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Module 2 - How Radar Imaging Works

E) Scattering Mechanisms

Objectives

  1. Students will be able to draw and label a diagram showing an example ofwhat happens to a transmitted pulse when it hits four types of surface.
  2. Students will be able to predict what the brightness will be, (high,medium or low) for each type of surface.
  3. Students will be able to list several examples of each type of scatteringmechanism.

Radar is reflected in the same manner as visible light. We refer to thisreflection as scattering. The radar pulses sent out by an imaging radar arescattered upon contact with the earth's surface. The way in which the energycontained in the pulse is scattered is known as a scattering mechanism. Animaging radar of the type we are discussing can only measure the energyscattered back towards it, i.e. the backscatter.

In this section, four different types of common scattering mechanism areintroduced, including scattering from four types of surface:

  1. Smooth surface
  2. Rough surface
  3. Double-bounce
  4. Vegetation Layer

Smooth Surface

The first type of scatter we shall consider is from a smooth surface. As canbe seen in the diagram below, most of the scatter from a smooth surface is inthe forward direction, away from the radar. Only a very small fraction of theenergy in the transmitted pulse is reflected back towards the radar.

[SMOOTH SURFACE BOUNCE]

Thus, backscatter from a smooth surface will tend to be very low (say less than-20dB) and will appear dark in the radar image. The exceptions to this are whenthe radar is pointing straight down, so that the smooth surface is at rightangles to the radar look direction; or when the surface is tilted towards theradar. In these cases, almost all the energy in the pulse will be reflectedback towards the radar, and the backscatter will be very high (white in theradar image). The types of surface that would demonstrate smooth surfacescattering are smooth water, dry lake beds, some types of smooth rock and baresoil. The effect of scattering off a smooth surface is like a pool ball hittingthe side of a pool table. The ball is "REFLECTED" at the same angle it hit theside, but in the opposite direction.

To demonstrate the same effect using visible light, use a flashlight and aflat piece of aluminum foil. Have the students stand above the aluminum foiland aim the flashlight at the foil. They will notice that most of the light isreflected in one direction, away from them.

Rough Surface

The second type of scatter we shall consider is from a rough surface. As seenin the diagram below, the scatter from a rough surface is in all directions.Some fraction of the energy in the transmitted pulse is reflected back towardsthe radar. In general, the rougher the surface is, the higher the backscatter.This can be understood if we consider that the rougher a surface is, the morelikely it is that some segments (or facets) of it will be inclined at rightangles to the radar look direction. Backscatter from a rough surface will tendto be greater than -20dB and will appear gray to white in the radar image. Thetypes of surface that would demonstrate rough surface scattering are waterblown by the wind or with waves, lava and other types of rock and bare soilwith clumps.

Surfaces may be rough at different scale lengths. Comparing the scale lengthwith the radar wavelength we can get an idea whether the surface will appearrough to the radar and thus whether it will give high or low backscatter. Forexample, if we look at a plowed field, we may find that the plowed furrows areabout 6cm high. This is one measure of the scale of the roughness for thatfield. With a roughness scale of 6cm, the field will appear rough (i.e. brightin the radar image) at C-band (6cm wavelength) and X-band (3cm wavelength), butnot at L-band (24cm wavelength).

To demonstrate the effect of scattering from a rough surface using visiblelight, use a flashlight and a crinkled piece of aluminum foil.

Double-bounce

The third type of scatter is from two surfaces, one flat on the ground(horizontal), the other upright (vertical). The reflected pulse hits bothsurfaces one after the other. This type of scattering is known asdouble-bounce. From the diagram below, most of the scatter for a double-bouncemechanism is back towards the radar, i.e. in the backscatter direction.Double-bounce backscatter will tend to be fairly high (HH, VV greater than-10dB) and will appear light gray to white in the radar image. Double-bouncescattering occurs commonly in urban areas, where there are plenty of verticalsurfaces (the sides of buildings) and horizontal surfaces (sidewalks, streets).It can also occur frequently in nature, whenever there are upright vegetationstems (stalks, trunks) and a relatively smooth (and flat) surface underneath.Examples are flooded forests, forests where the vegetation lying on the groundhas been removed, rice fields, corn fields, marshes and swamps.

To demonstrate the effect of double-bounce scattering using visible light, usea flashlight and two flat pieces of aluminum foil, one on a table top, theother held vertically next to it.

Vegetation Layer

The fourth type of scatter is from a layer of randomly oriented scatterers,which is common in many types of vegetation. This type of scattering is morecomplicated than the other three. The radar pulse penetrates the vegetationlayer, then is scattered after hitting one of the randomly oriented branches orleaves in the canopy. From the diagram below, the scatter for a vegetationlayer is in all directions, with some in the backscatter direction. Backscatterfrom a vegetation layer will vary. Some vegetation layers will be fairly low inbackscatter (HH, VV less than -10dB) and will appear dark gray in the radarimage. Other vegetation layers will have fairly high backscatter (HH, VV a fewdB above -10dB) and will appear light gray in the radar image. Most naturallyoccurring vegetation, such as forests, shrubs, and grassland have scatteringpatterns which are characteristic of vegetation layers, but vary depending onthe wavelength. Forests tend to have fairly high backscatter at all radarwavelengths, because forests are made up of objects of all different sizes(from leaves to trunks). Shrubs are smaller than forests and tend to havefairly high backscatter at short and medium wavelengths (i.e. less than say30cm). Short grass tends to have low backscatter at all wavelengths. Long grassmay have high backscatter at shorter wavelengths (say, less than 10cm) but lowbackscatter at longer wavelengths. Scattering from a vegetation layer anddouble-bounce backscatter can often be seen together, for example in a radarimage of a forest. We will see later how these two phenomena can bedistinguished.

