our effort which would help in spaceborne passive microwave sensor(i.e. radiometers) design. Additional aspect is that the emissivity and reflectivityinformations are contained in some form or the other in TB plots. Figure 4 shows TB versus ~plot again for both HH— and VV—polarizations. For VV—polarization higher brightness isobserved with lower moisture of background which is true for all ages of crops between a = 250

(7)76 S. K. Shanna and K. P. Singh

and 600 . Anomally is seen for TB less than a = 250 and greater than a = 600. So spacebornepassive sensor’s look angle between 250 and 600 would yield unambiguous results. Again lowermoisture containing background gives higher brightness temperature at lower a—values forHH-polarization . Therefore, if the choice be HH-polarization from spaceborne sensors thenpreferably near nadir measurements should be preferred for increasing the level ofconfidence as higher TB is seen around near nadir. However, with the advancement ofelectronics technology, any desired sensitivity of sensors can be achieved. So withsensitive systems available even HH—polarization gives significant identification marks athigher look angles, as can be seen from the shifting dips in curves from 600 to 650 to 700respectively corresponding to the age of the crop, i.e. 46 days, 74 days and 82 days.

+10 - SPINACH

FREQ- 979888

DOMING-MAR 14,88 320SPINACH

— +5 — SOW ARTA,08 — — —~~

o~ ~\‘~~4 200 HH-POLARIZATION VV-POLARIZA~~K

HH-POLARIZATION VV-POLARIZATION ~ APR 27,88 --~-- APR 30,88

—10 -IAPR 27,88 --~ APR 30,88 ~ —~— MAY 28,88 ~-4--MAY 38,88

—f MAY 28, 08 —4—- MAY 3068 -f- JUNG 86 4- JUN 4 98

—f—JUTI 6,88 --~-- JUN 408 160 3504~R ‘ ~, ‘

e-ANOLE OF INCIDENCE IDEGREEI—15 i I I I I

2O~ 30’ 40’ 50’ 60’ 70’0-ANGLE OF INCIDENCE (DEGREE)

Fig. 3. Angular response of ~0 (scattering Fig. 4. Angular response of TB (brightnesscoefficient) for spinach at different stages temperature) for spinach at different stagesof growth for HH— and VV-polarizations at of growth for HH—and VV—polarizations at9.79 GHz. 9.79 GHz.

CONCLUSIONS

1. Active microwave sensors can monitor spinach growth precisely if supplemented by passivemicrowave sensors. This is because of the fact that passive sensors can give prettyaccurate results as far as the brightness temperature is concerned. In turn, brightnesstemperature relates data with moisture content and surface temperature. With theseinformations readily available at hand, the job of monitoring spinach through activesensors becomes too easy.

2. Passive microwave sensors should be used with HH-polarization near nadir for brightnesstemperature measurements; whereas VV-polarization is better for background moistureevaluation and for the age of the crop between a= 25°to 600.

3. No single type of remote sensor can independently predict target characteristics withconfidence. Degree of identification confusion can be greatly reduced when two or moreremote sensors are used simultaneously.

REFERENCES -

1. R.E. Matthews, Active Microwave Workshop Report, NASA SP-376, Scientific and TechnicalInformation Office, National Aeronautics and Space Administration Washington (D.C.),(1975).

2. F.T.Ulaby, Radar Response to Vegetation, IEEETrans. Ant. Prop.AP-23, 36-45 (1975).

3. T.F. Bush and F.T. Ulaby, Radar Return from a Continuous Vegetation Canopy, IEEE Trans.

Ant. Prop. AP—24, 269-276 (1976).4. F.T. Ulaby, R.K. Moore and A.K. Fung, Microwave Remote Sensing (Active and Passive)’

Addison-Wesley Pub. Co.,1982.

5. K.P. Singh and K.K. Jha, Effect of soil moisture and surface roughness on microwavescattering signatures, in: Multiple Scattering of Waves in Random Media and Random RoughSurfaces,ed. V.V. Varadan and V.K. Varadan, The Penn. State University, College Park1985, p. 901.