The Shuttle Imaging Radar (SIR-B), the second synthetic aperture radar (SAR) to fly aboard a shuttle, was launched on October 5, 1984. One of the primary goals of the SIR-B experiment was to use multiple incidence angle radar images to distinguish different terrain types through the use of their characteristic backscatter curves. This goal was accomplished in several locations including the Chubut Province of southern Argentina. Four descending image acquisitions were collected providing a multiple incidence angle image set. The data were first used to assess stereo-radargrammetric techniques. A digital elevation model was produced using the optimum pair of multiple incidence angle images. This model was then used to determine the local incidence angle of each picture element to generate curves of relative brightness vs. incidence angle. Secondary image products were also generated using the multi-angle data. The results of this work indicate that: (1) various forest species and various structures of a single species may be discriminated using multiple incidence angle radar imagery, and (2) it is essential to consider the variation in backscatter due to a variable incidence angle when analyzing and comparing data collected at varying frequencies and polarizations

Brandani, Aldo

Casey, Daren

Cimino, Jobea

Domik, Gitta

Leberl, Franz

Wall, Stephen

English

NASA Technical Reports Server

N 87 - 17157
MULTIPLE INCIDENCE ANGLE SIR-B EXPERIMENT OVER ARGENTINA
JoBea Cimino, Daren Casey, and Stephen Wall
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, California
Aldo Brandani
Consejo Nacional de Investigaciones Cientificas y T_cnicas
and Centro de Geologia de Costas
Mar del Plata, Argentina
Gitta Domik and Franz Leberl
VEXCEL Corporation
Boulder, Colorado
I. INTRODUCTION
The Shuttle Imaging Radar (SIR-B), the second synthetic aperture
radar (SAR) to fly aboard a shuttle, was launched on October 5, 1984. One of
the primary goals of the SlR-B experiment was to use multiple incidence angle
radar images to distinguish different terrain types through the use of their
characteristic backscatter curves. This goal was accomplished in several
locations including the Chubut Province of southern Argentina. Four descending
image acquisitions were collected providing a multiple incidence angle image
set. The data were first used to assess stereo-radargrammetric techniques
(Leberl, et al., 1986a). A digital elevation model was produced using the
optimum pair of multiple incidence angle images. This model was then used to
determine the local incidence angle of each picture element to generate curves
of relative brightness vs. incidence angle (Cimino, et al., 1986). Secondary
image products were also generated using the multi-angle data (Domik, et al.,
1986).
II. OBSERVATIONS
A. Acquired Data
The images examined in this study were acquired on consecutive days
of the SIR-B mission with incidence angles from vertical of 33.0 ° , 44.7 °,
53.7 ° and 59.4 ° (Figure I and Table i). The center latitude and longitude of
the site are 43°20'S and 71 ° 27FW, respectively, near a peak named CordOn la
Grasa. The images were collected at about 6 p.m. local time between October 8
and ii, 1984. The width of the area of coincident coverage ranges from about
12 to 15 km.
B. Description of the Study Area
The majority of the study area imaged by SIR-B is within a sharp
boundary controlled by a strong east-west rainfall gradient which divides the
Andes ecosystems from the Patagonian desert. The ecotone includes plant
species from the Andean forests and from the Patagonian steppe (Figure ib).
The study area encompasses the Valle Rio Frio and the terrain just east and
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west of it. Corddn la Grasa, one of the series _ of ranges forming the Andes
Pre-Cordillera, dominates the western portion of the image. A pure Nothofagus
pumilio, or lenga, forest is found at higher elevations and along water
courses. Below the lenga to the Valle Rio Frio, there is a complex unit formed
by both pure Nothofagus antarctica, or _ire, forest between i000 m and 1050 m
elevation, and _ire mixed with maiten (Maytenus boaria), laura (Schinus pata-
gonicus) and several species of Berberis below i000 m. Locally, the ratio of
Eire, maiten, laura and berberis varies depending on slope, exposure, and
available moisture. The ecosystems are described in detail by Cimino, et al.,
1986.
III. ANALYSIS OF THE IMAGERY
a. Radargrammetric Analysis
The multiple incidence angle data set permits one to study the
capabilities and limitations of radargrammetric mapping in a manner previously
not possible due to a lack of available data. Figure 2(a) is a sketch of the
imaging configurations. The largest intersection angle for stereoscopic
analysis was 23 degrees (Table 2). This stereo model may present a different
quality of stereo-viewing than one using data with smaller intersection
angles. Generally one can expect the best visual stereo-matching when the
intersection angles are at a minimum. Unfortunately, however, this would lead
