CH Remote Sensing & Satellite Imagery 2 — Questions and Answers
Question 1: Converting raw satellite digital number values to surface reflectance is accomplished through:
- Georeferencing and map projection
- Radiometric calibration followed by atmospheric correction (Correct answer)
- Spatial resolution enhancement (pansharpening)
- Band ratioing and index calculation
Correct answer: Radiometric calibration followed by atmospheric correction
Radiometric calibration converts raw digital numbers to at-sensor radiance, and atmospheric correction then removes the effects of atmospheric scattering and absorption to yield true surface reflectance.
Question 2: ICESat-2, used for shallow water depth mapping, employs which primary technology?
- Radar altimetry with Ku-band signal
- Photon-counting green-wavelength LiDAR (ATLAS instrument) (Correct answer)
- Multispectral push-broom imaging
- Synthetic Aperture Radar interferometry
Correct answer: Photon-counting green-wavelength LiDAR (ATLAS instrument)
ICESat-2 carries the Advanced Topographic Laser Altimeter System (ATLAS), a photon-counting LiDAR using 532 nm green light capable of penetrating shallow, clear water to measure bathymetry.
Question 3: What is the primary limitation of optical satellite imagery when used for bathymetric surveys?
- Insufficient spatial resolution for most coastal features
- Water turbidity and depth restrict usable optical penetration (Correct answer)
- Inability to be corrected for atmospheric effects
- Revisit interval is too long for tidal cycle capture
Correct answer: Water turbidity and depth restrict usable optical penetration
Suspended sediment, algae, and colored dissolved organic matter absorb and scatter light rapidly, limiting optical satellite bathymetry to clear, shallow water typically shallower than 20–25 m.
Question 4: WorldView-2 is particularly valuable for coastal bathymetric mapping because it provides:
- SAR capability and 1-meter resolution in all weather
- Eight multispectral bands including a coastal blue band with sub-meter panchromatic resolution (Correct answer)
- Daily revisit time with 30-meter multispectral resolution
- Thermal infrared bands for sea surface temperature
Correct answer: Eight multispectral bands including a coastal blue band with sub-meter panchromatic resolution
WorldView-2's eight spectral bands — including the 400–450 nm coastal blue band ideal for water penetration — combined with approximately 0.5 m panchromatic resolution make it well suited for high-detail shallow water mapping.
Question 5: In multispectral satellite-derived bathymetry, which wavelength range penetrates deepest into clear oceanic water?
- Red band (~650 nm)
- Near-infrared band (~850 nm)
- Blue band (~450 nm) (Correct answer)
- Green band (~550 nm)
Correct answer: Blue band (~450 nm)
Water absorbs longer wavelengths (red, near-infrared) very rapidly; blue light (~450 nm) experiences the least absorption in clear oceanic water and therefore penetrates to the greatest depth.
Question 6: The primary purpose of atmospheric correction when processing satellite imagery for hydrographic applications is to:
- Increase the spatial resolution of the sensor data
- Remove the distorting effects of atmospheric scattering and absorption on measured radiance (Correct answer)
- Merge overlapping imagery from adjacent satellite swaths
- Convert geographic coordinates to a projected coordinate system
Correct answer: Remove the distorting effects of atmospheric scattering and absorption on measured radiance
Atmospheric correction reverses the effects of gases and aerosols that scatter and absorb electromagnetic radiation on its path between the Earth's surface and the satellite sensor, yielding true surface reflectance.
Question 7: With both Sentinel-2A and Sentinel-2B satellites operating, the approximate revisit interval at the equator is:
- 1 day
- 5 days (Correct answer)
- 16 days
- 26 days
Correct answer: 5 days
Each Sentinel-2 satellite has a 10-day repeat cycle, but with two satellites phased apart the combined revisit time is approximately 5 days at the equator, improving temporal monitoring capability.
CH Remote Sensing & Satellite Imagery 2
Converting raw satellite digital number values to surface reflectance is accomplished through: