1. RADAR Remote Sensing

2. Introduction A Quick Reminder Passive RS Systems: Active RS Systems
Record energy reflected (visible, NIR) and emitted (TIR) from the Earth’s surface
Active RS Systems
Transmit their own EM energy to Earth and record backscattered energy

3. Introduction Most widely used active RS systems RADAR LIDAR SONAR
Radio Detection and Ranging
Microwaves (long waves)
LIDAR
Light Detection and Ranging
Laser light waves (short waves)
Topographic mapping, etc.
SONAR
Sound Navigation and Ranging
Sound waves (long waves)
Bathymetric mapping, etc.

4. RADAR Basics Two types of RADAR data collection systems Active:
Record ‘artificially generated’ microwave energy
Passive:
Record microwave ‘naturally emitted’ microwave energy

5. History of RADAR RS James Clerk Maxwell (1831-1879)
Provided mathematical descriptions of magnetic and electric fields associated with EM radiation
Heinrich R. Hertz ( )
Provided fundamental knowledge of creation and propagation of EM energy in microwave and radio portions of EMS
His studies on interaction of radio waves with metallic surfaces eventually facilitated the invention of radios and radars
Guglielmo M. Marconi ( )
Used above fundamental physics principles to construct the first antenna to transmit and receive radio waves
1901: Radio waves across the Atlantic
1909: Novel Prize in physics

6. History of RADAR RS A. H. Taylor and L.C. Young 1922
Realized that radio signals might be useful for detecting the distance to ships at sea, etc. and both at night and in bad weather
Today, we use almost only microwaves but the term RADAR was never changed
1935
Combined antenna transmitter and receiver in one instrument
Groundwork for RADAR development during WW II for navigation and target location (back then: no RADAR imagery from air- or spacecraft)

7. History of RADAR RS RADAR Imagery SLAR: Side-Looking Air-borne Radar
Used since 1950s by military
Some systems declassified since the 1960s
Continuous-strip mapping capability
Reconnaissance over vast regions to the left or right of aircraft: Long-range standoff data collection
Radargrammetric measurement
Science of extracting quantitative geometric object information from RADAR imagery

8. History of RADAR RS Two SLAR Types
Real Aperture Radar (AKA “brute force radar”)
Use antenna of fixed length (e.g., 1 – 2 m)
Synthetic Aperture Radar (SAR) (aperture = antenna)
Also use ‘fixed-length” antenna but are able to synthesize a much larger antenna (by sensing a greater number of beams)
Example:
An 11-m antenna on an orbital platform can be synthesized electronically to have a synthetic length of 15 km
The longer the antenna, the better the resolving power

9. History of RADAR RS Since 1960s and 1970s
Various SARs launched for Earth resource reconnaissance
Particularly useful in areas that have a more or less perennial cloud cover (e.g., tropical areas)
1978: SEASAT (USA)
1981; 1984; 1994: SIR-A; SIR-B; SIR-C/X-SAR (USA)
1991: ALMAZ-1(USSR)
1991; 1994: ERS-1; ERS-2 (ESA)
1992: JERS-1(Japan)
1995; 2006: RADARSAT; RADARSAT2 (Canada)
2000: SRTM (USA)
2002: Envisat; ASAR (ESA)

10. Advantages of RADAR RS All-weather RS system
Certain microwave frequencies penetrate cloud cover
Provides synoptic views of large areas for mapping
1:10,000 – 1:400,000
Coverage can be obtained at user-specific times
Day and night
Provides different perspectives that cannot always be obtained using aerial photography
Permits imaging at shallow look angles
Angle of illumination can be controlled (has its own illumination)
Provides information on surface roughness, dielectric properties, and moisture content
Senses in wavelengths outside the optical regions of the EMS,

11. Advantages of RADAR RS
May penetrate vegetation, sand, and snow surface layers
Enables resolutions to be independent of distance to object
Size of a resolution cell may be as small as 1 m
May produce images from different types of polarized energy
HH, HV, VV, VH
Has multi-frequency potential
May operate simultaneously in several wavelengths (frequencies)
Can measure ocean wave properties, even from orbital altitudes
Can produce overlapping images suitable for stereoscopic viewing
Can produce overlapping images suitable for radargrammetry
Supports interferometric operation using two antennas
3-D mapping
Analysis of incident-angle signatures of objects

12. Disadvantages of RADAR RS
Can be very noisy and difficult to classify
Large amounts of geometric relief displacement
Geometric relief displacement inconsistent across the image

13. RADAR: How does it work?
Microwave energy is transmitted toward an area from an antenna in very short bursts or pulses
Energy is reflected off of the area (backscattered) and recorded at the same antenna after a period of time
Propagation of one radar pulse

14. RADAR: How does it work?
Strength (detection) and time delay (ranging) of return signals is measured
Range or distance between the transmitter and the area are inferred from time elapsed between transmission and reception
Resulting antenna return

15. Typical SLAR System Components
Canberra IFSAR
System Layout
(Source: Original figure by H.A.Malliot, High Altitude Mapping Missions, Inc.)

