The polarimetric Synthetic Aperture Radar (SAR) is a powerful sensor for high resolution ocean and land mapping and particularly for monitoring hydrological parameters in large watersheds. There is currently much research in progress to assess the SAR operational capability as well as to estimate the accuracy achievable in the measurements of geophysical parameters with the presently available airborne and spaceborne sensors. An important goal of this research is to improve our understanding of the basic mechanisms that control the interaction of electro-magnetic waves with soil and vegetation. This can be done both by developing electromagnetic models and by analyzing statistical relations between backscattering and ground truth data. A systematic investigation, which aims at a better understanding of the information obtainable from the multi-frequency polarimetric SAR to be used in agro-hydrology, is in progress by our groups within the framework of SIR-C/X-SAR Project and has achieved a most significant milestone with the NASA/JPL Aircraft Campaign named MAC-91. Indeed this experiment allowed us to collect a large and meaningful data set including multi-temporal multi-frequency polarimetric SAR measurements and ground truth. This paper presents some significant results obtained over an agricultural flat area within the Montespertoli site, where intensive ground measurements were carried out. The results are critically discussed with special regard to the information associated with polarimetric data

Baronti, S.

Luciani, S.

Moretti, S.

Paloscia, S.

Schiavon, G.

Sigismondi, S.

English

NASA Technical Reports Server

MAC-91: POLARIMETRIC SAR RESULTS ON MONTESPERTOLI SITE N95- 23940
S. Baronti (*), S . Luciani (**), S. Moretti ('**),
S. Paloscia (*), G. Schiavou (**) and S. Sigismondi (***)
(*) lstituto di Ricerca sulle Onde Elettromagnetiche - CNR
Via Panclatichi 64, 50127 Firenze (I)
(**) Dipartimento di lngegneria Elettronica, Universiti Tor Vergata
Via della Ricerca Scientifica, 00133 Roma (I)
(***) Dipartimento di Scienze della Terra, Universita di Firenze
Via La Pira 4, 50121 Firenze (I)
1. INTRODUCTION
The polarimetric Synthetic Aperture Radar (SAR) is a powerful sensor for high
resolution ocean and land mapping and particularly for monitoring hydrological
parameters in large watersheds. There is currently much research in progress to
assess the SAR operational capability as well as to estimate the accuracy achievable
in the measurements of geophysical parameters with the presently available
airborne and spaceborne sensors. An important goal of this research is to improve
our understanding of the basic mechanisms that control the interaction of electro-
magnetic waves with soil and vegetation. This can be done both by developing
electromagnetic models and by analyzing statistical relations between backscatte-
ring and ground truth data. A systematic investigation, which aims at a better
understanding of the information obtainable from the multi-frequency polari-
metric SAR to be used in agro-hydrology, is in progress by our groups within the
framework of SIR-C/X-SAR Project and has achieved a most significant
milestone with the NASA/JPL Aircraft Campaign named MAC-91. Indeed this
experiment allowed us to collect a large and meaningful data set including multi-
temporal multi-frequency polarimetric SAR measurements and ground truth.
This paper presents some significant results obtained over an agricultural flat area
within the Montespertoli site, where intensive ground measurements were carried
out. The results are critically discussed with special regard to the information
associated with polarimetric data.
2. EXPERIMENT DESCRIPTION
During the Multisensor Airborne Campaign (MAC-91), the site of Montespertoli
(Italy) was imaged on three different dates (22 June, 29 June and 14 July) by the
airborne JPL polarimetric SAR (AIRSAR) operating at P-, L- and C- band
(Canuti et al., 1992). Three passes were performed during each flight data, in
order to cover the most significant sub-areas with different incidence angles 0 be-
tween 20* and 50*. Nominal pixel sizes obtained on the 16 looks images were 6.6
m in range and 12 m in azimuth. The test site covers the basins of two small rivers
and contains a relatively large flat area, along the border of the Pesa river, which
includes agricultural fields of various crops (i.e. alfalfa, colza, corn, sorghum, sun-
flower, wheat) bare soils, olive-yard, vineyards and a few small forests. The area
was equipped with two trihedral corner reflectors of 180 cm and one of 240 cm,
supplied by the JPL, and deployed along the Pesa river. In addition, a few
"extended homogeneous targets", namely three plots of bare soil with different
surface roughness, one field of alfalfa and one of sunflower, had been specifically
