Active Remote Sensing for Elevation Mapping

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

2. Radar image: Himalayas

3. Radar derived elevation with TM draped over it

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

5. High-resolution LIDAR topography

6. Lidar vs. DEMs from topo sheets

9. Passive vs. Active Remote Sensing • Passive remote sensing uses the energy from the sun • Active remote sensing sends out its own energy and records how much bounces back • Imaging Radar uses microwave wavelengths • 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 on carried on aircraft or satellite (or space shuttle) • Send out pulses of microwave EMR • Measure time required for pulse to return to instrument • Can measure properties of the returning EMR (polarity, intensity, phase) • Useful for characterizing elevation, surface roughness, surface wetness

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 – 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 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 accurate 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. Accuracy of InSAR

29. Applications of InSAR • Earthquakes • Volcanic Activity • Land Surface Deformation • Movement of Glaciers • Water level Changes

30. InSAR Volcanic Inflation Image • Data Before and After Eruption • Provide Insight into: • Magma Dynamics • Structure • Plumbing • State of Restless Volcanoes

31. Interferogram Example Corresponding interferogramof Kahlua, showing topographic fringes (NASA/JPL-Caltech)

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

35. 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

36. 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

38. 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 fairly interactive human processing

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

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

42. Coral reef mapping (underwater) with Lidar

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