to poor vertical exaggeration of the stereo-model.
The radargrammetric measurements and computations were performed on
a computer-controlled photogram_etric analytical stereo-plotter. The analysis
procedure consists of several steps:
Ground control points are identified on existing topographic maps
and in each of the 6 pairs of overlapping radar images;
On the computer-controlled stereo instrument, 18 different stereo
models were set-up and measurements of coordinates were taken;
A digital contour map was created from the most accurate of the
radar stereo models;
A reference digital elevation matrix was obtained using the maps of
the study area; and
The radar- and map-derived digital elevation data were compared.
Figure 2(b) illustrates graphically the results of the comparison
in terms of the change in accuracy as a function of stereo-intersection
angle. The results at a 5 ° intersection are nearly as predicted; at 23 ° the
experimental data are poorer than those expected. Figure 2(b) is based on the
Cord6n la Grasa data set and on results from another SIR-B multiple angle data
set of Mt. Shasta (Leberl et al., 1986b).
It is evident that height values are less accurate than theoretical
prediction based on range resolution. This difference becomes more distinct
as stereo-intersection angles increase. Limitations in accuracy at large
intersection angles may be due to
effects of "specular point migration" and migration of edges due to
differences in illumination;
166
changes in backscatter due to differences in incidence angles;
variations_in radar azimuth angle and resulting edge migration;
effects of noise.
B. Generation of Color Composite Image
To assess visually the value of a multiincidence angle image set
over that of a single image for vegetation classification, a color composite
was generated using three of the four SIR-B images. Registered, geometrically
corrected digital images (Domik, et al., 1986) from three of the four
acquisitions of the CordOn la Grasa area were used to produce the color
composite by assigning each image a color. The 33.0 ° incidence angle image
was assigned to a blue color, the 44.7 ° image to a green and the 59.4 ° image
to red. The composite (see Cimino, et al., 1986) clearly delineates the
forest units which may be discriminated when the multiple incidence angle data
sets are combined.
Co Generation of Relative Brightness Curves
Although SIR-B was designed as a calibrated system, images in the
resulting data set resist quantitative interpretation because of an unexpected
breakdown in the insulation of the cabling connecting the transmitter/receiver
unit to the antenna during the flight. The effect of the breakdown was to
modulate the effective antenna gain; the frequency spectrum of this modulation
is unknown. However, short term variations in azimuth are strongly damped by
the correlation process, which effectively averages returned echoes over a
2-second interval. Such averaging insures that comparisons of data separated
by 15 km or less are legitimate.
Day-to-day variations in transmitted power are compensated by the
use of ratios--that is, all multiple-angle data are presented in terms of
brightness relative to a reference forest unit.
For selected forest units, several areas uniform in image
brightness were chosen for analysis (Figure 3a). The digital values or data
numbers (DN) of the pixels in each box were averaged to remove speckle. The
averages were corrected for all known receiver and processing gain variations
and plotted as a function of local incidence angle using topographic
information generated by Leberl, et al., 1986a. The curves for each vegeta-
tion type were then ratioed to the low _ire forest curve (unit 8). Error bars
are the root-sum-square of the speckle noise and the overall relative
calibration error (SIR-B Science Plan). In general, the overall relative
calibration error dominates.
IV. RESULTS AND DISCUSSION
The resulting curves of relative brightness vs incidence angle
(Figure 3b) describe in a more quantitative fashion the results displayed in
the color composite. The lenga forest has a significantly stronger response at
the smaller incidence angles than the surrounding _ire forest. At 59 ° , the
curves of the lenga and the _ire forests on the west side of CordOn la Grasa
167
converge. The lenga in the Andes also has a muchstronger response than the
nire but a slightly weaker response than the lenga on CordOnla Grasa.
The _ire forests on the east and west sides of Cord6n la Grasa have
very similar responses at the three smallest incidence angles. The regenerated
_ire (unit 3) has a slightly lower response than the other Eire forests. At
the largest incidence angle, however, the _ire on the west side of Cord6n la
Grasa have a muchstronger response than the low _ire forests (unit 8) on the
east side. The wind damaged_ire (unit 7) has a response similar to the low
_ire. The grazed mallines have a much lower response at all three larger inci-
dence angles than the forests, however at the smallest incidence angle, their