prepared, in the same sub area, to act as "distributed" calibrators. The ground data,
which were collected on selected fields at the same time as the remote sensing
measurements, regarded all the significant vegetation and soil parameters, such as
leaf area index (LAI), plant water content (PWC), dimensions of leaves and stalks,
soil moisture content and roughness. During the experiment the average
gravimetric soil moisture of the first 5 cm layer changed from about 15-20 % (on
https://ntrs.nasa.gov/search.jsp?R=19950017520 2020-06-16T08:38:19+00:00Z
June 22) to about 10% (on July 14). At the same time the leaf area index (m2/m 2)
of sorghum and corn increased from ~ 0.5 to ~ 3.5, that of sunflower from ~ 0.5
to ~ 3.5. Alfalfa leaf area index ranged from ~ 0 to ~ 4. Wheat and colza were in
the ripening stage and therefore their LAI was almost 0; some of these fields were
harvested at the time of the last flight.
3. DATA ANALYSIS
Calibrated (amplitude and phase) polarimetric data, collected over the agricultural
area on alfalfa, corn, colza, sunflower, sorghum, bare soil, olives, vineyard and on
a few forested areas, at incidence angles of 35* and 50", have been analyzed.
Background - Scattering sources may be grouped in broad categories, each
associated to typical scattering behaviors, as shown in recent works (e.g. Ulaby et
al. 1990, Van Zyl 1989). In particular: a) Soil response is a surface scattering
effect. This effect is important for bare soils and, in general, at low frequencies,
where many agricultural crops are rather transparent. O*RL is appreciably higher
than o*RR and o*HV is low. At lower frequencies (P and L band) O*vv> O*HH,
as predicted by the Small Perturbation surface scattering model (Ulaby et al.
1982), while at higher frequencies (C band) O*vv _ o*nH. b) Vertical structures,
like forest trunks and crop stalks, produce double-bounce scattering. In general,
this mechanism is important at P band for forests and at L band for some
agricultural crops like corn and sunflower, o*rl v is low, as in the soil scattering
case, but, differently from the soil, O*nH is generally higher than O*vv and the
large difference between a*RL and O*rtR disappears, c) Ensembles of inclined
cylindrical structures, like forest branches and crop stems, produce volume
scattering with an appreciable presence of multiple scattering. The differences
O*HH - o*HV and O*vv - O*rlv are much lower than those for soil and vertical
structures. In circular polarization, O*RL = O*RR. d) Ensembles of inclined planar
structures, like leaves, also produce volume scattering; however, an appreciable
amount of single "facet" scattering is present. O*nH - O*nV and O*vv - o*HV
differences are relatively low, similarly to the cylinder case, but at circular
polarization, the "facet" effect generates appreciable positive O*RL - o*RR
differences. The scatterer dimensions have important effects too. Let branches and
stems be represented as ensembles of cylinders with the length proportional to the
radius. For a canopy of equal cylinders, the backscatter coefficient changes with
the cylinder length (expressed in wavelengths) and shows a maximum (Karam et
al. 1992). It follows that, for each band, there is a range of cylinder dimensions
generating a dominant contribution to the backscatter. For leaves, which can be
described as discs, scattering is dependent on thickness and moisture content
(Ferrazzoli et al. 1993).
4. SURFACE TYPE DISCRIMINATION
Some significant backscattering features of agricultural surfaces can be inferred
from Fig. 1 which represents the cross-polarized backscattering coefficient (o°HV)
of well developed (Plant Water Content - PWC > 1 Kg/m 2) crops measured at L-
band and 0 = 35*, as a function of the corresponding o°rlv at C-band. At L-
band there is a continuous increase of backscatter as the dimensions of scattering
elements increase from the 'small leaf' crops such as alfalfa (A) and sorghum (S)
to the bigger olive-yards (O) and forest trees (T), passing through 'broad leaf"
crops such as sunflower (F) and coiza (R). At C-band, bare soil can be discrimi-
nated from crops among which colza, which has a well ramified structure, has the
highest backscattering value.The relatively strong capability of C-band backsc-
attering to discriminate agricultural surfaces is improved if circular polarization
data are used as well (Baronti et ai. 1993). Indeed, bare soils and plants character-
ized by relatively large leaves, like sunflower, corn, sorghum, show a "facet" sca-
ttering with O*RL > o*aR. while, in the other fields, O°RL = O°RR . However
colza crops show rather high backscatter
at both RR and RL polarizations, due to
their scattering elements (stems, pods)
which are numerous and relatively large,
