GEOG Fall 2003 Overview of Microwave Remote Sensing (Chapter 9 in Jensen) from Prof. Kasischke’s lecture October 6,2003

Visible-thermal-microwave

Radar is an acronym for RAdio Detection And Ranging

‘ Speckle’

‘Speckle’

Figure 1-18 from Elachi, C., Introduction to the Physics and Techniques of Remote Sensing, 413 pp., John Wiley & Sons, New York, Microwave energy is largely unaffected by the atmosphere, e.g., it has 100% transmission

RADAR – Radio Detection and Ranging 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 Energy 1.All EM energy travels at the speed of light (c = 3 x 10 8 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,, & f c = * f f = c / f = c / = c / f = c / f

Common Radar Bands BandFrequencyWavelength (most common) X 8 to 12 GHz2.5 to 4.0 cm (3.0 cm) C4 to 8 GHz4 to 8 cm (6.0) L1 to 2 GHz15 to 30 cm (24.0) P0.3 to 1 GHz30 to 100 cm (65 cm)

Key Components of a Radar System 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 with 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 Shadowing

Figure 9.4 from Jensen

Radar Foreshortening and Layover

Radar 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 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 surface scattering Surface roughnessSurface roughness Surface dielectric constantSurface dielectric constant

Surface Reflectance or Scattering Specular reflection or scatteringSpecular reflection or scattering Diffuse reflection or scatteringDiffuse reflection or scattering

Specular Reflection or Scattering Occurs from very smooth surfaces, where the height of features on the surface << wavelength of the incoming EM radiation

Diffuse Reflectors or Scatterers Most surfaces are not smooth, and reflect incoming EM radiation in a variety of directions These are called diffuse reflectors or scatterers

Figures from mgddf/chap5/f5-4f.gif Radar backscattering is dependent on the relative height or roughness of the surface

Microwave scattering as a function of surface roughness is wavelength dependent

Figure from mgddf/chap5/f5-4f.gif Microwave scattering is dependent on incidence angle

Radar sensing of forests L-band (23 cm) is best

Microwave Scattering from 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 = 24 cm = 3 cm = 24 cm

Smooth area – no wind = 3 cm = 24 cm

Factors controlling surface scattering Surface roughnessSurface roughness Surface dielectric constantSurface dielectric constant

Dielectric 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 Common 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 cross section Stealth fighter = <0.02

Radar cross section <dialectric constant v. smooth surfaces radar absorbing materials

Camouflage true color & radar B-1 F-111 F-117 AV-8

Minimize visual, thermal, and radar emissions