1 Lecture 16 – Active Microwave Remote Sensing 2 December 2008.

Lecture 16 Active Microwave Remote Sensing

2 December2008

Recommended ReadingsChapter 7 in Campbell

Figure 1-18 from Elachi, C., Introduction to the Physics and Techniques of Remote Sensing, 413 pp., John Wiley & Sons, New York, 1987.Active microwave systems operate at wavelengths (3 to 70 cm) that are not influenced by the atmosphere, e.g., these wavelengths have 100% transmission

Lecture 16 TopicsDefinition of RADARMeasurements made by a RADAR - Range to the target, Azimuth resolution, Range resolution, Intensity of the returned pulse,Microwave or Radar backscatterFactors controlling microwave backscatterSurface roughnessBragg scatteringVariations in dielectric constantSpaceborne Radar Systems and ApplicationsAltimetersScatterometersSynthetic Aperture Radar (SAR)

The basic concept of RADAR was discovered by scientists working at the Naval Research Laboratory who were investigating using microwave EM energy as a source for radio transmissions

RADAR Radio Detection and RangingConcept behind radars discovered in 1923RADARs systems invented in the 1930sA 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 receiverThus, RADARs initial function was to detect and determine the range to a target

Microwave Transmitter / ReceiverAntennaMicrowave EM energy pulse transmitted by the radarMicrowave EM energy pulse reflected from a target that will be detected by the radarTarget

Key Components of a Radar SystemMicrowave Transmitter electronic device used to generate the microwave EM energy transmitted by the radarMicrowave Receiver electronic device used to detect the microwave pulse that is reflected by the area being imaged by the radarAntenna electronic component used through which microwave pulses are transmitted and received

Common Radar Bands

Radar systems control the polarization of both the transmitted and received microwave EM energyFigure 9.6 from Jensen

Radar System DesignationRadar systems typically have a 3 letter designation to describe the frequency-polarization of operation:First letter denotes the radar frequency and wavelength (e.g., X,C, L,P see slide 10)The second letter denotes the polarization of transmitted EM waves (H for horizontal, V for vertical)The third letter denotes the polarization of the received EM waves (H for horizontal, V for vertical)

For example, an C-VH radar is one that transmits EM radiation at a C-band wavelength (between 4 and 8 cm), it transmits horizontally polarized EM energy and it receives vertically polarized EM energy

Lecture 16 TopicsDefinition of RADARMeasurements made by a RADAR - Range to the target, Azimuth resolution, Range resolution, Intensity of the returned pulseMicrowave or Radar backscatterFactors controlling microwave backscatterSurface roughnessBragg scatteringVariations in dielectric constantSpaceborne Radar Systems and ApplicationsAltimetersScatterometersSynthetic Aperture Radar (SAR)

Measurements made with a simple radarRange to the targetRange resolution Azimuth resolutionIntensity of the returned pulse

Microwave Transmitter / ReceiverAntennaMicrowave EM energy pulse transmitted by the radarMicrowave EM energy pulse reflected from a target that will be detected by the radarTarget

Microwave Transmitter / Receiver1. Transmitted pulse travels to the targetTarget2. The target reflects the pulse, and the reflected pulse travels back to the microwave antenna / receiver, where it is DETECTED3. The radar measures the time (t) between when the pulse was transmitted and when the reflected signal reaches the receiver The time it takes the pulse to travel from the radar to the target and back is used to estimate the RANGEAntenna

Radar range - RThe distance, R, from the antenna to the target is calculated as R = ct / 2

where c is the speed of light (3 x 10-8 m sec -1)t is the time between the transmission of the pulse and its reception by the radar antenna

Pulse Duration (p) p Radars send out pulses of EM energy, e.g., a burst of energy that lasts for a very short time period, the pulse duration

Pulse Duration (p) and Pulse LengthRadar systems transmit microwave pulses with of specific durations - pThe pulse length of the system defines the range resolution (r) of the radar

