Active Microwave Remote Sensing

Lecture 8 Oct 22, 2007

Recap: passive and active RS

Passive: uses natural energy, either reflected sunlight (solar energy) or emitted thermal or microwave radiation.

Active: sensor creates its own energy Transmitted toward Earth or other targets Interacts with atmosphere and/or surface Reflects back toward sensor (backscatter) Advantages: all weathers and all times

Widely used active RS systems RADAR: RAdio Detection And Ranging

Long-wavelength microwaves (1 – 100 cm)

LIDAR: LIght Detection And Ranging Short-wavelength laser light (UV, visible, near IR)

SONAR: SOund Navigation And Ranging: (very long wave, low Hz) Sound can not travel through vacuum Earth and water absorb acoustic energy far less than EMR energy Seismic survey use small explosions, record the reflected sound Medical imaging using ultrasound Sound waves through a water column. Sound waves are extremely slow (300 m/s in air, 1,530 m/s in sea-water) Bathymetric sonar (measure water depths and changes in bottom topography ) Imaging sonar or sidescan imaging sonar (imaging the bottom topography and

bottom roughness)

Types of radar

Nonimaging radar Traffic police use handheld Doppler radar system

determine the speed by measuring frequency shift between transmitted and return microwave signal

Plan position indicator (PPI) radars use a rotating antenna to detect targets over a circular area, such as NEXRDA

Satellite-based radar altimeters (low spatial resolution but high vertical resolution)

Imaging radar Usually high spatial resolution, Consists of a transmitter, a receiver, one or more

antennas, GPS, computers

Microwaves

Band Designations(common wavelengths Wavelength () Frequency ()shown in parentheses) in cm in GHz_______________________________________________Ka (0.86 cm) 0.75 - 1.18 40.0 to 26.5K 1.18 - 1.67 26.5 to 18.0Ku 1.67 - 2.4 18.0 to 12.5X (3.0 and 3.2 cm) 2.4 - 3.8 12.5 - 8.0C (7.5, 6.0 cm) 3.8 - 7.5 8.0 - 4.0S (8.0, 9.6, 12.6 cm) 7.5 - 15.0 4.0 - 2.0L (23.5, 24.0, 25.0 cm) 15.0 - 30.0 2.0 - 1.0P (68.0 cm) 30.0 - 100 1.0 - 0.3

Two imaging radar systems

In World War II, ground based radar was used to detect incoming planes and ships (non-imaging radar).

Imaging RADAR was not developed until the 1950s (after World War II). Since then, side-looking airborne radar (SLAR) has been used to get detailed images of enemy sites along the edge of the flight field. SLAR is usually a real aperture radar. The longer the antenna (but there is limitation), the better the spatial resolution

Real aperture radar (RAR) Aperture means antenna A fixed length (for example: 1 - 15m)

Synthetic aperture radar (SAR) 1m (11m) antenna can be synthesized electronically into a 600m (15 km)

synthetic length. Most (air-, space-borne) radar systems now use SAR.

1. RAR: Principle of SLAR

waveform

Radar Nomenclature and Geometry

Radar NomenclatureRadar Nomenclature

• • nadirnadir•• azimuth (or flight) directionazimuth (or flight) direction•• look (or range) directionlook (or range) direction•• range (near, middle, and far)range (near, middle, and far)•• depression angle (depression angle ())•• incidence angle (incidence angle ())•• altitude above-ground-level, altitude above-ground-level, HH•• polarizationpolarization

Azimuth flight direction

Flightline groundtrack

Look/Range direction

Far range

Near range

Slant-range vs. Ground-range geometry

Radar imagery has a different geometry than that produced by most conventional remote sensor systems, such as cameras, multispectral scanners or area-array detectors. Therefore, one must be very careful when attempting to make radargrammetric measurements.

• Uncorrected radar imagery is displayed in what is called slant-range geometry, i.e., it is based on the actual distance from the radar to each of the respective features in the scene.

• It is possible to convert the slant-range display into the true ground-range display on the x-axis so that features in the scene are in their proper planimetric (x,y) position relative to one another in the final radar image.

Most radar systems and data providers now provide the data in ground-range geometry

Range (or across-track) Resolution

cos2ctRr

Pulse duration (t)= 0.1 x 10 -6 sec

t.c called pulse length. It seems the short pulse length will lead fine range resolution.

However, the shorter the pulse length, the less the total amount of energy that illuminates the target.

t.c/2 t.c/2

Azimuth (or along-track) Resolution

DSRa

The shorter wavelength and longer antenna will improve azimuth resolution.

The shorter the wavelength, the poorer the atmospheric and vegetation penetration capability

There is practical limitation to the antenna length, while SAR will solve this problem.

2. SAR

A major advance in radar remote sensing has been the improvement in azimuth resolution through the development of synthetic aperture radar (SAR) systems. Great improvement in azimuth resolution could be realized if a longer antenna were used. Engineers have developed procedures to synthesize a very long antenna electronically. Like a brute force or real aperture radar, a synthetic aperture radar also uses a relatively small antenna (e.g., 1 m) that sends out a relatively broad beam perpendicular to the aircraft. The major difference is that a greater number of additional beams are sent toward the object. Doppler principles are then used to monitor the returns from all these additional microwave pulses to synthesize the azimuth resolution to become one very narrow beam.

