GEOG 372 - Fall 2003GEOG 372 - Fall 2003Overview of MicrowaveOverview of Microwave

Remote Sensing Remote Sensing (Chapter 9 in Jensen) (Chapter 9 in Jensen)

from Prof. Kasischke’s from Prof. Kasischke’s lecture October 6,2003lecture October 6,2003

Visible-thermal-Visible-thermal-microwavemicrowave

Radar is an acronym for RAdio Detection And

Ranging

 ‘ ‘SpeckleSpeckle’’

‘‘SpeckleSpeckle’’

Figure 1-18 from Elachi,

C., Introduction

to the Physics and Techniques of Remote

Sensing, 413 pp., John

Wiley & Sons, New York,

1987.

Microwave energy is largely unaffected by the atmosphere, e.g., it has 100%

transmission

RADAR – Radio Detection and RADAR – Radio Detection and RangingRanging

• Concept behind radars discovered in 1923• RADARs systems invented in the 1930s

– A high powered, radio transmitter/receiver system was developed that would transmit a signal that was reflected from a distant object, and then detected by the receiver

– Thus, RADAR’s initial function was to detect and determine the range to a target

Fundamental Characteristics of

Electromagnetic Energy1. All EM energy travels at the speed

of light (c = 3 x 108 m sec -1) in a vacuum in a wave form.

2. All EM energy has a wavelength - - the distance between crests of successive waves.

3. All EM energy has a frequency – f - the number of wave crests that pass a stationary point per unit time.

Relationships between c, Relationships between c, , & , & ff

c = c = * * f f

ff = c / = c /

= c / = c / ff

Common Radar BandsCommon Radar BandsBandBand FrequencyFrequency Wavelength Wavelength

(most common)(most common)

X 8 to 12 GHz 2.5 to 4.0 cm (3.0 cm)

C 4 to 8 GHz 4 to 8 cm (6.0)

L 1 to 2 GHz 15 to 30 cm (24.0)

P 0.3 to 1 GHz 30 to 100 cm (65 cm)

 

Key Components of a Radar Key Components of a Radar SystemSystem

• Microwave Transmitter – electronic device used to generate the microwave EM energy transmitted by the radar.

• Microwave Receiver – electronic device used to detect the microwave pulse that is reflected by the area being imaged by the radar.

• Antenna – electronic component used through which microwave pulses are transmitted and received.

Measurements made Measurements made with a simple radarwith a simple radar

• Range to the targetRange to the target• Intensity of the returned pulseIntensity of the returned pulse• Azimuth resolutionAzimuth resolution• Range resolutionRange resolution

The Radar Equation

Radar ShadowingRadar Shadowing

Figure 9.4 from Jensen

Radar Foreshortening and Radar Foreshortening and LayoverLayover

Radar ForeshorteningRadar Foreshortening

Radar foreshortening refers to the compression of the range dimension of an elevated object towards

the direction the radar is looking

Radar look direction

Radar Backscatter - Radar Backscatter -

• Radar backscatter is the amount of energy received from the area of interest by a radar relative to the energy received from a metal target with a specified area

= energy from study area energy from calibrated target

Factors controlling Factors controlling surface scatteringsurface scattering

• Surface roughnessSurface roughness• Surface dielectric constantSurface dielectric constant

Surface Reflectance or Surface Reflectance or ScatteringScattering

•Specular reflection or Specular reflection or scatteringscattering

•Diffuse reflection or Diffuse reflection or scatteringscattering

Specular Reflection or Specular Reflection or ScatteringScattering

• Occurs from very smooth surfaces, where the height of features on the surface << wavelength of the incoming EM radiation

Diffuse Reflectors or Diffuse Reflectors or ScatterersScatterers

• Most surfaces are not smooth, and reflect incoming EM radiation in a variety of directions

• These are called diffuse reflectors or scatterers

Figures from http://pds.jpl.nasa.gov/ mgddf/chap5/f5-4f.gif

Radar Radar backscattering is backscattering is dependent on the dependent on the relative height or relative height or roughness of the roughness of the

surfacesurface

Microwave Microwave scattering scattering

as a as a function of function of

surface surface roughness roughness

is is wavelength wavelength dependentdependent

Figure from http://pds.jpl.nasa.gov/

mgddf/chap5/f5-4f.gif

Microwave Microwave scattering is scattering is dependent on dependent on

incidence angleincidence angle

Radar Radar sensing sensing

of forestsof forestsL-band L-band (23 cm) (23 cm) is bestis best

Microwave Scattering Microwave Scattering from a Water Surfacefrom a Water Surface

Water has a dielectric constant of 80

• All scattering from water bodies in the Microwave region of the EM Spectrum is from surface scattering as no EM energy penetrates the water surface

= 3 cm = 3 cm = = 24 cm24 cm

Smooth area – no wind

= 3 cm = 24 cm

Factors controlling Factors controlling surface scatteringsurface scattering

• Surface roughnessSurface roughness• Surface dielectric constantSurface dielectric constant

Dielectric ConstantDielectric Constant

• The dielectric constant is a measure of the electrical conductivity of a material

• Degree of scattering by an object or surface is proportional to the dielectric constant of the material – ~ dielectric constant

• To some degree, dielectric constants are dependent on microwave wavelength and polarization

Dielectric Constants of Dielectric Constants of Common MaterialsCommon Materials

• Soil – 3 to 6• Vegetation – 1 to 3• Water – 80

–For most terrestrial materials, the moisture content determines the strength of scattering of microwave energy

Radar Radar cross cross sectiosectio

nnStealth fighter = Stealth fighter = <0.02<0.02

Radar cross sectionRadar cross section<dialectric constant<dialectric constantv. smooth surfacesv. smooth surfaces

radar absorbing materialsradar absorbing materials

CamouflageCamouflagetrue color & true color &

radarradarB-1

F-111

F-117AV-8

Minimize visual,

thermal, and radar

emissions