Antenna Beamwidth - Microwave Transmitter / ReceiverAntennaThe microwave energy transmitted by a radar is focused into a beam, with an angular dimension,

Antenna Beamwidth - If the length of the antenna is L, and the microwave wavelength is , then

= / L

Azimuth Resolution - a Microwave Transmitter / ReceiverAntennaThe direction parallel to the antenna length is called the azimuth dimensiona is the azimuth resolution of the radar, e.g., the distance 2 targets have to be separated in order to be distinguished by the radar

Azimuth Resolution - a Real Aperture RadarMicrowave Transmitter / ReceiverAntennaRaa = R

Synthetic aperture radar (SAR) is a specific type of imaging radar system

A SAR operates by continuous transmitting and receiving pulses reflected from a target the entire time the target is within the beamwidth of the system

The pulses transmitted and received by a SAR are linearly swept in frequency, e.g., the frequency of the pulse is lower at the beginning of the pulse than at the endA single target results in thousands of pulses that are detected and recorded by the SAR

These pulses are specially processed using fourier transforms to recreate a single point on the image representing the imaged target

By collecting data over a very long time, a SAR creates an synthetically long antenna or aperture (Ls) hence the term synthetic aperature radar

As a result, the azimuth resolution of a SAR is independent of range to the target:

a = L /2

where L is the actual length of the antenna

LsSlides 26 to 27 are for background only know material on this slideAzimuth resolution for a Synthetic Aperture Radar

The Radar Equation

Microwave (Radar) BackscatterWhen microwave EM energy transmitted by a RADAR system reaches the earth surface, some is absorbed by the surface and the remainder is reflected in multiple directionsIn microwave remote sensing, surface reflection is referred to as scattering of microwave EM energyThe microwave EM energy that is scattered in the Radars direction of transmission is the only EM energy that is detected by the radar this EM energy is referred to as microwave or radar backscatter

Radar cross section - Radar cross section is the area of a theoretical, perfect reflector of EM energy (e.g., a metal sphere) that would reflect the same amount of energy back to the radar as the actual target resulting in the microwave EM energyTo determine the radar cross section for a detected microwave signature, engineers build targets with known cross section and use these to calibrate radar image intensity valuesThe units for radar cross section is m2

Radar scattering coefficient - The radar scattering coefficient is used to describe the radar intensity per unit area of the image pixel

= / A

where A is the area of the pixel

The Decibel unit (dB)Radar scattering coefficient is typically described using decibels, where

(dB) = 10 log ()

Surface Reflectance or ScatteringSpecular reflection or scattering

Diffuse reflection or scattering

Specular Reflection or ScatteringOccurs from very smooth surfaces, where the height of features on the surface

Diffuse Reflectors or ScatterersMost surfaces are not smooth, and reflect incoming EM radiation in a variety of directionsThese are called diffuse reflectors or scatterers

Figures from http://pds.jpl.nasa.gov/ 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 http://pds.jpl.nasa.gov/ mgddf/chap5/f5-4f.gif Microwave scattering is dependent on incidence angle

As incidence angle increases, radar backscatter decreases for all surface roughnesses

Variation in MW backscatter from a rough surface (grass field) as a function of wavelength As the wavelength gets longer, the backscattering coefficient drops

Microwave Scattering from a Water Surface Bragg ScatteringWater has a dielectric constant of 80All 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 Small surface or capillary waves present on a water surface these waves are generated by wind

Smooth area no wind

L-band airborne SAR Image of ship and its wake from previous slideWhy do you have backscatter at L-band from an ocean surface?

