InSAR measurements show a section of a city is subsiding at 2 cm per year. What physical principle allows SAR to measure such small ground movements from 700 km away?
AThe SAR measures changes in backscatter intensity caused by surface compression
BInterferometry compares the phase of radar signals from two passes; the phase difference encodes the change in sensor-to-surface distance with sub-wavelength precision
CThe SAR detects changes in surface roughness caused by subsidence
DDoppler frequency shifts from the moving ground surface are measured directly
InSAR compares the phase of radar returns from two or more passes. If the ground moved between passes, the path length changes, producing a measurable phase difference. Since phase can be measured to a fraction of the wavelength (e.g., 5.6 cm for C-band), displacements of millimeters to centimeters are detectable.
Question 2 True / False
SAR images appear similar to photographs and can be interpreted using the same visual principles.
TTrue
FFalse
Answer: False
SAR imagery looks fundamentally different from optical imagery. Backscatter depends on surface roughness, not color. Smooth surfaces (calm water) appear dark. Urban areas show extreme brightness from corner reflectors. Layover, foreshortening, and shadow artifacts distort terrain geometry. Speckle noise creates a granular texture absent from optical images.
Question 3 Short Answer
Why does C-band SAR (~5.6 cm) interact primarily with leaves and small branches, while L-band SAR (~23 cm) penetrates the canopy to interact with trunks?
Think about your answer, then reveal below.
Model answer: Radar backscatter is strongest when scattering elements are comparable in size to the wavelength. C-band interacts with canopy elements of similar dimensions -- leaves and twigs -- and is largely scattered by the upper canopy. L-band wavelengths pass through small elements and interact with larger structures (trunks, major branches) and the ground beneath. This wavelength-dependent penetration is why L-band is preferred for forest biomass estimation.
Short-wavelength radar scatters off small canopy elements while longer wavelengths penetrate to interact with larger structures below.