Microwave Remote Sensing - Part 2 Flashcards
Explain why radar imagers are side-looking and what problem a synthetic aperture radar sensor solves
- to address the left-right ambiguity problem
- there is no left-right ambiguity and the phase shift of the pulses can be used to set the proper range of the objects
Define backscatter
- backscatter is a physical quantity that is a function of the brightness or intensity of the returned microwave signal and the local incidence angle
explain how the target geometric and electric properties affect the scattering mechanisms
- scattering is affected by the electrical properties of the surface, which are strongly influenced by moisture, and geometric properties characterized by roughness (1. smooth surfaces will backscatter very little energy and thus will appear dark in a SAR image of the area 2. if the surface is rough, more of the energy will be scattered back towards the sensor and the measured intensity of the microwave radiation will thus be higher, resulting in targets that appear brighter in the image)
surface roughness is relative to…
the wavelength of the microwave radiation - the longer the wavelength, the deeper the microwave radiation will penetrate into virtually all Earth objects
Explain why speckle occurs in microwave images
noise in the backscatter - unavoidable consequence of the way microwaves interact with real Earth objects (radiation will interact with plants, others with soil, and other scene constituents)
These individual scatters will reflect the microwave radiation in essentially random ways, and it is the sum of these interactions that is sensed as the returned microwave energy at the sensor
ways to deal with speckle
- If the application allows multiple microwave images of the same place to be used, then so-called multi-looking procedures can all but eliminate this effect
- apply filtering processes that attempt to smooth the noise out using statistical properties of multiple pixel windows
general comment about speckle
This is just part of life when it comes to microwave remote sensing. Unless the entire area comprising a pixel element and surrounding area is perfectly homogenous, then we will end up with interactions between different scattering elements that will tend to lead to random increases and decreases in the signal as the multiple out of phase waves interact
Describe the unique types of imaging distortions caused by topography when using side-looking radar systems
terrain distortions
- Foreshortening: when the backscatter from a large area on the sensor facing side of the slope return to the sensor at around the same time
- Layover: returns from the front and back sides of the slope are received at roughly the same time
- Shadow: steep slopes relative to the look angle prevent microwaves from interacting with the opposite side of the mountain at all
Define InSAR (SAR Interferometry) and describe how it works
- InSAR uses the phase information from two SAR images of the same area collected at different times. By comparing shifts in the phase for the same place, the displacement of the surface can be measured extremely accurately, down to less than a centimeter
- measure changes that may accompany changes in the surface morphology caused by earthquakes for example
- can measure surface changes less than a centimeter from 700 km away in space
microwave sensors measure…
and the intensity or amplitude of the signal, and the phase of the signal (allows us to record where on earth the object doing the reflecting is)
left-right ambiguity
If the transmitter was pointed straight down, then two objects that are the same distance from the transmitter, but on opposite sides of the transmitter would return the microwave energy at precisely the same time. Thus, there’d be no way of locating the pulse as coming from the left or right hand side of the transmitter
the spatial resolution that is achievable with imaging radar systems is …
inversely proportional to the size of their antenna
The reason that we can still achieve moderate to high spatial resolution imaging of Earth with practical antenna sizes is through …
the magic of synthetic aperture radar systems
(because the sensor and transmitter system is moving, each target object will be exposed to multiple different pulses of microwave energy. The modestly sized antenna that is actually onboard the sensor will record the information from the same target multiple times, and these measurements can be combined together to simulate what the measurement would’ve been if the antenna was much larger)
phase
tells us how far away the target was in the direction of the transmitted microwave
phase of the signal used to put the measurements in the right place along the slant range, and eventually, with corrections along the ground range in constructing an image
phase information can also be used to measure changes in the range overtime
