1. An Introduction to Radar and Lidar Remote Sensing
Credit to: Weile Wang
With materials from Drs. Jeff Dozier (UCSB), Howard Zebker (Stanford), Jacob van Zyl (JPL), Alan Strahler (Boston U.), Ralph Dubayah (U. Maryland), Michael Lefsky (U. Colorado), Guoqing Sun (U. Maryland), and many others.
Gustav Klimt ( ), Der Park

2. Outline Radar Basics, PPI, SLAR, SAR, InSAR;
Radar Equation, Imaging Geometry, Geometric Distortion, Speckle, Polarization, Interferometry;
Lidar, Waveform, Footprint, Forest Structure Measurement;
SRTM, LVIS, SMAP, GRACE, DESDynI, and Echidna.

3. 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)

4. Common active remote sensing systems
Radar (RAdio Detection And Ranging)
long-wavelength microwaves (1-100cm)
recording the amount of energy back-scattered from the terrain
Lidar (LIght Detection And Ranging)
short-wavelength laser light (e.g., 0.90 µm)
recording the light back-scattered from the terrain or atmosphere
Sonar (SOund Navigation And Ranging)
sound waves through a water column
recording the amount of energy back-scattered from the water column or the bottom

5. Microwave Bands

6. What is Radar? RADAR = Radio Detection And Ranging
Since radar pulses propagate at the speed of light, the difference to the “target” is proportional to the time it takes between the transmit event and reception of the radar echo

7. Ranging: Distance Measurement? ?
c = speed of light
= 3.00 × 108 m/s

8. Mapping Multiple Objects: PPI Radar Display
PPI=Plan Position Indicator

9. The Radar Equation (1)   Gt is the “Antenna Gain”;
σ is the “cross section” of the target.

10. The radar equation (2)
The radar equation represents the physical dependences of the transmit power, that is the wave propagation up to the receiving of the echo-signals. The power PE returning to the receiving antenna is given by the radar equation, depending on the transmitted power Pt, the slant range R, and the reflecting characteristics of the aim (described as the radar cross-section σ). At known sensibility of the radar receiver the radar equation determines the achieved by a given radar set theoretically maximum range. Furthermore one can assess the performance of the radar set with the radar equation.
Suggest reading:

11. The radar equation (3)
antenna gain: Since a spherical segment emits equal radiation in all direction (at constant transmit power), if the power radiated is redistributed to provide more radiation in one direction, then this results an increase of the power density in direction of the radiation. This effect is called antenna gain.

12. Imaging Radar: Side-Looking Airborne Radar

13. Imaging Geometry
azimuth refers to the along-track dimension parallel to the flight direction.
Swath width refers to the strip of the Earth’s surface from which data are collected by
a side-looking airborne radar (SLAR)

14. Forming an image

15. Radar Reflections from Flat Ground
The Earth plane surrounding a radar antenna has a significant impact on the vertical polar diagram. The combination of the direct and re-reflected ground echo changes the transmitting and receiving patterns of the antenna.

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

17. Radar geometry

18. Range resolution
Calculate Rr

19. Side-looking airborne radar (SLAR)
H is the height of the antenna,    (height of the airplane) L is the geometric length of the antenna, λ is the wavelength of the transmitted pulses, and θ is the incidence angle(1) L · cos θ

20. Azimuth resolution Question:
Why is wavelength important in determining Ra?

21. The equation shows, that with increasing altitude decreases the azimuthal resolution of SLAR. A very long antenna (i.e., large L) would be required to achieve a good resolution from a satellite. Synthetic Aperture Radar (SAR) is used to acquire higher resolution.

22. For an SLAR with the following characteristics: λ = 1 cm, L = 3 m, H = 6000 m, θ = 60°, and tp = 100 ns, has got a resolution of Ra = ??? and Rr = ??? m
Note: The same SLAR on a platform in a height of 600 km would achieve an azimuth-resolution of Ra = ???.

23. Synthetic aperture radar (SAR)

24. A Synthetic Aperture Radar (SAR), or SAR, is a coherent mostly airborne or spaceborne sidelooking radar system which utilizes the flight path of the platform to simulate an extremely large antenna or aperture electronically, and that generates high-resolution remote sensing imagery.
Read

25. The SAR works similar of a phased array, but contrary of a large number of the parallel antenna elements of a phased array, SAR uses one antenna in time-multiplex. The different geometric positions of the antenna elements are result of the moving platform now.
The SAR-processor stores all the radar returned signals, as amplitudes and phases, for the time period T from position A to D. Now it is possible to reconstruct the signal which would have been obtained by an antenna of length v · T, where v is the platform speed. As the line of sight direction changes along the radar platform trajectory, a synthetic aperture is produced by signal processing that has the effect of lengthening the antenna. Making T large makes the „synthetic aperture” large and hence a higher resolution can be achieved.
As a target (like a ship) first enters the radar beam, the backscattered echoes from each transmitted pulse begin to be recorded. As the platform continues to move forward, all echoes from the target for each pulse are recorded during the entire time that the target is within the beam. The point at which the target leaves the view of the radar beam some time later, determines the length of the simulated or synthesized antenna. The synthesized expanding beamwidth, combined with the increased time a target is within the beam as ground range increases, balance each other, such that the resolution remains constant across the entire swath.
The achievable azimuth resolution of a SAR is approximately equal to one-half the length of the actual (real) antenna and does not depend on platform altitude (distance).

