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

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1 meter resolution 250 meter resolution

IKONOS image of Gunnison River Basin, CO1

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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

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Imag

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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

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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