Module 1 - Mission to Planet Earth

D) Zoom-In on Los Angeles

Objectives

Students will identify at least three differences in the amount of detail visible from different elevations.
Students will identify at least two structures which show decidedly different reflectivity.
Students will be introduced to Radar Imaging.
Students will locate specific portions of an area using X,Y coordinates and will pick out gray scale values in radar images showing highly vegetated (mountainous) and paved (urban) areas.

Image data for Module 1 contained in the /MODS1TO5/MODULE01/IMAGES directory should be copied over to your computer before beginning this module. The files you will need are in this directory are LASEASAT.gif, LAZoomIn.gif and LASIRC.gif

SEASAT

Remote sensing saw a big leap forward with the arrival of SEASAT in 1978. The SEASAT payload included an imaging radar which transmitted electromagnetic waves at 24 cm wavelength (known as L-band) with a horizontal-horizontal (HH for short) polarization. The images produced by these impulses had a resolution of 25 meters and a pixel size of 12.5 meters. SEASAT operated for roughly 100 days beginning in July of 1978. Its mission was to understand the physical processes taking place on the ocean surface through analysis of its radar images. These radar images gave scientists an invaluable tool for looking at the Earth, over both ocean and land. Since the radar instrument uses light generated by the instrument itself, radar imaging can produce images at night. Also, radar has the capacity to transmit through even the heaviest cloud cover. This is because clouds are effectively transparent to radar waves. Optical images of a planet's surface, such as those in the color photographs taken by the Galileo spacecraft, can not be taken when an area is cloud-covered or dark. Because radar makes it possible for images to be taken at night, the number and completeness of images can be greatly augmented. Also, areas such as rain forests, which can have cloud cover up to 95% of the year, can now be charted.

Images

Taken by SEASAT as it passed in 1978, the next image students will display is one of the Los Angeles basin.

This image of Los Angeles and its surrounding cities [LASEASAT.gif] was taken from the west (left on the image). The Pacific Ocean is visible on the left of the radar image and the San Gabriel Mountains are clearly shown in the upper right corner. An in-depth discussion of why particular aspects of the landscape appear as dark or light is included in the next section. Briefly, however, the more impulses returning to the radar receiver, the lighter the image. Since the radar impulses are sent at an angle toward the earth's surface, flat surfaces such as streets, parking lots, and calm water reflect the energy away from the receiver, like so:

All of these structures generally appear as dark areas in radar images. The angled roofs and uneven surfaces of vegetation and land surfaces tend to reflect a greater amount of energy back at the radar antenna and thus appear as lighter areas.

At X,Y coordinates 350, 750 is the city of Burbank, which appears as a very light square. Burbank is unique in this image in that its streets are oriented differently from the remainder of the cities in Los Angeles. The streets are lined up in such a way that the incoming radar pulses are able to bounce off the streets and then bounce again off the buildings (called a double-bounce) and directly back towards the radar instrument, as shown in the graphic below:

Just below Burbank, you can see the Hollywood Hills, home of the "HOLLYWOOD" sign. These hills continue to the west, becoming the Santa Monica Mountains. At the foot of the San Gabriel Mountains, at X, Y coordinates 500, 720 lies the Jet Propulsion Laboratory. The Palos Verdes peninsula juts out at about 120, 360. In the center of the image, you will locate Dodger stadium. The many dark lines criss-crossing the image are the renowned L.A. freeway system. The runways of the Los Angeles International Airport are also shown as two dark strips bordering the Pacific at about 190,400 on the image. Once again, there are no clouds present because clouds do not reflect radar impulses and are thus invisible to radar sensors.

The next image of Module 1 is of Dodger Stadium [LAZoomIn.gif] and the surrounding L.A. community. In this image, students can view downtown Los Angeles in the center of the image, ringed by freeways. Dodger stadium sits at the top center of the image, surrounded by Elysium Park. Both the freeways and the individual city blocks can be seen with greater resolution in this image. The stadium itself appears darker than the surrounding areas because the flat surfaces of the playing field and parking lots reflect the radar impulses away from the radar receiver.

The final image in Module 1 is a SIR-C image of Los Angeles, collected in October 1994. This is again a black-and white image of Los Angeles, taken using the same type of radar (same wavelength and polarization) as the one flown on SEASAT. The main difference between the two images (LASEASAT.gif and LASIRC.gif) is of course that they were taken 16 years apart, but they were also taken at different angles. SEASAT flew in a near-polar orbit, observing Los Angeles at it flew from South to North. SIR-C flew in a different orbit, observing Los Angeles at it flew from almost South-East to almost North-West. When you look at things from different directions they tend to appear different. Have one of your students face the class straight-on, then in profile to demonstrate this concept. See what differences your students can identify between the two radar images.

Teacher's Guide - Table of Contents

Converted to the IBM-PC by Al Wong, sirced03@southport.jpl.nasa.gov

Jet Propulsion Laboratory
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Pasadena, CA 91109