Bragg Scattering from Water SurfacesWind creates small waves on the ocean surface (capillary waves) which in the absence of wind will continue to propagateIf wind continues, waves will grow in size and increase in wavelength and height to become ultra-gravity waves and eventually gravity wavesA water surface affected by wind will have a spectrum of surface waves, e.g., multiple wavelengths and heights

Bragg Scattering from Water SurfacesMicrowave EM energy has been shown through wave tank experiments to constructively interfere or resonate with surface capillary and ultra-gravity waves This phenomenon is known as Bragg Scattering

Bragg Scattering

Backscatter dependence on wind speedL-HH MeasurementsupwindIncidence angleWave tank studies show that variations in radar backscatter from water surfaces is proportional to wind speed

Dielectric ConstantThe dielectric constant is a measure of the electrical conductivity of a materialDegree of scattering by an object or surface is proportional to the dielectric constant of the material ~ dielectric constantTo some degree, dielectric constants are dependent on microwave wavelength and polarization

Dielectric Constants of Common MaterialsSoil 3 to 6Vegetation 1 to 3Water 80For most terrestrial materials, the moisture content determines the strength of scattering of microwave energy

At microwave wavelengths, the refraction coefficient (n) is determined by the dielectric constant of the material ()

n = square root ()

The reflection coefficients are determined from the refraction coefficient (see slides 30-36 in Lecture 15)

At an incidence angle of 0, the reflection coefficients are

rv = rh = [(n-1) / (n+1)]2

Dielectric constant as a function of soil moisture = 21.4 cmFigure E.47 from Ulaby, Moore, and Fung, Microwave Remote Sensing, Volume III.

Radar backscatter as a function of soil moistureFigure 21.26 from Ulaby, Moore, and Fung, Microwave Remote Sensing, Volume III. = 3.3 cm

Satellite AltimetersAltimeters are radars that measure the height of the surface of the earthTransmit a radar pulse which is reflected from the earths surfaceMeasure the time it takes for the pulse to travel to the earth and back (t)Height of the satellite (H) H = ct/2 where c is the speed of lightThe altitude of the satellite is carefully measured using GPS and ground-based laser systems

AltimetersAltimeters measure round-trip travel time of microwave radar pulse to determine distance to sea surfaceFrom this (and additional info) we can determine h the dynamic sea surface topography

Altimeter MissionsNASA GEOS-3, 1975-1978NASA Seasat, 1978NAVY Geosat, 1985-1989 (first 2 years classified)ESA ERS-1/2, 1991-1996 and 1995-NASA/CNES TOPEX/Poseidon, late 1992-NASA/CNES Jason-1, late 2000-

ERS ScatterometerResolution = 50 km

Obtains measurements looking upwind, cross-wind, and downwind

Empirical Algorithms used to estimate wind speed and direction

Backscatter dependence on wind speed: L-HH Measurementsupwind

ERS Scatterometer Accuracy

Spaceborne SAR SystemsERS-1/2 SARSLaunched in 1991 and 1995 by the European Space Agencies, still in operationC-VV system (6 cm wavelength)100 x 100 km image, 25 m pixel sizeRepeat frequency every 2 weeksENVISAT ASARsLaunched in 2002 as follow-on to ERS 1/2 SARsMultiple swath widths and pixel sizes30 to 1000 m pixels, 100 to 400 km swath widthsC band SAR with multiple polarizations VV, HH, HV, VH

Spaceborne SAR SystemsRadarsat 1/2 SARSCanadian Space AgencyLaunched in 1995, 2007C band (6 cm) SAR, HH polarization100 km swath, 25 m pixel standard mode, multiple other modes (variable pixel size, swath widths

Spaceborne SAR SystemsJERS SARLaunched by Japanese Space Agency in 1992Operated for 4 yearsL-band HH SAR30 m pixel, 75 km swath, 44 day repeat cycle

ALOS PALSARLaunched by Japanese Space Agency in 2006L-HH or L-VV SAR7 -100 m pixel size40 to 350 km swath widthRepeat frequency of 40 days

