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Sensor Resolution
Outline for 4/14/2003
• Digital imagery - follow-on from last lecture
• Spatial Resolution
• Spectral Resolution
• Temporal
• Radiometric
• Instrument sensitivity
Digital Number
imaging opticsdetectors
electronics
at-sensorradiance DN
DN is proportional to at-sensor radiance
Resolution and InstrumentResponse Functions
• Sensor has finite precision
• Input signals vary in time and space
• Sensor has “response function” (spatial,spectral)
inputsignal
outputsignal
convolutionw/responsefunction
Spatial Resolution• “A measure of the smallest angular or linear
separation between two objects that can beresolved by the sensor”. (Jensen, 2000)
• Resolving power is the ability to perceive twoadjacent objects as being distinct– size– distance– shape– color– contrast characteristics– sensor characteristics
• Instantaneous field of view (IFOV) is theangular field of view of the sensor,independent of height
• IFOV is a relative measure because it is anangle, not a length
b
GIFOV
• Ground-projected instantaneous field of view(GIFOV) depends on satellite height (H)
†
GIFOV = 2H tan IFOV2
Ê
Ë Á
ˆ
¯ ˜
1 meter resolution 250 meter resolution
IKONOS image of Gunnison River Basin, CO1
kilo
met
er
Spectral Resolution
• The width and number of spectral intervals inthe electromagnetic spectrum to which aremote sensing instrument is sensitive
• Allows characterization based on geophysicalparameters (chemistry, mineralogy,etc.)
Spectral Resolution
• Determined by:– the number of spectral bands
– spectral response function of each band
– full-width at half-maximum (FWHM)
AVIRIS image of Moffat Field, CA
224 channels from 0.4 - 2.5 mm10 nm bandwidth
• Surface components with very distinctspectral differences can be resolved usingbroad wavelength ranges
vegetation spectral signatures from Jasper Ridge
Subtle differences require finer spectral resolution
Radiometric Resolution
• Number of digital levels that a sensor can useto express variability of brightness within thedata
• Determines the information content of theimage
• The more levels, the more detail can beexpressed
Radiometric Resolution
• Determined by the number of bits ofwithin which the digital information isencoded
22 = 4 levels28 = 256 levels212 = 4096 levels
2 b
it r
adio
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8 b
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Imag
e B
right
ness
Scene Brightness
DynamicRange
ActualSensorResponse
IdealResponse
DarkCurrentSignal
Saturation
Temporal Resolution
• The frequency of data acquisition overan area
• Depend on:– the orbital parameters of the satellite
– latitude of the target
– swath width of the sensor
– pointing ability of the sensor
• Multi-temporal imagery is important for– infrequent observational opportunities (e.g.,
when clouds often obscure the surface)
– short-lived phenomenon (floods, oil spills,etc.)
– rapid-response (fires, hurricanes)
– detecting changing properties of a feature todistinguish it from otherwise similar features
Breakup of the Larsen B Ice Shelf
MODISimagery fromJanuary 31, 2002-March 6, 2002
Courtesy of Ted Scambos, NSIDC
Signal Strength
• Depends on– Energy flux from the surface
– Altitude of the sensor
– Spectral bandwidth of the detector
– IFOV
– Dwell time
Signal-to-Noise Ratio (SNR)Sensor responds to a both target brightness
(signal) and electronic errors from varioussensor components (noise)
SNR = signal to noise ratio
signal = the actual energy reaching the detectornoise = random error in the measurement (all
systematic noise has been removed)
To be effective, sensor must have high SNR†
signalnoise
†
Noise = iDN - mDN( )2
n -1i=1
n
Â
200201199203202201200
50%
Mean DN = 201Noise = 1.345SNR = 201/1.345 = 149
Noise Equivalent Radiance orReflectance
• A measure of the lowest signal that can bedetected just before the signal falls below thelevel of the noise
NEDL or NEDr = the standard deviationof the Mean (of a set of measurements)that produces a SNR of 1
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