Radar Backscatter Characteristics Of Scattering Mechanisms

The following is a summary of approximate characteristics ofbackscatter for the four different surface types:

Smooth Surface

Overall backscatter very low (HH, VV < -20dB), except when lookingstraight down

Rough Surface

Overall backscatter low to high (HH, VV > -20dB)
VV > HH
HV < -20dB
Phase Difference within 30 degrees of zero
Correlation coefficient > 0.7

Double-bounce

Overall backscatter high (HH, VV > -10dB)

Phase Difference within 80o of +180o

Vegetation Layer

HH and VV within 1dB of each other

HV > HH - 7dB or HV > VV - 7dB

Correlation coefficient between 0 and 0.5

These radar backscatter characteristics can be compared with the mean values inthe "STATISTICS" results generated by Sigma0. The above "RULES-OF-THUMB"should separate most different types of scatterers, but will be wrong insome cases (there is always an exception to every rule).

Hint for the Classroom:

Use this sheet to compare against Statistics obtained usingSigma0.

Mapping Vegetation Type

Given the characteristics of the scattering mechanisms described above, it ispossible to estimate the dominant (strongest) type of scatter for each pixel ina radar image data set where all the polarizations are available. We might forexample, classify a pixel as being one of three types:

  1. Surface Scatter
  2. Double-bounce
  3. Vegetation Layer
Suppose that we have three wavelengths of radar data available to us. We canclassify data from each wavelength separately into one of the above three typesof backscatter. We may get a different answer for each wavelength. It is thenpossible to use these different answers to classify the image into differentvegetation or ground cover types.

The NASA/JPL AIRSAR system is an imaging radar which produces radar image dataat three wavelengths: C-band (6cm wavelength); L-band (24cm wavelength); andP-band (68cm wavelength). At each wavelength, images for all polarizations aregenerated. Three frequency AIRSAR data can be found on the SIR-CEDCD-ROM in the Pre-SIRC folders as files of type Filename.stk.This data can be classified as to the dominant scattering mechanism asdescribed above.

Also on the SIR-CED CD-ROM can be found vegetation maps, generated fromNASA/JPL AIRSAR data, using an algorithm developed by A. Freeman of JPL. Thevegetation maps are stored on the CD-ROM as image files namedVegmpbyt.gif in Pre-SIRC folders. Each pixel in a Vegmpbyt.gif file is classified as corresponding to an urban area (e.g. a city ortown); a forest; medium vegetation, such as shrubs or a mature corn field; lowvegetation, such as grassland, scrub or low-lying agricultural crops (e.g.wheat or barley); no vegetation (rocks, bare soil and water surfaces). Withinthe three vegetated classes (forest, low and medium vegetation), there is afurther subdivision into, for example, forest which has a significant amount ofdouble-bounce [Forest (D)] and forest which doesn't [Forest]. Theclassification is based on a model of the scattering mechanisms expected fromthese different ground cover types at all three AIRSAR frequencies and wasfirst made to work on AIRSAR data from a few typical scenes. Some pixels areunclassified and are designated as bright, medium or dark. These pixels do notmatch the models.

Classification Rules for Vegetation Maps

The vegetation maps found in the Vegmpbyt.gif files were generatedfollowing rules broadly along the lines of those described below:

  1. No Vegetation - surface scatter dominant at all three frequencies
  2. Low Vegetation - vegetation layer scatter dominant at C-band, but not at L-and P-band.
  3. Low Vegetation (D) - classified as low vegetation, but with asignificant amount of double-bounce (implied by large HH-VV phase difference)at C-band.
  4. Medium Vegetation - vegetation layer scatter dominant at L-band, but notat P-band.
  5. Medium Vegetation (D) - classified as medium vegetation, but with asignificant amount of double-bounce (implied by large HH-VV phase difference)at L-band.
  6. Forest - vegetation layer scatter dominant at L-band and P-band.
  7. Forest (D) - classified as forest, but with a significant amount ofdouble-bounce (implied by large HH-VV phase difference) at P-band.
  8. Urban - Double-bounce scatter dominant at all three wavelengths.
Some pixels do not match any of the scattering mechanism models or the rulesdescribed above. These are unclassified pixels. To differentiate betweenthese unclassified pixels, three levels are given:

  1. Bright - Unclassified pixels which have high backscatter at L-band andP-band.
  2. Medium- Unclassified pixels which have high backscatter at L-band butnot at P-band.
  3. Dark- Unclassified pixels which have do not have high backscatter atL-band and P-band.
WARNING - the vegetation maps in the Vegmpbyt.gif files are ourbest estimate of the ground cover class based on current models andunderstanding of radar backscatter. They may be wrong in some cases.

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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