Remote Sensing
1
Active Remote Sensing
Radio Detection and Ranging = Radar
Light Detection and Ranging = Lidar
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
o Imaging Radar uses microwave wavelengths
o Lidar uses visible wavelength (laser) or sometimes NIR
Oil spill off NW coast of Spain
IKONOS image
Oil reaching shore
1
2
3
2
Radar image of oil spill off NW coast of Spain
(Black areas = oil)
Side-looking Radar
• Most radar systems do look off to the side (off-nadir)
o For military applications allows planes to fly over friendly territory and look into enemy territory
o More importantly, gives us more info about surface characteristics
• Radar is sensitive to surface roughness
Side-looking Radar
4
5
6
3
Side-looking Radar
Radar Terminology
• Swath width = range
• Direction of flight = azimuth
• Backscatter = reflectance
• Angle of view = depression angle
• Etc.—unique terminology
Depression Angle
Radar Geometry
7
8
9
4
Radar Geometry
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)
Radar Disadvantages
• Developed by military, less civilian experience so far than passive remote sensing
• Difficult to interpret—complicated causes of reflectance
• Geometric distortions caused by side looking geometry
10
11
12
5
Radar bands were originally code names assigned by the military:
P-band
L-band
S-band
C-band
X-band
K-band
Radar Bands
Radar penetration increases with wavelength
Radar Bands
Interpretation of Radar Data
• Surface “smoothness” or “roughness” with respect to radar depends on wavelength and incident angle
o A smooth surface reflects in one direction (specular)
o A rough surface scatters radiation in all directions (Lambertian or diffuse)
o Rough surfaces tend to depolarize radiation
13
14
15
6
Surface Roughness
Shorter wavelength radar can resolve roughness more finely
Interpretation of Radar Data
Type of Backscatter (Reflection)
• Diffuse o Vegetation canopies
o Cross polarization (use HH and HV)
• Specular o Microwaves reflected away (not scattered) (e.g., oil slick)
• Corner reflectors o Tree trunks and buildings
o can make objects very “bright” or very “dark” depending on orientation
Surface roughness is described as a function of wavelength and the angle of incidence of the incoming radiation
Interpretation of Radar Data
16
17
18
7
Interpreting Radar Data
• Longer wavelength bands (P and L bands) o Penetrate canopy and reflects off of standing tree trunks
o Can detect amount of wood in a forest
o Estimate forest biomass
• Shorter wavelengths (C and X bands) o X band can detect leaves
o C (and L) band can detect twigs
Resolving Objects with Radar
• Radar resolution determined by…
o Size of area “illuminated” by microwaves at one time
• Depends on antenna length (longer antenna = better spatial resolution)
o But…difficult physically for airplane or satellite to carry a very large antenna.
• Depends on length of radar pulse (longer pulse = lower spatial resolution)
Pulse length in part determines radar resolution
Interpretation of Radar Data
19
20
21
8
Real Aperture vs. Synthetic
Aperture Radar (SAR) • Real aperture radar actually uses a single
antenna of a given length – resolution limited to what plane or satellite can carry.
• Synthetic Aperture Radar (SAR) can simulate a large antenna by taking advantage of the Doppler effect o 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.
Synthetic Aperture Radar – Doppler Effect
Interpretation of Radar Data
Radar Sensors • There are many imaging radar sensors available,
both airborne and on satellites o Most aircraft use SAR
o All satellites use SAR (to achieve reasonable spatial resolution)
22
23
24
9
Radar Sensors
Radar Applications • Forest inventory
• Oceanography
• Archaeology
• Sea ice studies
• Digital Elevation Models (DEMs)
• Urban mapping
• Wildfire studies
Radar Image – Angkor, Cambodia. Purple spots are possible buried ruins
Radar Applications
25
26
27
10
Radar image of oil slicks
Radar Applications
Radar derived elevation with TM draped over it
Radar Applications
Kamchatka Peninsula – Shuttle Radar Topography Mission (SRTM)
(Mission generated detailed topographic data for 80% of earth’s land surface)
Radar Applications
28
29
30
11
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
High-resolution
LIDAR topography
Lidar Remote Sensing
Lidar vs. DEMs from topo sheets
31
32
33
12
landslides
Southern Bainbridge Island
34
35