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

2. Radar image: Himalayas

3. Kamchatka Peninsula – Shuttle Radar Topography Mission (SRTM)
(Mission generated detailed topographic data for 80% of earth’s land surface)

4. Radar derived elevation (SRTM) with Landsat draped over it
Cape of Good Hope, South Africa

5. High-resolution LIDAR topography

6. Lidar vs. DEMs from USGS topo map

9. 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

10. 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.

11. Radar bands were originally code names assigned by the military

12. 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)

13. 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

14. 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

16. Radar Terminology Direction of flight = azimuth
Backscatter = reflectance
Angle of view = depression angle
etc.—whole new terminology

17. Radar Geometry
Depression Angle

19. 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)

21. 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.

22. 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)

24. 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

25. 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)

26. Radar Interferometry (2 or more passes)

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

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

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

32. 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

34. 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

35. 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

37. 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

38. Lidar Applications
Detailed (1 meter horizontal) resolution “bare earth” elevation surface

39. Lidar derived flood plane
Topo derived flood plane
More precise elevation data allows better prediction of flood damage

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

42. Coral reef mapping (underwater) with Lidar

43. 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.