Radar and Lidar Fundamentals and Applications

Active Remote Sensing for Elevation Mapping

Radar image: Himalayas

Kamchatka Peninsula – Shuttle Radar Topography Mission (SRTM)

(Mission generated detailed topographic data for 80% of earth’s land surface)

Radar derived elevation (SRTM) with Landsat draped over it

Cape of Good Hope, South Africa

High-resolution LIDAR topography

Lidar vs. DEMs from USGS topo map

Passive vs. Active Remote SensingPassive remote sensing uses the energy

from the sun Active remote sensing sends out its own

energy and records how much bounces backImaging Radar uses microwave wavelengths

RAdio Detection And RangingPassive microwave measures emitted long wave

radiationLidar uses visible and NIR wavelengths (laser)

Light Detection and Ranging

Radar instruments on carried on aircraft or satellite (or space shuttle)

Send out pulses of microwave EMRMeasure time required for pulse to return to

instrumentCan measure properties of the returning EMR

(polarity, intensity, phase)Useful for characterizing elevation, surface

roughness, surface wetness

Radar

Radar bands were originally code names assigned by the military

Radar AdvantagesCan penetrate cloudsActive, so can use day or nightLess of a radiance vs. reflectance problem

since you know exactly how much energy you send out and can measure what you get back—and atmosphere not a problem

Can penetrate dry soil and get subsurface characteristics (e.g., archaeology)

Radar DisadvantagesDeveloped by military, less civilian experience

so far than passive remote sensingDifficult to interpret—complicated properties

of ground affect reflectanceGeometric distortions caused by side looking

geometryNot much spectral information

Side-looking RadarMost radar systems do not look straight down

but instead off to the sideFor military applications allows planes to fly

over friendly territory and look into enemy territory

Gives us more info about surface than when radar looks straight down because differences in surface roughness become more apparent

Radar TerminologyDirection of flight = azimuthBackscatter = reflectanceAngle of view = depression

angleetc.—whole new terminology

Depression Angle

Radar Geometry

Interpretation of Radar DataSurface “smoothness” or “roughness” with

respect to radar depends on wavelength and incident angle

A smooth surface reflects in one direction (specular)

A rough surface scatters radiation in all directions (Lambertian or diffuse)

Real Aperture vs. Synthetic Aperture Radar (SAR)Real aperture radar actually uses a single

antenna of a given length – resolution limited to what a plane or satellite can physically carry.

Synthetic Aperture Radar (SAR) can simulate a large antenna by taking advantage of the Doppler effectDoppler shift allows sensor to identify

electromagnetic waves from ahead and behind the platform and therefore track an object for longer than it otherwise could, as if the antenna were longer.

Radar SensorsThere are many imaging radar sensors

available, both airborne and on satellites

Most aircraft use SARAll satellites use SAR (to achieve reasonable

spatial resolution)

Two general strategiesSingle pass: Data from one Radar flight used

to map surfaceUse time for radar signal to go out and come back

to calculate distance to groundMust know location of radar instrument very

accurately through time (inertial navigation systems + GPS)

Radar Interferometry: Use 2 radar flights of same area to calculate distance to surfaceAllows more accurate calculation of elevation

Mapping Elevation with Radar

Distance from plane to target is given by:

Distance = 0.5 * c * t

Where c = speed of light (2.98 x 108 m/s) t = time required for pulse to go out

and come back (seconds)

Single pass ranging

Radar Interferometry (2 or more passes)

Applications of InSAREarthquakesVolcanic ActivityLand Surface

DeformationMovement of GlaciersWater level Changes

InSAR Volcanic Inflation Image

•Data Before and After Eruption

•Provide Insight into:

•Magma Dynamics

•Structure

•Plumbing

•State of Restless Volcanoes

Interferogram Example

Corresponding interferogram of Kahlua, showing topographic fringes (NASA/JPL-Caltech)

Lidar Remote SensingLike radar but sends laser pulses instead of

microwave/radio pulsesCan collect extremely accurate elevation data

quickly (vs. ground survey)Typically flown on aircraft

Same as for single pass radar – use time for pulse to go out and return and speed of light to calculate distance

Like radar, depends on inertial navigation systems and GPS

More accurate than radar

Calculating elevation from Lidar

Generally better than radar resolution because:Radar has a pulse-based (wave) footprint that

is usually broad (pulse radiates outward away from sensor)

Lidar has a beam-based (photon) footprint that is usually narrow (pulse width stays narrow away from sensor)

Lidar uses shorter wavelength light and therefore it is reflected by smaller objects than radar

Lidar resolution

Lidar derived surface models include top of vegetation canopy, buildings, etc.

Lidar derived bare earth elevation must have all of those removed

Lidar for hydrologic flows requires bare earth in some places but not in othersE.g., water doesn’t typically flow through

buildings

Requires fairly interactive human processing

Lidar for different surfaces

Lidar ApplicationsDetailed (1 meter horizontal) resolution “bare earth” elevation surface

Lidar derived flood plane

Topo derived flood plane

More precise elevation data allows better prediction of flood damage

Coral reef mapping (underwater) with Lidar

Radar and Lidar are active remote sensing techniques

Can operate day or nightMany applications, but most important is

development of accurate elevation surfacesElevation data are critical for many other

application, from vegetation mapping to hydrology to geology and others.

Summary