Readings on SAR applicationsKasischke, E.S., K.B. Smith, L.L. Bourgeau-Chavez, E.A. Romanowicz, S.M. Brunzell, and C.J. Richardson, Effects of Seasonal Hydrologic Patterns in South Florida Wetlands on Radar Backscatter Measured from ERS-2 SAR Imagery, Remote Sens. Env., 88, 423-441, 2003.Kasischke, E.S., L.L. Bourgeau-Chavez, and J.F. Johnstone, Assessing spatial and temporal variations in surface soil moisture in fire-disturbed black spruce forests using spaceborne synthetic aperture radar imagery - implications for post-fire tree recruitment, Rem. Sens. Environ., 108, 42-58, doi:10.1016/j.rse.2006.10.020, 2007.Harrell, P.A., E.S. Kasischke, L.L. Bourgeau-Chavez, E. Haney, and N.L. Christensen, Evaluation of approaches to estimating of aboveground biomass in southern pine forests using SIR-C imagery, Remote Sensing Env., 59, 223-233, 1997.

Sources of backscatter / attenuationc - volumetric scattering from the canopys direct scattering from the ground surfacem multiple bounce scattering between the ground and canopy c attenuation coefficient for the canopycBecause of their long wavelengths, EM energy from a SAR has multiple interactions with a land surface covered by vegetation

With short, herbaceous vegetation, EM has multiple scattering sources

Case study 1Monitoring burned forests in Alaska

See Kasischke et al. 2007

Case study 2

Monitoring of hydrologic conditions in southern Florida wetlands

See Kasischke et al. 2003

ERS SARImagery

Site 4 Marl PrairieSite 5 Cypress DomeSite 6 Pine FlatwoodSite 11 Wet MarshSite 12 Hatrack CypressExamples of Wetland Types in South Florida

When water covers the ground surface, there is no surface backscatter (s)

As water depth increases, the amount of canopy backscatter (c) and multipath backscatter (m) decrease

Backscatter vs. Soil MoistureWhen the wetlands dry out and standing water is absent, backscatter is proportional to soil moisture

Case Study 3SAR Monitoring of forests

See Harrell et al. 1997

ctIn forested canopies, additional backscatter terms are present

t - direct backscatter from tree trunks

d multi-path or double bounce-scattering between the ground and tree trunk

Landsat TM (June)Wetlands as Viewed with VIS/IR and Synthetic Aperture Radar ImageryRadarsat (C-HH)Double bounce scattering leads to higher radar scattering in flooded forests

SAR backscatter is quite sensitive to variations in aboveground biomass in forests.

In our studies, biomass ranged from 0.1 to 44.4 kg/sq m

See Harrell et al. 1997

An important characteristic of radars is that the energy they transmit and receive are largely unaffected by atmospheric moisture and particles

Transmission through the normal atmosphere is 100% at microwave wavelengths

In addition, all microwave wavelengths are unaffected by clouds, and for wavelengths > 1 cm, it takes a very heavy rate of rainfall to influence the radar signature

Note, weather radars use wavelengths that are < .5 cm, and are sensitive to rainfall

Imaging radars operate at wavelengths > 1 cmThe initial focus of radar systems was to detect ships and airplanes

Radar was actually simaltaneously invented in the U.S. and England

In the U.S., scientists at the Naval Research Lab actually were transmitting radio waves between two laboratories separated by the Anacosta and Potomac Rivers

They noted that when a ship came between the two systems, the signal was blocked

They then placed a received at the same place as the transmitter, and found that when a ship passed, they actually received a signal at the receiver itself, e.g., the ship were reflecting the radio waves

Now, lets talk about the basic way in which a radar operates

Draw picture on the next page

Radars therefore are much different than radiometers that detect and record EM energy in the visible, SW IR and thermal (LW) IR regions

These systems have optical lenses to define the field of view of the system, and focus the EM energy onto a detector

These radiometers are called optical systems, while microwave systems have no optics what-so-everHere we have a photograph of one of the earliest radar installations constructed by England during WWII

This detection was particularly important to England because of WWII and the air attacks by Germany

Eventually, England built a series of land-based radar systems that were used to detect German aircraft out to a distance of 50 miles

A series of stations were built along the English coast. The early radars did not tell direction, only that a signal was being reflected at a certain distance.

