Module 2 - How Radar Imaging Works

F) Interferometry

Objectives

1. Students will learn that the technique of radar interferometry can produce accurate maps of surface heights, similar to a topographic map.
2. Students will be able to explain the significance of measuring surface heights for at least one application.

The Lesson Plan for this component does not go into detail about how interferometry works, but stresses the significance of the topographic maps generated using this technique. You will find an explanation of how interferometry works below.

In Radar Interferometry, radar measurements from two radars are combined to give a measurement of the height of the surface.

The two measurements can be obtained by using two radars, observing the ground at exactly the same time, but from slightly different vantage points. The separation between the two radars is often quite small, from a few meters to at most a kilometer. An alternative is to use only one radar, and obtain the two measurements on different orbit tracks which are closely spaced but a day or longer apart. This is the technique used for SIR-C: it is called Repeat-pass Interferometry. It relies on the Earth's surface not changing between passes, so it doesn't work all the time.

Each radar measures the distance to a point on the ground very precisely. The two radars, and any object on the ground, form a triangle. If we know the distance from each radar to the object on the ground (r1 and r2), the distance between the two radars (the baseline B) and the angle of that baseline, (with respect to the horizontal), we can use trigonometry (cosine rule) to calculate the height, h, of one of the radars above the position of the object on the ground.

Here's how it works. From the above diagram, we know that:

and, by the cosine rule,

which is the same as,

We solve for , then for h (using ). This calculation has to be repeated for every point on the ground within the image, i.e. for every pixel in the image. Fix the position of one of the radars above sea level, and we can produce a map of the surface heights.

As you might imagine, generating a height map from two radar measurements in practice is not straightforward. First we have to form an image from each radar, then we have to make sure the two images are registered. Then it turns out that we can only precisely measure the relative distance from the phase difference between each pair of radar measurements. This can be related to the height h after some more complicated algebra and some processing on the computer. But the above explanation gives the basic idea.

Radar Interferometry is a technique which combines information from two radar images to produce a map of surface heights for the area being investigated. A map of surface heights can be made into a map with contours, showing how high mountains are, just like the type you might use when hiking, or into an image, showing the mountains, hills and valleys. Information on surface heights for any particular area can be very useful to scientists who might want to study how water flows in that area (usually downhill), for example.

Run the Quicktime Movie Player and do a File | Open. Go to the directory /ROADMAP/MOVIES/QTMOVIES on the CD. Select the file named GalapmSP.mov. You should see a window that looks something like this:

Click on the arrow at the bottom left-hand corner to play the movie. You will see a radar-generated fly-by of the Galapagos Islands, which are a chain of volcanic islands in the Pacific off the coast of Ecuador. In the movie, interferometry was used to produce the height information. The gray-scale information is a radar image of the islands laid on top of the surface information. The bright 'splash' on the side of the volcano which is closest to you at the beginning is a patch of lava from a recent eruption.

Radar Interferometry can produce accurate height maps and very accurate measurements of height changes. Scientists use height maps in modeling the Earth. Height maps can be used in planning a new town, or a dam, or a canal, or a new road. The Army uses height maps as information to help get soldiers from Point A to Point B on the ground. Very accurate measurements of height changes can be used to monitor earthquake damage, volcanoes, landslides and glaciers, all of which cause the Earth's surface to shift.

Teacher's Guide - Table of Contents

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

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