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 (active) uses reflected microwaves
Passive microwave measures emitted long wave radiation
Lidar (active) uses reflected visible and NIR wavelengths
RADAR = RAdio Detection And Ranging
Lidar = LIght Detection And Ranging

10. Radar
Radar instruments are 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 also 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
Atmospheric interference less of a problem than with shorter wavelengths
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

14. Side-looking Radar Most radar systems are oblique
For military applications, allows planes to fly over friendly territory and look into enemy territory
Surface roughness is more apparent when viewed obliquely

16. Radar Terminology Direction of flight = azimuth
Backscatter = reflectance
Angle of view = depression angle
Etc.—different 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 uses an antenna of a given length – spatial resolution limited by antenna length (longer is better).
Synthetic Aperture Radar (SAR) can simulate a long antenna by taking advantage of the Doppler effect
Illustration: Christian Wolff, Creative Commons: (Link)

22. Radar Sensor Timeline

23. Mapping Elevation with Radar
Two strategies
Single pass: Data from one radar flight used to map surface elevation based on time for signal to return to sensor
Must know location of radar instrument very accurately through time (inertial navigation systems + GPS)
Radar Interferometry: Uses 2 radar flights of same area to calculate distance to surface based on wave phase
Allows more precise calculation of elevation

24. 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 return (seconds)

25. Radar Interferometry (2 or more passes)

26. Applications of InSAR Earthquake Movement Volcanoes
Land Surface Deformation
Movement of Glaciers
Water level Changes
InSAR data for Yellowstone (link) for ground deformation (USGS).

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

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

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

31. 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
Usually more precise than radar

32. 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

34. 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

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

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

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

39. Coral reef mapping (underwater) with Lidar

40. 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.