An Introduction to Radar and Lidar Remote Sensing

Credit to: Weile Wang

Gustav Klimt (1862-1918), Der Park

With materials from Drs. Jeff Dozier (UCSB), Howard Zebker (Stanford), Jacob van Zyl (JPL), Alan Strahler (Boston U.), Ralph Dubayah (U. Maryland), Michael Lefsky (U. Colorado), Guoqing Sun (U. Maryland), and many others.

Radar Basics, PPI, SLAR, SAR, InSAR; Radar Equation, Imaging Geometry, Geometric Distortion,

Speckle, Polarization, Interferometry; Lidar, Waveform, Footprint, Forest Structure Measurement; SRTM, LVIS, SMAP, GRACE, DESDynI, and Echidna.

Outline

Active and passive remote sensing

• Passive: uses natural energy, either reflected sunlight or emitted thermal or microwave radiation

• Active: sensor creates its own energy– Transmitted toward Earth– Interacts with atmosphere and/or surface– Reflects back toward sensor (backscatter)

Common active remote sensing systems

• Radar (RAdio Detection And Ranging)– long-wavelength microwaves (1-100cm)– recording the amount of energy back-scattered from the terrain

• Lidar (LIght Detection And Ranging)– short-wavelength laser light (e.g., 0.90 µm)– recording the light back-scattered from the terrain or atmosphere

• Sonar (SOund Navigation And Ranging)– sound waves through a water column– recording the amount of energy back-scattered from the water

column or the bottom

8 -1

Frequency

where speed of light

=

Useful

3.00

tric

10 m s

30GHzcm

k

c

c

Microwave Bands

What is Radar?

TRANSMITTER

RECEIVER

CIRCULATOR

RADAR PULSE

"TARGET"

• RADAR = Radio Detection And Ranging• Since radar pulses propagate at the speed of light, the difference to the “target”

is proportional to the time it takes between the transmit event and reception of the radar echo

Ranging: Distance Measurement

??

c = speed of light

= 3.00 × 108 m/s

Mapping Multiple Objects: PPI Radar Display

PPI=Plan Position Indicator

The Radar Equation (1)

• Gt is the “Antenna Gain”;

• σ is the “cross section” of the target.

The radar equation (2)• The radar equation represents the physical dependences of

the transmit power, that is the wave propagation up to the receiving of the echo-signals. The power PE returning to the receiving antenna is given by the radar equation, depending on the transmitted power Pt, the slant range R, and the reflecting characteristics of the aim (described as the radar cross-section σ). At known sensibility of the radar receiver the radar equation determines the achieved by a given radar set theoretically maximum range. Furthermore one can assess the performance of the radar set with the radar equation.

• Suggest reading: http://www.radartutorial.eu/01.basics/rb13.en.html

The radar equation (3)

• antenna gain: Since a spherical segment emits equal radiation in all direction (at constant transmit power), if the power radiated is redistributed to provide more radiation in one direction, then this results an increase of the power density in direction of the radiation. This effect is called antenna gain.

Imaging Radar: Side-Looking Airborne Radar

Imaging Geometry

azimuth refers to the along-track dimension parallel to the flight direction.Swath width refers to the strip of the Earth’s surface from which data are collected by

a side-looking airborne radar (SLAR)

Forming an image

Radar Reflections from Flat Ground

• The Earth plane surrounding a radar antenna has a significant impact on the vertical polar diagram. The combination of the direct and re-reflected ground echo changes the transmitting and receiving patterns of the antenna.

Nomenclature• nadir• azimuth flight direction• look direction• range (near and far)• depression angle (γ)• incidence angle (θ)• altitude above-ground-

level, H• polarization

Radar geometry

Range resolution

pulse length speed of light

2cos 2cos depression anglercR

Calculate Rr

Side-looking airborne radar (SLAR)

H is the height of the antenna,   (height of the airplane)L is the geometric length of the antenna,λ is the wavelength of the transmitted pulses, andθ is the incidence angle(1) L · cos θ

cos

L

HRa

Azimuth resolution

slant range wavelengthantenna length

aSR

L

Question:

Why is wavelength important in determining Ra?