Azimuth resolution is constant = D/2, it is independent of the slant range distance, , and the platform altitude. So the same SAR system in a aircraft and in a spacecraft should have the same resolution. There is no other remote sensing system with this capability.

Animation of the Doppler Effect

Animation of the Doppler Effect

Animation of the Doppler Effect

Animation of the Doppler Effect

Animation of the Doppler Effect

Animation of the Doppler Effect

Animation of the Doppler Effect

Animation of the Doppler Effect

9 8 7 6 5 4 3 2 1

time n

time n+4time n+3

time n+2

pulses of microwave energy

interference signal

radar hologram

a.b. c.

d. e.

8 7

6.5 7

9 9 8 9 8 7

78 9 78 9 6.5 6.5 7

time n+1object is a

constant distance from the flightline

At time n+3, the shortest distance and area of zero Doppler shift

Speckle noise

Using SAR, we can get high spatial resolution in the azimuth dimension (direction). But the coherently recording returned echoes (SAR) also causes speckle noise.

For one-single channel SAR system, the speckle noise has a multiplicative nature for the amplitude and an additive nature for the phase.

For multi-dimensional (or polarimetric) SAR (or PolSAR) system, speckle noise is even complicated.

There are two ways to remove speckle noise: Using several looks, i.e., averaging takes place, usually 4 or 16 looks

(N). But lose resolution: Azimuth resolution = N(D/2) Modeling the noise, then remove them.

Backscatter

The portion of the outgoing radar signal that the target redirects directly back towards the radar antenna.

When a radar system transmits a pulse of energy to the ground (A), it scatters off the ground in all directions (C). A portion of the scattered energy is directed back toward the radar receiver (B), and this portion is referred to as "backscatter".

Amount of backscatter per unit area

http://earth.esa.int/applications/data_util/SARDOCS/spaceborne/Radar_Courses/Radar_Course_III/parameters_affecting.htm

Fundamental radar equation

t

sin8h

Intermediate wrong

L-band 23.5 cm

C-band 5.8 cm

X-band 3 cm

a. b. c.

Response of A Pine Forest Stand to X-, C- and L-band Microwave EnergyResponse of A Pine Forest Stand to X-, C- and L-band Microwave Energy

Penetration ability to forest

Polarization

Unpolarized energy vibrates in all possible directions perpendicular to the direction of travel.

The pulse of electromagnetic energy is filtered and sent out by the antenna may be vertically or horizontally polarized.

The pulse of energy received by the antenna may be vertically or horizontally polarized

VV, HH – like-polarized imagery

VH, HV- cross-polarized imagery

Penetration abilityinto subsurface

SIR-C/X-SAR SIR-C/X-SAR Images of a Portion Images of a Portion

of Rondonia, of Rondonia, Brazil, Obtained on Brazil, Obtained on

April 10, 1994April 10, 1994

Penetration abilityto heavy rainfall

Radar Shadow

Shadows in radar images can enhance the geomorphology and texture of the terrain. Shadows can also obscure the most important features in a radar image, such as the information behind tall buildings or land use in deep valleys. If certain conditions are met, any feature protruding above the local datum can cause the incident pulse of microwave energy to reflect all of its energy on the foreslope of the object and produce a black shadow for the backslope

Unlike airphotos, where light may be scattered into the shadow area and then recorded on film, there is no information within the radar shadow area. It is black.

Two terrain features (e.g., mountains) with identical heights and fore- and backslopes may be recorded with entirely different shadows, depending upon where they are in the across-track. A feature that casts an extensive shadow in the far-range might have its backslope completely illuminated in the near-range.

Radar shadows occur only in the cross-track dimension. Therefore, the orientation of shadows in a radar image provides information about the look direction and the location of the near- and far-range

Shuttle Imaging Radar (SIR-C) Image of MauiShuttle Imaging Radar (SIR-C) Image of Maui

Shadows and look direction

Major Active Radar Systems

Seasat, June 1978, 105 days mission, L-HH band, 25 m resolution SIR-A, Nov. 1981, 2.5 days mission, L-HH band, 40 m resolution SIR-B, Oct. 1984, 8 days mission, L-HH band, about 25 m resolution SIR-C, April and Sept. 1994, 10 days each. X-, C-, L- bands multipolarization

(HH, VV, HV, VH), 10-30 m resolution JERS-1, 1992-1998, L-band, 15-30 m resolution (Japan) RADARSAT, Jan. 1995-now, C-HH band, 10, 50, and 100 m (Canada) ERS-1, 2, July 1991-now, C-VV band, 20-30 m (ESA) ASAR on EnviSat, 2002-now, C band (ESA) AIRSAR/TOPSAR, 1998-now, C,L,P bands with full polarization, 10m

NEXRAD, 1988-now, S-band, 1-4 km, TRMM precipitation radar, 1997, Ku-band, 4km, vertical 250m (USA and Japan)

Active Radar Systems for Mars

MARSIS (Mars advanced radar for subsurface and ionosphere sounding) of Mras Express, 2003, 1.8-5 MHz, up to 5 KM deep (ESA)http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=34826&fbodylongid=1601

SHARAD (shallow subsurface radar) of MRO, 2005, 15-25 MHz, up to 1Km deep (ISA-NASA)http://mars.jpl.nasa.gov/mro/mission/sc_instru_sharad.html