ESM 266: Active microwave remote sensing

1. 1 ESM 266: Active microwave remote sensing Jeff Dozier

2. 2 Active and passive remote sensing Passive: uses natural energy, either reflected sunlight or emitted thermal or microwave radiation Active: sensor creates its own energy Transmitted toward Earth Interacts with atmosphere and/or surface Reflects back toward sensor (backscatter)

3. 3 Widely used active remote sensing systems Active microwave (radar) long-wavelength microwaves (1-100cm) recording the amount of energy back-scattered from the terrain Lidar short-wavelength laser light (e.g., 0.90 �m) recording the light back-scattered from the terrain or atmosphere Sonar sound waves through a water column recording the amount of energy back-scattered from the water column or the bottom

4. 4 Frequency-wavelength relation Generally in the microwave part of the spectrum we use frequency instead of wavelength Typically measured in s�1, called Hertz (Hz) Most often Gigahertz (GHz) = 109Hz

5. 5 Microwave band codes

6. Sending and receiving a pulse of microwave radiation

7. SIR-C/X-SAR images of Rondonia, Brazil

8. 8 Advantages of radar All weather, day or night Some areas of Earth are persistently cloud covered Penetrates clouds, vegetation, dry soil, dry snow Sensitive to water content, surface roughness Can measure waves in water Sensitive to polarization and frequency Interferometry (later) using 2 receiving antennas

9. 9 Disadvantages of radar Penetrates clouds, vegetation, dry soil, dry snow Signal is integrated over a depth range and a variety of materials Sensitive to water content, surface roughness Small amounts of water affect signal Hard to separate the volume response from the surface response Sensitive to polarization and frequency Many choices for instrument, expensive to cover range of possibilities The math can be formidable

10. How it works Pulses of active microwave electromagnetic energy illuminate strips of the terrain at right angles (orthogonal) to the direction of travel called the range or look direction The terrain illuminated nearest the aircraft is the near-range The farthest point of terrain illuminated is the far-range

11. How it works (cont.) Aircraft or satellite travels in a straight line: the azimuth direction Pulses of microwave electromagnetic energy illuminate strips of the terrain orthogonal to direction of travel: the range or look direction Terrain illuminated nearest the sensor in the line of sight is the near-range The farthest point of terrain illuminated by the pulse of energy is the far-range Generally, objects that trend (or strike) in a direction orthogonal (perpendicular) to the range or look direction are enhanced much more than those objects in the terrain that lie parallel to the look direction Consequently, linear features that are imperceptible in a radar image using one look direction may appear bright in another radar image with a different look direction.

12. Nomenclature nadir azimuth flight direction look direction range (near and far) depression angle (?) incidence angle (?) altitude above-ground-level, H polarization

13. Variability with look direction

14. Depression angles and incidence angles Depression angle (g): between a horizontal plane extending out from the sensor and the electromagnetic pulse of energy from the antenna to a specific point on the ground Incidence angle (q): between the radar pulse and the normal to Earth�s surface When surface is flat, q = 90��g

15. Polarization 1st letter is transmitted polarization, 2nd is received Can have VV, HH (like) HV, VH (cross)

16. Polarization with visible light In this case, incoming radiation (sunlight) is not polarized (or is polarized in both directions) Vertically polarized light is reflected from surface At this view angle, horizontally polarized light is not So horizontal filter allows us to see the bottom

17. Polarization with radar

18. Radar geometry � is weird, not like cameras or multispectral sensors Uncorrected radar imagery is displayed in slant-range geometry, based on the distance from the radar to each of the respective features in the scene But can also display in ground-range geometry, so that features in the scene are in their proper planimetric (x,y) positions Radar resolution has 2 dimensions, range and azimuth

20. Range resolution

21. Azimuth resolution

22. Foreshortening, layover, shadow

23. Foreshortening In flat terrain, easy to convert a slant-range radar image into a ground-range radar image � but with trees, tall buildings, or mountains, you get radar relief displacement the higher the object, the closer it is to the radar antenna, and therefore the sooner (in time) it is detected on the radar image Terrain that slopes toward the radar will appear compressed or foreshortened compared to slopes away from the radar

24. Foreshortening

25. Layover Extreme case of foreshortening, when incidence angle is less than slope angle toward radar (i.e. ?<a) cannot be corrected got to be careful in the mountains

26. Shadow When slope away from radar is steeper than the depression angle, i.e. �a > ?

27. Speckle Grainy salt-and-pepper pattern in radar imagery Caused by coherent nature of the radar wave, which causes random constructive and destructive interference, and hence random bright and dark areas in a radar image Reduced by multiple looks processing separate portions of an aperture and recombining these portions so that interference does not occur

28. Synthetic aperture radar (SAR) Major advance in radar remote sensing to improve azimuth resolution by synthesizing a long antenna

29. Synthetic aperture radar (SAR)

30. 30 Based on Doppler principle Frequency (pitch) of a wave changes if the receiver and/or source are in motion relative to one another Train whistle has a increasing pitch as it approaches, highest when it is directly perpendicular to the listener (receiver) Point of zero Doppler After train passes by, its pitch will decrease in frequency in proportion to the distance it is from the listener (receiver) This principle is applicable to all harmonic wave motion, including the microwaves used in radar systems

31. Synthetic aperture radar

32. Creation of SAR image

33. 33 Radar equation

34. 34 Radar backscatter coefficient Primary signal of interest Percentage of electromagnetic energy reflected back to the radar from within a resolution cell Depends on terrain parameters like geometry, surface roughness, moisture content, and radar system parameters (wavelength, depression angle, polarization, etc.)

35. Roughness

36. Nile River, Sudan

37. Sources of radar backscattering from a vegetation canopy Subscripts t trunk s soil c leaves m multiple

38. Types of scattering from a pine stand

39. Strength of scattering from a pine stand depends on frequency

40. RADARSAT-2 (launched Dec 2007) C-band radar (5.4 GHz) with HH, VV, HV, and VH polarizations

41. 41 SIR-C/X-SAR web site at JPL SIR-C Spaceborne Imaging Radar-C (following SIR-A in 1981 and SIR-B in 1984) X-SAR X-band Synthetic Aperture Radar (built by Germans) Flew on Shuttle, 2 10-day missions in 1994