However, since the English knew where the German air raids were coming from, the could deduce that the blips being detected were indeed bombers

In reality, the common radar bands span a range of frequencies, hence wavelengths

Here we have a table of the four common wavelengths used in imaging radar systemsAnother characteristic of imaging radars is that they control the polarization of the transmitted and received radiation

Typically, two polarizations are dealt with vertical (V) and horizontal (H)

Vertical polarizations are those that are perpendicular to the earths surface

Horizontal polarizations are those that are parallel to the earths surface

Discuss common nomenclature for identifying radar systems

Imaging radars are typically identified by a 3 letter code, I.e.,

First letter is the band (X, C, L, P)Second letter is transmitted polarizationThird letter is received polarization

e.g.,

C-VV is a 6 cm wavelength radar that transmits and receives vertically polarized EM energy

L-HH is a 24 cm radar that transmits and receives horizontally polarized energy

P-HV is a 60 cm radar that transmits horizontal polarization and receives vertical polarizationDefine radar range as

R = ct / 2

So basically, the function of our crude radar is to detect and determine the range to the targetNow, how do radars tell if one target is closer than the other, e.g.,

How far apart do targets have to be in the range dimension for a radar to tell them apartor

How do we determine the range resolution of a radar?

Microwave transmitters can actually send out a short burst of energy for a specified period of time,

This the length of time over which a microwave transmitter sends its pulse, p

Here we have a schematic diagram of a transmitted pulse.

Note that the time over which it is transmitted is the pulse duration

Note that there will be a number of waves or cycles transmitted during the pulse duration

The number of waves transmitted during the pulse is called the bandwidth B

Note B = 1 / p

One of the reasons we use Bandwidth instead of pulse duration is that microwave transmitters can easily measure the number of cycles that have been transmitted, whereas measuring time is more difficult

Often, when discussing range resolution of a system, it is defined in terms of the bandwidth of the systemTwo objects have to be Pr apart in the range dimension before they can be discriminated by the radar systemThe next thing we want to talk about is the azimuth, or along track resolution of a radar system

The energy transmitted by a radar is actually focused into a beam, with an angular dimension of - draw picture on next pageThe beamwidth of the radar is dependent on two things

The radar wavelength () and the length (L) of the antenna in the azimuth direction

= / L

For example, lets say that we have a radar with a wavelength of 5 cm, and an antenna length of 5 m, then the beamwidth =

5 / 500 = 0.01

Now, the azimuth resolution, a, is the azimuth resolution of the radar, e.g., the distance 2 targets have to be separated in order to be distinguished by the radar in the azimuth direction

Draw picture on next page

You can see that as R gets larger, a gets larger as well

a = R

Go back to our original example, with a 5 cm radar and a 5 meter antenna length,

Let us say that two targets are 10 km away or 10,000 m

These targets would have to be Ra = .01 x 10,000 or 100 m apart for the for radar to tell there are two targets

In addition to the range to the target, there are a number of additional characteristics that radars can measure

Build a model of the radar equation based upon the next page

What a radar actually measures is the power received, Pr

What you want to do is relate Pr to Pt, the power that the radar transmits

Now, a radar transmits energy through an antenna

In turn, the antenna has two fundamental roles on the transmitted energy

First, the antenna is not a perfect transmitter it looses some energy in transmitting the energy

The Gain of the antenna is defined as the ratio of power transmitted out of the antenna relative to the amount of energy transmitted into the antenna

Energy leaving the antenna = PtG

Once the energy leaves the antenna, it travels equally in all directions, therefore, the amount of energy actually traveling towards the target is a fraction of the energy actually transmitted by the antenna This is called isotropic spreading

The amount of spreading that occurs is defined by the distance to the target as= 1/4R2

So the amount of energy that reaches the target is

PtG / 4R2

Now, the amount of energy that is reflected back to the radar is defined as the radar cross section - of the target

Thus, the amount of energy leaving the target is PtG / 4R2

Now this energy also undergoes isotropic spreading as well, e.g., it looses energy at a rate of 1/4R2

