Active Remote Sensing for Elevation Mapping Radar and Lidar Fundamentals and Applications

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 Sensing Passive remote sensing uses the energy from the sun Active remote sensing sends out its own energy (EMR) and records how much reflects back Imaging Radar uses microwaves RAdio Detection And Ranging Passive microwave measures emitted long wave radiation Lidar uses visible and NIR wavelengths (laser) Light Detection and Ranging

Radar Radar instruments carried on aircraft or satellite (or space shuttle) Send out pulses of microwave EMR Measure time required for pulse to go to target and return to instrument Can measure properties of the returning EMR (polarity, intensity, phase) Useful for characterizing elevation, surface roughness, surface wetness, vegetation structure, etc.

Radar bands were originally code names assigned by the military

Radar Advantages Can penetrate clouds Active, so can use day or night Less 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 Disadvantages Developed by military, less civilian experience so far than passive remote sensing Difficult to interpret—complicated properties of ground affect reflectance Geometric distortions caused by side looking geometry Not much spectral information

Side-looking Radar Most radar systems do not look straight down but instead off to the side For 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 Terminology Direction of flight = azimuth Backscatter = reflectance Angle of view = depression angle etc.—whole new terminology

Radar Geometry Depression Angle

Interpretation of Radar Data Surface “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 – spatial resolution limited by antenna length. Synthetic Aperture Radar (SAR) can simulate a long antenna by taking advantage of the Doppler effect Doppler 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 Sensors There are many imaging radar sensors available, both airborne and on satellites Most aircraft use SAR All satellites use SAR (to achieve reasonable spatial resolution)

Mapping Elevation with Radar Two general strategies Single pass: Data from one Radar flight used to map surface Use time for radar signal to go out and come back to calculate distance to ground Must 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 surface Allows more precise calculation of elevation

Single pass ranging 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)

Radar Interferometry (2 or more passes)

Applications of InSAR Earthquake Movement Volcanoes Land Surface Deformation Movement of Glaciers Water level Changes

InSAR Volcanic Inflation Image Provide insights into: Magma Dynamics Structure Plumbing

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

Lidar Remote Sensing Like radar but sends laser pulses instead of microwave/radio pulses Can collect very accurate elevation data quickly (vs. ground survey) Typically flown on aircraft

Calculating elevation from Lidar 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

Lidar resolution 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 for different surfaces 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 others E.g., water doesn’t typically flow through buildings Requires interactive human processing

Lidar Applications Detailed (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

Biomass distribution in rain forest canopy. La Selva, Costa Rica

Coral reef mapping (underwater) with Lidar

Summary Radar and Lidar are active remote sensing techniques Can operate day or night Most important application is development of accurate elevation surfaces Elevation data are critical for many other applications, from vegetation mapping to hydrology to geology and others.