• The equation shows, that with increasing altitude decreases the azimuthal resolution of SLAR. A very long antenna (i.e., large L) would be required to achieve a good resolution from a satellite. Synthetic Aperture Radar (SAR) is used to acquire higher resolution.

• For an SLAR with the following characteristics:λ = 1 cm,L = 3 m,H = 6000 m,θ = 60°, andtp = 100 ns,has got a resolution ofRa = ??? andRr = ??? m

• Note: The same SLAR on a platform in a height of 600 km would achieve an azimuth-resolution of Ra = ???.

Synthetic aperture radar (SAR)

• A Synthetic Aperture Radar (SAR), or SAR, is a coherent mostly airborne or spaceborne sidelooking radar system which utilizes the flight path of the platform to simulate an extremely large antenna or aperture electronically, and that generates high-resolution remote sensing imagery.

• Read http://www.radartutorial.eu/20.airborne/ab07.en.html

• The SAR works similar of a phased array, but contrary of

a large number of the parallel antenna elements of a phased array, SAR uses one antenna in time-multiplex. The different geometric positions of the antenna elements are result of the moving platform now.

• The SAR-processor stores all the radar returned signals, as amplitudes and phases, for the time period T from position A to D. Now it is possible to reconstruct the signal which would have been obtained by an antenna of length v · T, where v is the platform speed. As the line of sight direction changes along the radar platform trajectory, a synthetic aperture is produced by signal processing that has the effect of lengthening the antenna. Making T large makes the „synthetic aperture” large and hence a higher resolution can be achieved.

• As a target (like a ship) first enters the radar beam, the backscattered echoes from each transmitted pulse begin to be recorded. As the platform continues to move forward, all echoes from the target for each pulse are recorded during the entire time that the target is within the beam. The point at which the target leaves the view of the radar beam some time later, determines the length of the simulated or synthesized antenna. The synthesized expanding beamwidth, combined with the increased time a target is within the beam as ground range increases, balance each other, such that the resolution remains constant across the entire swath.

• The achievable azimuth resolution of a SAR is approximately equal to one-half the length of the actual (real) antenna and does not depend on platform altitude (distance).

Radar Image Elements

Roughness

Smooth 25sin

Rough 4.4sin

h

h

Sources of radar backscattering from a

vegetation canopy

Question:

Does the strength of the backscattering vary with frequencies?

Strength of scattering from a pine stand depends on frequency

Polarization

• 1st letter is transmitted polarization, 2nd is received– Can have

VV, HH (like)

– HV, VH (cross)

Polarization with radar

a.

b.

look direction

N

Ka - band, HH polarization

Ka - band, HV polarization

Polarization with radar

• RADARSAT, C-band radar (5.4 GHz) with HH, VV, HV, and VH polarizations

Geometric Distortions

Geometric Distortions(or: Slant-range distortion)

• Foreshortening• Layover• Shadow

See handout

Slant-range distortionThe slant-range distortion occurs because the radar is measuring the distance to

features in slant-range rather than the true horizontal distance along the ground. This results in a varying image scale, moving from near to far range.

• Foreshortening occurs when the radar beam reaches the base of a tall feature tilted towards the radar (e.g. a mountain) before it reaches the top. Because the radar measures distance in slant-range, the slope (from point a to point b) will appear compressed and the length of the slope will be represented incorrectly (a' to b') at the image plane.

• Layover occurs when the radar beam reaches the top of a tall feature (b) before it reaches the base (a). The return signal from the top of the feature will be received before the signal from the bottom. As a result, the top of the feature is displaced towards the radar from its true position on the ground, and „lays over” the base of the feature (b' to a').

• The shadowing effect increases with greater incident angle θ, just as our shadows lengthen as the sun sets.