Thus, the total amount of energy that makes it back to the radar is

PtG / (4)2 R4

The amount of energy that is actually received, depends on the antenna size, A

So finally,

Pr = A PtG / (4)2 R4

This is called the radar equationThis means that the energy received falls off very quickly as R increases

In order to detect this energy, radars have to either: (a) have large Pt or very sensitive receiversEarlier we discussed two types of surface reflectance specular and diffuse

The descriptors used for reflectance also apply to scattering of microwave energy

You have either diffuse scattering or specular scatteringNote that in specular reflection, the angle of the outgoing radiation is equal to the angle of the incoming radiation

Reflection and scattering are wavelength dependent

Very few objects are specular reflectors

Water and smooth glass surfaces are good examples of specular reflectors

Some surface objects almost reflect all incoming radiation, are called near-perfect specular reflectorsSurfaces with any degree of roughness reflect energy in a variety of directions

Most objects are diffuse reflectors

This is why we are able to see most objects, because they are reflecting light in a diffuse manner

This illustrates the fact that scattering is dependent on wavelength

It takes more roughness for a surface to be a diffuse scatterer at longer wavelengths than at shorter wavelengthsIn these images, I want you to focus on the dark areas in the L-band images

These are grass fields

These fields appear smooth are dark at the longer wavelength L-band (24 cm), but are rougher (and brighter) at the shorter X and C band wavelengths (e.g., 3 and 6 cm wavelengths)In the presence of wind, most water surfaces have small waves that are the result of wind blowing along the surface

These waves are called capillary waves

If the wind keeps blowing, the waves get bigger, and are called ultra-gravity waves

TRANSITION 1 and 2

From a radar scattering viewpoints, for short EM wavelengths (e.g., 1 to 6 cm) these small surface waves are certainly either slightly rough or moderately rough surfaces, and

therefore it should not be surprising that they result in backscattering of energy to the radar

TRANSITION AGAIN ALL Stuff

However, these small waves are still smooth to longer wavelengths, e.g., Lband, 24 cm wavelengths

Thus, we would note expect for an L-band SAR to detect anything on an ocean surface, except when you have considerable wind and larger capillary/ultragravity waves

Here we have an example of an ocean surface that has both very smooth surfaces because of no wind and

rough surfaces where there are some capillary wavesHere we have an L-band SAR image that was collected over this ship at the time the aerial photo in the previous slide was collected Clearly, the wind roughened surface is resulting in considerable radar backscatter at L-band

What is responsible for this backscatter?Write on board Bragg Scattering description from next pageWe can see from this table that Bragg scattering is not only wavelength dependent, but also dependent on incidence angle

This means that the ocean surface changes its scattering characteristics as incidence angle increases

E.g., it is not a lambertian type surface where scattering is constantThere is a definite dependence of wind speed on incidence angle

For incidence angles > 20 degrees, as wind speed increases, radar backscatter increases as well

It is a logrithmic relationship

Note that this relationship changes as a function of incidence angleVariations in plant and soil moisture is an extremely important surface component

In the case of bare soils, two sources of variation

Surface roughness

Soil moistureStudies have been carried out to look at the dielectric constants of a number of different soil types at a number of different wavelengths

Here we have an example for 21.4 cm wavelength

Studies show

Dielectric constant increases as soil moisture increases

Variations between different soil types is not greatThe relationship of increasing DC as a function of soil moisture translates into a linear relationship between soil moisture and radar backscatterHere are the key characteristics of altimetersNote that the footprint of altimeters is fairly large, on the order of an 8 km diameter surface

So, altimeters do not measure topography per se, but over elevations of large areasA number of altimeters have been deployed, beginning in 1975

Provided a lot of informationOne of the things to remember about Altimeters is that they are not imaging sensors

Essentially collect a transect of data that is 8 km wideWhat has been produced from these data is average Sea surface topography

These maps are created through the extrapolation of the transect data

A number of satellite-based radar altimeters have been in operation over the past 10 years

