9 Radar Remote Sensing Flashcards
Radar (Radio detection and ranging)
- Radar is an object detection system
- It uses electromagnetic wave in microwave range
- Determines range, angle, or velocity of objects
Radar remote sensing
•Active sensors
–Has its own source of energy
–Independent of sun illumination
•Large wavelength of microwave allows penetration through clouds and (partially) canopy, soil, and snow
- notes
- longer wavelength of microwave can do better penetration
- the big issue in optical RS is cloud coverage
Two types of Radar Remote Sensor
- Radar altimeters
2. Imaging radar
Radar Remote Sensor
Radar altimeters
•Radar altimeters use nadir looking radar to measure
–Sea level height
–Wave height
–Wind speed / Sea surface roughness
•Sentinel 6 is a radar altimeter sensor
*notes
the difference between radar altimeter and the sensors we talked previously like optical sensor is that radar alitimeters dont provide images. As they travel along their orbit, they measure points. Along the line, we have several point measurements of sea level height, for instance.
Radar altimeters use nadir looking radar to measure:
–Sea level height # the average time that it takes for the signal to go 2 ways path will determine the water level
–Wave height # Waves that are recorded by the sensor is the sum of all these pulses that bounce from the water. the higher the water level is, the shorter the time interval. from the green line, we get the first returning signal to the sensor. and from the red line, we get the last pulse. Based on the slope (blue), we can determine how high the waves are. # We have higher waves here, so it take longer time for the sensor to record all theses waves form.
–Wind speed / Sea surface roughness # Low wind results in smooth surface while strong wind results in rough surface. because smooth surface acts like a mirror, almost all signal that is sent, bounce back and received by the sensor, making high magnitude of power recorded by the sensor. # When the sea surface is rough, signal that is sent bounce to many directions, resulted in low magnitude recorded by the sensor. # Comparing the power magnitude, we can determine the sea surface roughness
Imaging radar
•Imaging radar is a side looking
–Left and right areas are not distinguishable for nadir looking radar
-imaging radar sends pulse off nadir direction. it has angle.
- swath = width area that is illuminated by the sensor
- ground range = in ground range we can improve the spatial resolution of sensor easier.
- While in azimuth direction, the spatial resolution depends on 3 parameters (wavelength Lamda, antenna length L, range distance R). Azimuth direction = same as flight direction
Synthetic Aperture Radar (SAR)
•Azimuth resolution depends on the wavelength ( l ), antenna length (L), Range distance (R)
•SAR sensors use the motion of the radar platform to synthesize a large antenna and
improve the azimuth resolution (can be better than 1 m)
Synthetic Aperture Radar (SAR)
Two quantities are measured by the antenna
–SAR Amplitude
•Function of Surface properties (Topography, roughness, soil moisture,
–SAR Phase
•Function of the distance between sensor and ground
•Proportional to the two way travel distance divided by the wavelength (measured the phase difference between transmitted and response pulses)
•is measured modulo 2phi
SAR distortions
Because of the side-looking geometry and topography of the terrain, SAR imaging is affected by distortions
–Shadow
terrain that do not illuminated by the sensor (appear dark)
–Foreshortening
Terrain that exposed to sensor illumination, get so much energy, appears to be so bright and squeezed
–Layover
area closer to the sensor is projected in a way it is farther from the sensor. appears bright, high energy, and difficult to distinguish to Foreshortening
SAR amplitude
High amplitude
- if you have corner, much amplitude will return to the sensor.
- Urban area, rough surface have high amplitude.
Low amplitude
- Water, wet soil have low amplitude.
- area appears dark in the radar image bcs not much amplitude return to sensor
Application of SAR amplitude
Clement
Flood monitoring
#Soil moisture monitoring from the graph, lower amplitude when the soil is dry and higher amplitude when the soil is wet (???)
Differential Interferometric SAR (DInSAR)
used to measure the distance between sensor and the ground
- DInSAR uses two images acquired from almost the same point in the space
- Amplitude are used to coregister two images
- Phase is used to generate the interferogram
Interferogram = PMaster - PSlave
•Topography map generation
–Large perpendicular baseline is desirable
*notes
to make topography maps, select image where we have large topography. Resulting in more sensitive interferogram to topography change and more phase changing -> more accurate topography maps
•Displacement map generation
–Small perpendicular baseline is desirable
–Each color cycle (fringe) = ½ wavelength displacement
*notes
if we want to know deformation, then we have to remove topography -> smaller perpendicular baseline is desirable
InSAR Applications
# earthquake #land subsidence #volcanic eruption
Multi-temporal InSAR
- Increase in temporal/spatial baselines reduces the quality of interferograms
- Stacking several interferograms can increase signal to noise
*notes
persistant scatterers = big building e.g.
small scatterers
InSAR time series analysis
• Permanent/Persistent scatterers Interferometry (PSI) approach
- Works for urban areas where many strong scatterers are found
• Small Baseline Subset (SBAS) approach
- For non-urban areas where distributed scatterers are found