16. Wavelength, Frequency, Pulse Length
Pulse of EM radiation transmitted through antenna
Has a specific wavelength
Much longer than TIR
Measured in centimeters (cm)
Names of radar wavelengths:
Alphabetic descriptor, not actual wavelength or frequency
Relicts of secret work on radar RS in WW II
Has a specific duration
Pulse length
Measured in microseconds (msec)

17. Wavelength, Frequency, Pulse Length
RADAR Band l
(cm) n
(GHz) Ka
0.75 – 1.18
40 – 26.5 K
1.19 – 1.67
26.5 – 18 Ku
1.67 – 2.4
18 – 12.5 X
2.4 – 3.8
12.5 – 8 C
3.9 – 7.5
8 – 4 S
7.5 – 15
4 – 2 L
15 – 30
2 – 1 P
30 – 100
1 – 0

18. Wavelength, Frequency, Pulse Length
Ka, K, and Ku bands:
Shortest wavelengths
Partially absorbed by water vapor, thus limiting cloud penetration
Used by ground-based weather radars to track cloud cover and precipitation
X-band (SIR-C; SRTM)
Shortest wavelength used by any orbital/ sub-orbital imaging radar
C-band (SIR-C; ERS 1 & 2; RADARSAT 1 & 2; SRTM; Envisat, ASAR)
S-band (ALMAZ-1)
L-band (SEASAT, SIR-A, B, C; JERS-1)
P-band
Longest radar wavelengths

19. Azimuth, Range Direction, etc.
Ka, K, and Ku bands:
Shortest wavelengths
Partially absorbed by water vapor, thus limiting cloud penetration
Used by ground-based weather radars to track cloud cover and precipitation
X-band (SIR-C; SRTM)
Shortest wavelength used by any orbital/ sub-orbital imaging radar
C-band (SIR-C; ERS 1 & 2; RADARSAT 1 & 2; SRTM; Envisat, ASAR)
S-band (ALMAZ-1)
L-band (SEASAT, SIR-A, B, C; JERS-1)
P-band
Longest radar wavelengths

20. Other Important Parameters
Azimuth Direction; Range Direction; Depression Angle; Look Angle; Incident Angle; Polarization Φ
Look angle θ
Incident angle
Slant
range
Nadir
Swath

21. Other Important Parameters
Pulse length determines range resolution
Beam width determines azimuth resolution
System
Resolution
Nadir
Swath Φ
Look angle θ
Incident angle
Slant
range

22. Other Important Parameters
Azimuth Direction
Straight line in which the aircraft travels
Range / Look Direction
Direction in which active microwave energy illuminates strips of the terrain (orthogonal to the aircraft’s azimuth direction)
Near-range: terrain illuminated nearest the aircraft
Far-range: terrain illuminated farthest away from the aircraft
Significantly affects feature interpretation
Objects that trend (or strike) in a direction perpendicular to the range direction are more enhanced/emphasized than objects that lie parallel to it

23. Other Important Parameters
Effect of Range Direction on Radar Imagery
Example: X-band image of the Kaduna State in Nigeria
East-west azimuth direction with RADAR looking north
East-west azimuth direction with RADAR looking south
Note: Orient image so that the look direction is toward you. That way, the shadows fall toward you, keeping you from experiencing pseudoscopic illusion.

24. Other Important Parameters
Depression Angle (γ)
Angle between a horizontal plane extending out from the aircraft fuselage and the EM pulse of energy from the antenna along the radar line-of-sight to a specific point on the ground
Varies from a near-range to a far-range depression angle
Look Angle (Φ)
Angle between the vertical from the antenna to the ground and the radar line-of-sight
Varies from a near-range to a far-range look angle
Complement of the depression angle

25. Other Important Parameters
Incident Angle (θ)
Angle between the radar pulse of energy and a line perpendicular to the Earth’s surface where it makes contact
Complement of the depression angle where the terrain is flat (often assumed in radar studies)
Incident angle in terms of the relationship between radar beam and surface slope:
Vertical
Normal
to surface
Radar
wave
Incident angle, θ
Local slope angle, α
Local
incident
angle
Scattering surface

26. Other Important Parameters
Polarization
Unpolarized energy vibrates in all possible directions perpendicular to the direction of travel
Radar antennas send and receive polarized energy ( Pulse of energy is filtered)
Four Polarizations
Like-polarized (co-polarized):
Send and receive horizontally polarized energy (HH)
Send and receive vertically polarized energy (VV)
Cross-polarized:
Send horizontal and receive vertically polarized energy (HV)
Send vertical and receive horizontally polarized energy (VH)

27. RADAR Polarization
Cinder cone and basalt lava flow, AZ

28. Slant- vs. Ground Range Geometry
Uncorrected imagery is displayed in slant-range geometry
Based on actual distance from the radar to features in the scene
Near-range objects are more compressed than far-range objects
May be converted to ground-range geometry so that features in the scene are in their proper planimetric position (See book.)

29. RADAR Issues Geometric distortions exist in almost all RADAR imagery
No problem in flat terrain
Simple algorithms for converting slant-range to ground-range geometry
Problem in variable terrain
RADAR image is formed in the range (cross-track) direction
The higher the object, the closer it is to the antenna, and the sooner it is detected on the radar image  Displacement toward the antenna (vs. air photos where relief displacement is away from the PP)
Foreshortening & Layover = Elevation-induced distortions
Shadows
Speckle

30. RADAR & Topography Foreshortening
In high relief areas, slopes facing the sensor will appear compressed and their lengths will be incorrect

31. RADAR & Topography Layover Extreme case of foreshortening
When the return signal from the top of a feature is received before that of the base
Object will appear to lean toward the sensor

32. RADAR & Topography Radar Shadow
The down range dimension of a tall object is not illuminated by the sensor so that no energy is available to be backscattered

33. RADAR & Topography Speckle
Grainy salt-and-pepper pattern in RADAR imagery
Due to coherent nature of the RADAR wave, which causes random constructive and destructive interference, hence random bright and dark areas in an image
Can be reduced but at the cost of degraded resolution (LAB)
1-look radar image
4-look radar image
16-look radar image
Radar speckle reduction using multiple-look techniques

34. ? Any questions ?