Lab 18: Introduction to Synthetic Aperture Radar Data (SAR)
In this lab you will work with two Synthetic Aperture Radar (SAR) images acquired over
Bonanza Creek Experimental Forest – Long Term Ecological Research (LTER) site near
Fairbanks, Alaska. There is also a scanned aerial photo mosaic of the area to help you
with the interpretation. Note that interpretation of radar imagery is difficult and
sometimes ambiguous, so don’t worry if you have some trouble figuring these images
out. Ideally one would spend some time in Alaska getting a feel for the landscape there!
Thanks to Amy Parker-Williams (former Botany grad student) for developing this
exercise.
Turn in your lab with answers to the questions that are interspersed throughout.
You will use three files that are in the class data directory: bc13.img, bc19.img, and
bonanza.img. These files contain the following bands (note that bands are in different
order in the two radar images!):
Image
bc13.img
Band
Date
1. L-band HH 13 March 1988
2. L-band HV
3. L-band VV
4. P-band HH
5. P-band HV
6. P-band VV
Conditions
Unseasonably warm
bc19.img
1. C-band HH 19 March 1988
2. C-band HV
3. C-band VV
4. L-band HH
5. L-band HV
6. L-band VV
Well below freezing,
snow, ice on river
bonanza.img
Scanned air photo
1. Open bonanza.img in an Erdas Viewer window as a “True-Color” image by assigning
bands 1 to red, 2 to green and 3 to blue (should open this way by default, but you can
confirm). Inspect this air photo and familiarize yourself with the characteristics of the
area (use the zoom tool, etc. to look more closely).
The forests along the flood plain are primarily young successional stands of white spruce
(evergreen) and balsam poplar (deciduous). The upland forest at the top of the image is
primarily black spruce (evergreen). Areas along the river are dominated by mud banks
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and sand bars devoid of vegetation. Notice all of the clear cuts that are visible in the
image as lighter, geometrically shaped areas.
2. Leave the air photo (bonanza.img) open for reference. Open a second Viewer and
open the bc13 image (bc13.img) as a color composite by assigning bands 1, 2 and 3 to
red, green, and blue, respectively (this is a composite of the three L-band polarizations).
This SAR image is only the center portion of the air photo you observed earlier. Orient
yourself to the river and the vegetation types you found in the air photo. You can rotate
the air photo to match the orientation of the radar image from your Home Tab by going
to the Scale and Angle box and clicking repeatedly on the rotation arrows. (Worksheet
question #1).
3. Now open a third Viewer window and open bc19.img as a “True Color” composite
but assign band 4, 5 and 6 to red, green and blue respectively. This is a color composite
of the L-band’s 3 polarizations just as in #2 above, but on a later date. Compare the two
images by putting the two windows side by side and sizing them so that they are about
the same size (you can undock images by clicking on the little push-pin icon in each
view). (Remember the weather change that occurred). (Worksheet question #2)
4. You will now examine which wavelengths (C-band, L-band or P-band in order of
increasing wavelength) and which polarizations (HH, HV, VV) have the greatest return
between dates and between vegetation types.
Open a separate spectral profile tool for each image and arrange the windows so that
you can see all four windows (2 images + 2 charts) (make the chart for each image
relatively small so that you can keep the images relatively large). You can now select
individual pixels for each image and compare them. You can also link the two images so
that you can pick the same pixel when necessary. Select points from the same areas on
each image so that you can compare the two charts. Be sure to choose examples in the
river, on the river floodplain, and in the black spruce upland forest. Do one point from
each image and then compare charts before adding the next point. Refer back to the
beginning of these instructions for the lists of what each band in each image captures.
(Worksheet question #3).
5. Explore the individual radar bands in each image and experiment with different color
composites. Can you find band combinations that are especially good at highlighting
various ground features in the image (e.g., spruce)? (Worksheet question #4)
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Name____________________________
Worksheet
Lab 18: Introduction to Radar
1. Answer the following regarding the bc13 image that you opened in lab
instruction 2.
a. Why is part of the river black and part magenta colored?
b. Why are some vegetated areas bright (whitish) and others magenta?
c. Are the clear cuts visible in the radar data? Why or why not?
2. These questions refer to the bc19 image and comparisons of it to the bc13
image.
a. Describe any significant differences that you notice between the two
images.
b. Why are details less clear in the March 19th image (bc19)? For example,
examine the clear cuts to the left of the main central meander in each
image.
c. Why is the color of portions of the river different in the two images?
3. Answer the following regarding the radar returns from the two dates for various
materials.
a. Which band (wavelength) has the highest return on each of the two
dates?
b. Which polarization has the highest return?
c. Why are the two dates different?
d. Why are different points within an image different?
e. What kinds of backscattering (or reflectance) may be occurring (e.g.,
diffuse, specular, corner)?
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4. Paste in an images from each date that highlight black spruce, and tell me what
band combination you used in each. Why do you think this combination worked
well (if you could find one)?
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