Foreshortening

Layover

• Extreme case of foreshortening, when incidence angle is less than slope angle toward radar (i.e. θ<α)– cannot be

corrected– got to be careful

in the mountains

Shadow • When slope away from radar is steeper than the depression angle, i.e. –α > γ

Speckle: Random Interference

• Grainy salt-and-pepper pattern in radar imagery– Caused by coherent nature of the

radar wave, which causes random constructive and destructive interference, and hence random bright and dark areas in a radar image

• Reduced by multiple looks– processing separate portions of

an aperture and recombining these portions so that interference does not occur

a.

b.

c.

1 - Look radar image

4 - Look radar image

16 - Look radar image

InSAR: Adding the Z-dimension

Landsat overlaid on topography from SRTM – Malaspina Glacier, Alaska

22

interferometric phase incidence angle antenna angle baseline length wavelength range

From interferometry 2

2 sin

cost p

B

BB

h h

InSAR Geometry

Can you derive the equation? Extra credit (point)

The following materials are FYI. Not required for exam.

Shuttle Radar Topography Mission

Links to movies

SRTM Global Coverage

SRTM Elevation + Landsat Imagery

Perspective with Landsat Overlay: Antelope Valley, California

From Radar to Lidar

• LIDAR = Light Detection And Ranging

• Using laser instead of microwave

Measuring Forest Structure

Continuous Waveform, Large Footprint

Discrete Waveform, Small Footprint

Canopy Topography

Ground-Based Lidar (Echidna)

A real Echidna—in the forest

Data Examples

Airborne Lidar Instrument: LVIS

Space-borne: ICESat and GLAS

Synthesis of Lidar, Radar, and optical sensors

Global Map of Tree Height

Other Relative Sensors: GRACE

GRACE: Gravity Recovery And Climate Experiment

GRACE: The 2012 Drought over US

Soil Moisture Active & Passive (SMAP)

Radar• Frequency: 1.26 GHz • Polarizations: VV, HH, HV • Data collection:

• High-resolution/high-rate data collected for ground SAR processing • Low-resolution real-aperture data collected continuously

Radiometer• Frequency: 1.41 GHz • Polarizations: H, V, U • Relative accuracy: 1.3 K • Data collection: Continuous over full scan

SMAP Instruments

DEformation, Ecosystem Structure, and Dynamics of Ice

DESDynI Instruments4. Instrument Design & Performance

~350kmFlight Direction

Interferometric SARDual-Pol 3-BeamsQuad-Pol 6-BeamsRight or Left Point

L-Band Synthetic Aperture Radar

Lasers

LaserRadiators

Star Tracker

Multi-beam Lidar

Beam Spacing1 km

Radar Basics, PPI, SLAR, SAR, InSAR; Radar Equation, Imaging Geometry, Geometric Distortion,

Speckle, Polarization, Interferometry; Lidar, Waveform, Footprint, Forest Structure Measurement; SRTM, LVIS, SMAP, GRACE, DESDynI, and Echidna.

Summary

• For an SLAR with the following characteristics:λ = 1 cm,L = 3 m,H = 6000 m,θ = 60°, andtp = 100 ns,has got a resolution ofRa = 40 m andRr = 17.3 m

• Note:

Homework-8 1. Derive the radar equation. 2. Derive the raindrop size equation from the radar equation you derived in

question 1. 3. The SLAR on a platform in a height of 600 km would achieve an

azimuth-resolution of Ra = ?. (other needed variable are the same given in class activity)

4. (extra credit) NASA Tropical Rainfall Measuring Mission (TRMM) has a single frequency radar at the Ku-band 13.8 GHZ particularly sensitive to moderate rain rates. With a single frequency, the TRMM radar is able to retrieve drop size. Assume that raindrops range from 1/100 inch (.0254 centimeter) to 1/4 inch (.635 centimeter) in diameter. Antenna Gain is 1.698, instrument size is 0.5 m, plot the relation of ratio of Pt/Pr vs. raindrop size, assume height of rain is 1 km.