These systems have a very narrow footprint, on the order of 8 km

So it is necessary to collect a number of different maps and then interpolate the data

Here is one such map produced by the TOPEX/Poseidon altimeter systems

These systems, produce very valuable information on sea surface height

Here we have a typical satellite-based wind scatterometer system

What it does is it scans across a swath 500 km wide

It looks at a beam of about 50 km wide

It measures at three directions e.g., that match different relative wind directions

An algorithm then estimates the wind speed and direction based on the differences between the three look directionsThe relationship between radar backscatter and wind speed also depends on the direction of the wind relative to the look direction of the radar

E.g., TRANSITION, backscatter changes when you look upwind versus down wind

The changes are small, but still substantial

They also change when you look cross wind

Scatterometers can be used not only to estimate wind speed, but also wind direction based on the scattering differences based on wind direction relative to radar look direction.Here we have a plot of ground based wind speed and direction compared to the satellite estimated values

You can see the satellites do a very good job

Also have to remember that wind does not continuously blow at the same intensity or the same direction It is continuously changing

So, the accuracy of the scatterometer systems is very adequate for what is really going on.Here we have a ERS/AMI wind scatterometer product for a single days worth of data collected in June of 2001

Show wind scatterometer movie hereSo here we have the sources of backscatter in this situation

Note that each term will depend on

Moisture content of the material (e.g., soil and vegetation)

Wavelength and polarization of the incoming radiation

For example, 24 cm wavelength MW energy will not be affected by grass stalks, either scattered or attenuated

In contrast, 3 cm wavelength MWE is likely to be both heavily scattered and attenuated by the vegetation

Polarization vertically polarized energy will be affected more than horizontally oriented because the EM energy will be aligned with the stalks that are vertical themselves

Microwave scattering is a complex processMost land surface remote sensing today is carried out using either visible/Near IR or thermal IR satellite systems

However, there are a class of systems that operate in the microwave region of the EM spectrum, including

imaging radars, Which are active systems

And Microwave radiometers, which are passive systems

During the first part of this course, we will be reviewing these two types of systems, beginning with imaging radar systemsSoil moisture is an effect that I have discussed before

Here we see a series of ERS radar images collected over a burned forest in Alaska

These images were collected during the 1st year after this fireHere are some results from a more recent study.

In this case, we used an electric probe that directly senses the dielectric constant of the soil and converts this measure to soil moisture

A much less time consuming and more accurate approach

Shows the same trend

Now, we are working on approaches to convert radar backscatter into soil moisture estimatesThere is a natural historical wetlands found in southern Florida based on surface water that flows from Lake Okeechobee in a southerly direction

Earlier, I showed you an example of monitoring Florida wetlands using ERS SAR imagery

At that time, I only showed you two images

In fact, we have collected multiple images over this region from multiple years

Here you can see backscatter changes on these images during different parts of the growing season

We have images from the Early Dry season, late dry season, and wet seasons

You can see that these images represent the complex processes of

Flooding during the wet season

Higher soil moistures in the early dry season

Drying out of the soils in the later dry seasonWith woody vegetation, the presence of flooding presents a very unique case

In situations where the canopy attenuation does not dominate (e.g., at longer wavelengths or when leaves are absent),

Then the d term kicks in, e.g., you get enhanced scattering (increased backscatter) because of the double bounce scattering

Here we see an example of this increased scattering in forested wetlands along river banks

From the landsat scene, the forested area area are primarily in red

This is a river (ask megan) that is near the Chesapeake Bay

Note the rive flowing N/S

TRANSITION

Here we have a radar image of the same area

Radarsat SAR C-HH

Forested areas are brighter than the adjacent agricultural areas that are dark

You can see that the areas along the river that are flooded are much brighter than the adjacent upland forest

Due to the double-bounce scattering

1 Lecture 16 – Active Microwave Remote Sensing 2 December 2008.

Documents

Transcript of 1 Lecture 16 – Active Microwave Remote Sensing 2 December 2008.