26. Radar Image Elements

27. Geometric Distortions (or: Slant-range distortion)
Foreshortening
Layover
Shadow
See handout

28. Slant-range distortion
The slant-range distortion occurs because the radar is measuring the distance to features in slant-range rather than the true horizontal distance along the ground. This results in a varying image scale, moving from near to far range.
Foreshortening occurs when the radar beam reaches the base of a tall feature tilted towards the radar (e.g. a mountain) before it reaches the top. Because the radar measures distance in slant-range, the slope (from point a to point b) will appear compressed and the length of the slope will be represented incorrectly (a' to b') at the image plane.
Layover occurs when the radar beam reaches the top of a tall feature (b) before it reaches the base (a). The return signal from the top of the feature will be received before the signal from the bottom. As a result, the top of the feature is displaced towards the radar from its true position on the ground, and „lays over” the base of the feature (b' to a').
The shadowing effect increases with greater incident angle θ, just as our shadows lengthen as the sun sets.

29. Foreshortening

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

31. Shadow
When slope away from radar is steeper than the depression angle, i.e. –α > γ

32. Speckle: Random Interference
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

33. Roughness

34. Sources of radar backscattering from a vegetation canopy
Question:
Does the strength of the backscattering vary with frequencies?

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

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

37. Polarization with radar

38. Polarization with radar
RADARSAT, C-band radar (5.4 GHz) with HH, VV, HV, and VH polarizations

39. InSAR: Adding the Z-dimension
Landsat overlaid on topography from SRTM – Malaspina Glacier, Alaska

40. InSAR Geometry
Can you derive the equation? Extra credit (point)

42. The following materials are FYI. Not required for exam.

43. Shuttle Radar Topography Mission
Links to movies

44. SRTM Global Coverage

45. SRTM Elevation + Landsat Imagery
Perspective with Landsat Overlay: Antelope Valley, California

46. From Radar to Lidar LIDAR = Light Detection And Ranging
Using laser instead of microwave

47. Measuring Forest Structure

48. Continuous Waveform, Large Footprint

49. Discrete Waveform, Small Footprint

50. Canopy Topography

51. Ground-Based Lidar (Echidna)

52. A real Echidna—in the forest

53. Data Examples

54. Airborne Lidar Instrument: LVIS

55. Space-borne: ICESat and GLAS

56. Synthesis of Lidar, Radar, and optical sensors

57. Other Relative Sensors: GRACE
GRACE: Gravity Recovery And Climate Experiment

58. Soil Moisture Active & Passive (SMAP)

59. SMAP Instruments Radar Frequency: 1.26 GHz Polarizations: VV, HH, HV
Data collection:
High-resolution/high-rate data collected for ground SAR processing
Low-resolution real-aperture data collected continuously
Radiometer
Frequency: 1.41 GHz
Polarizations: H, V, U
Relative accuracy: 1.3 K
Data collection: Continuous over full scan

60. DEformation, Ecosystem Structure, and Dynamics of Ice

61. DESDynI Instruments Multi-beam Lidar L-Band Synthetic Aperture Radar
4. Instrument Design & Performance
L-Band Synthetic Aperture Radar
Lasers
Laser
Radiators
Star Tracker
Multi-beam Lidar
Beam Spacing
1 km
Interferometric SAR
Dual-Pol 3-Beams
Quad-Pol 6-Beams
Right or Left Point
Flight
Direction
~350km

62. Summary Radar Basics, PPI, SLAR, SAR, InSAR;
Radar Equation, Imaging Geometry, Geometric Distortion, Speckle, Polarization, Interferometry;
Lidar, Waveform, Footprint, Forest Structure Measurement;
SRTM, LVIS, SMAP, GRACE, DESDynI, and Echidna.

63. For an SLAR with the following characteristics: λ = 1 cm, L = 3 m, H = 6000 m, θ = 60°, and tp = 100 ns, has got a resolution of Ra = 40 m and Rr = 17.3 m
Note:

64. Homework-8 Derive the radar equation.
Derive the raindrop size equation from the radar equation you derived in question 1.
The SLAR on a platform in a height of 600 km would achieve an azimuth-resolution of Ra = ?. (other needed variable are the same given in class activity)
(extra credit) NASA Tropical Rainfall Measuring Mission (TRMM) has a single frequency radar at the Ku-band 13.8 GHZ particularly sensitive to moderate rain rates. With a single frequency, the TRMM radar is able to retrieve drop size. Assume that raindrops range from 1/100 inch (.0254 centimeter) to 1/4 inch (.635 centimeter) in diameter. Antenna Gain is 1.698, instrument size is 0.5 m, plot the relation of ratio of Pt/Pr vs. raindrop size, assume height of rain is 1 km.