while the elements of wheat and alfalfa
crops produce lower backscattering. It
must be noted that, unlike L-band,
discrimination is feasible also in the case
of moderate growth. Figure 2 shows
o*HV versus PertH measured at P-band
and 0 = 35*. According to the previous
considerations the scattering in HV pol.
is mainly due to inclined and relatively
large cylindrical elements, like branches
of forest, oliveyard and vineyard, while
the agricultural vegetation is rather
transparent. Three classes of **By val-
ues may be identified corresponding to:
forests (T), olive-yard and vine-yard (O
and V) and agricultural fields (A). Ac-
cordingly P-band backscattering, which
is
durt_ 22, _99!
-10
voBoo .
,
C Band (HV)
Figure 1 - Backscattering Coefficient
(o*) at L-band vs.** at C-band (HV
pol. 0=35*) Labels: B=bare soil, T=-
forest, V=vine-yard, *=olive-yard,
W=wheat, A= alfalfa, S=sorghum,
F=sunflower, R=colza, I=uncropped
-5
-lff
-15,
I
0
-20-
t2_
-25-
-5O
-30 -5
V
V V V
vY Vo
vv v _,.Voo
el: ,,
-_s -_0 :'_s -_o
SigmaO HH (dB)
Figure 2 - O*Hv vs. O°HH at P-band,
0=35*
dominated by the tree branches and the
soil trunk interaction, appears rather
effective in discriminating broad classes
of surfaces (forest, olive-yard, vine-yard,
agricultural fields) but their utility in
discriminating the various agricultural
crops is poor. At L-band, where scat-
tering is mainly due to stems and large
leaves, the SAR data appear more suitable
for separating the 'broad leaf' crops (corn
and colza) from other crops; this discrim-
ination is improved using C-band data
which are particularly sensitive to small
stems and leaves (Coppo et al. 1989). Note
that a discrimination between alfalfa and
wheat fields does not appear feasible at
C-band; however, recent works indicate
that it becomes possible by means of both
active and passive remote sensing systems
if higher frequencies (X- and K- bands) are used (Ferrazzoli et al. 1992).
SENSITIVITY TO SOIL MOISTURE AND VEGETATION BIOMASS
Soil Moisture - The measurement of soil moisture (SMC) with radar has been the
subject of much research over the past years. However, a reliable extraction of
such information from SAR data is still questionable in that the signal is strongly
influenced by surface roughness and vegetation cover. In our case data collected
at L and C-band show very little correlation with SMC even at 0 = 35*. A slightly
better result has been obtained at P-band where the o* an has shown a sensitivity
to SMC equal to 1.0 dB/% SMC and a correlation coefficient equal to 0.7.
Herbaceous crops - In this experiment the agricultural vegetation was rather
transparent at P-band while, at C-band, it generated considerable scattering
effects even in the case of moderate growth. Indeed, the highest correlations with
plant growth have been noted at L-band. On the other hand a*xv, which is low
for bare soils and is mainly associated to
the presence of inclined stems and large -16
leaves, is a relatively good growth indi-
cator. Figure 3 shows L-band o°HV as
a function of the Plant Water Content _'-2o
(PWC, kg/m2). Samples of sorghum (S),
corn (C), sunflower (F) and bare soil x>
-94-have been included. Very rough (ploug- o
o
hed) bare soils are labeled 'PB', while E
moderately rough (tilled or rolled) bare '-_
co -28
soils are labeled 'TB'. The correlation is
generally good; the lowest O'av values
are associated with moderately rough -32
bare soils and with sorghum fields at a -
very early stage of growth, while the
highest values are associated with well
developed sunflower crops.Ploughed
bare soils show relatively high val-
ues,indicating that the roughness plays a
PB C
F
SS
f cs
S
F F
F
pwc
Figure 3- O'lav vs. PWC (Kg/m 2) at
L-band, 0=50"
role similar to that of vegetation growth. The dynamic range is notable, in the
order of 10 dB. The plant growth may be also related to a*HH - O*VV at L-band.
In fact, for bare soils, OOHH< O*VV while, for developed crops, the soil plant
double bounce scattering, which may be rather important at L-band, produces a
positive OeHH - O*VV difference.
CONCLUSIONS
Polarimetric SAR measurements carried out over forests, oliveyards, vineyards and
agricultural fields indicate that P-band data are effective in discriminating among
broad surface categories, L-band data allow identification of well developed sun-
flower, corn and colza crops, and C-band data discriminate among different kinds
of herbaceous crops even in the case of moderate growth. A fairly good sensitivity
of oeHV and O*nH - O*VV to the Plant Water Content of some agricultural species
is observed at L-band.
ACKNOWLEDGMENT
This work was partially supported by the Italian Space Agency
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MAX-91: Polarimetric SAR results on Montespertoli site

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