responses are similar to the regenerated _ire. The bare rocks and snow at the
peak of CordOnla Grasa have the weakest response at all incidence angles.
At the smallest incidence angle the lenga forest has the strongest
backscattered return. The relative openness of the canopy suggests that a
significant amount of radiation is reaching the surface. The extreme
brightness may be due to a high density of trees and branches littering the
ground below the canopy. For the horizontally polarized radar these
horizontal branches are likely to be very good scatterers. At higher
incidence angles the incident radar beam must penetrate increasingly greater
amounts of canopy before breaking through to the understory, until at the
highest incidence angle little radiation reaches the surface and the lenga
backscatter is reduced to that of the _ire. The branching structures of the
lenga and _ire tree canopies are very similar, and in situations where most of
the radiation is either scattered or absorbed within the upper levels of the
canopies, such as at 59°, it is reasonable to assume these two forests look
the sameto the radar.
The_ire forests have a wide variety of structures depending on the
elevation, exposure to the sun and wind and the stage of regeneration if
burning has occurred. The shrub-like _ire (unit 8) is a weaker scatterer than
the tree-like _ire at the largest incidence angle. A possible explanation for
this relatively low backscatter is that the canopy is so dense that it scatters
like a smooth surface. At other incidence angles the two types of _ire have
essentially the samebrightness.
The results of this work indicate that (i) various forest species
and various structures of a single species maybe discriminated using multiple
incidence angle radar imagery and (2) it is essential to consider the
variation in backscatter due to a variable incidence angle when analyzing and
comparing data collected at varying frequencies and polarizations.
ACKNOWLEDGEMENTS
The work described in this paper was carried out under contract
with the National Aeronautics and Space Administration at the Jet Propulsion
Laboratory, California Institute of Technology.
168
REFERENCES
Cimino, J., A. Brandani, D. Casey, J. Rabassa, and S. Wall, 1986, "III.
Multiple Incidence Angle SIR-B Data over Argentina: Mapping of
Vegetation Units," IEEE, July issue.
Domik, G., F. Leberl and J. B. Cimino, 1986, II. Multiple Incidence Angle
SIR-B Experiment Over Argentina: Generation of Secondary Image
Products, IEEE, July issue.
Leberl, F., G. Domik, J. Raggams, J. Cimino, M. Kobrick, 1986a, I. Multi-
ple Incidence Angle SlR-B Experiment Over Argentina:
Stereo-Radargrammetric Analysis, IEEE, July issue.
Leberl, F., J. Raggam, G. Domik, M. Kobrick, 1986b, "Radar Stereo Mapping
Techniques and Application to SIR-B Images at Mt. Shasta, IEEE,
July issue.
SIR-B Science Plan, 1984, JPL Publication 82-78, Pasadena, CA.
Table I. Multiple incidence angle SlR-B data set
Data
take
Center Azimuth
incidence Date On Duration look
angle (1984) GMT (min) direction
56.4
72.4
88.4
104.4
33.0 ° Oct. 8 282/22:31:0 4 N42E
44.7 ° Oct. 9 283/22:14:0 2 N40E
53.7 ° Oct. I0 284/21:57:0 2 N40E
59.4 ° Oct. Ii 285/21:39:0 4 N39E
Table 2. Intersection angles
Case Data takes Look angles Intersection angles
I
II
III
IV
V
VI
104188 56o151 ° 5 °
88/72 51°143 ° 8 °
72156 43°/33 ° i0 °
104/72 56°/43 ° 13 °
88/56 51°/33 ° 18 °
104/56 56o/33 ° 23 °
169
_ pOOR
ILLUMINATION
DATA TAKE: 104.4, SCENE 006
CENTER INCIDENCE ANGLE: 59.4 °
DATA TAKE: 88.4, SCENE 008
CENTER INCIDENCE ANGLE: 53.7 °
DATA TAKE: 72.4, SCENE 001
CENTER INCIDENCE ANGLE: 44.7 °
Figure
DATA TAKE: 56.4, SCENES 010, 009 _N t I
CENTER INCIDENCE ANGLE" 33.0 ° _k 10 km
la. The four multiple incidenc_ angle images of the CordOn
region.
la Grasa
170
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171
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• MOUNTAINOUS AREAS
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Z_
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LOOK ANGLE DIFFERENCE (degrees)
(a) Look angle geometry of _ stereo models formed by the 4 images
of Figure la; (b) Height accuracy versus stereo intersection
angles, both as predicted and obtained experimentally.
172
OEIGI;,;AL PAGE IS
OF POOR QUALITY
2.0
1.8
1.6
1.4
z 1.2
o 1.0
<=
0.8
0.6
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Figure 3.
33.0
44.7 °
53.7 °
[ I I I
d_ REGENERATEDNIRE_
I_ARE SNOW AND ROCK
I I I I
20 30 40 50 60 70
INCIDENCE ANGLE (degrees)
O LENGA, UNIT 2 (BOX I, 2, 3,4, 5, 6)
• ANDES LENGA (BOX 10, 11)
[D LOW FIRE, UNIT 8, REFERENCE (BOX ?, 8)
A TALL NIRE, UNIT 4 (18, 19)
• REGENERATEDFIRE, UNIT 3 (BOX 20, 21, 22, 23)
A WIND DAMAGED FIRE, UNIT 7 (BOX 12, 13)
O PURE NIRE, UNIT 5(BOX 14, 15, 16, 17)
O GRAZED MALLIN, UNIT 10 (BOX 24)
• BARE SNOW AND ROCK, UNIT 1 (BOX 9)
(a) SIR-B image showing
relative brightness curves;
of local incidence angle.
location of areas used to
(b) Relative brightness as a
59.4 °
generate
function
173.


Multiple incidence angle SIR-B experiment over Argentina

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