Outline for 4/14/2003

Transcription

1 Sensor Resolution

2 Outline for 4/14/2003 Digital imagery - follow-on from last lecture Spatial Resolution Spectral Resolution Temporal Radiometric Instrument sensitivity

3 Digital Number at-sensor radiance imaging optics detectors electronics DN DN is proportional to at-sensor radiance

4 Resolution and Instrument Response Functions Sensor has finite precision Input signals vary in time and space Sensor has response function (spatial, spectral) input signal output signal convolution w/response function

5 Spatial Resolution A measure of the smallest angular or linear separation between two objects that can be resolved by the sensor. (Jensen, 2000) Resolving power is the ability to perceive two adjacent objects as being distinct size distance shape color contrast characteristics sensor characteristics

6 Instantaneous field of view (IFOV) is the angular field of view of the sensor, independent of height IFOV is a relative measure because it is an angle, not a length b

7 GIFOV Ground-projected instantaneous field of view (GIFOV) depends on satellite height (H) Ê GIFOV = 2H taná IFOV Ë 2 ˆ

8 IKONOS image of Gunnison River Basin, CO 1 kilometer 1 meter resolution 250 meter resolution

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10 Spectral Resolution The width and number of spectral intervals in the electromagnetic spectrum to which a remote sensing instrument is sensitive Allows characterization based on geophysical parameters (chemistry, mineralogy,etc.)

11 Spectral Resolution Determined by: the number of spectral bands spectral response function of each band full-width at half-maximum (FWHM)

12 AVIRIS image of Moffat Field, CA 224 channels from mm 10 nm bandwidth

13 Surface components with very distinct spectral differences can be resolved using broad wavelength ranges

14 Subtle differences require finer spectral resolution vegetation spectral signatures from Jasper Ridge

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16 Radiometric Resolution Number of digital levels that a sensor can use to express variability of brightness within the data Determines the information content of the image The more levels, the more detail can be expressed

17 Radiometric Resolution Determined by the number of bits of within which the digital information is encoded 2 2 = 4 levels 2 8 = 256 levels 2 12 = 4096 levels

18 2 bit radiometric resolution 8 bit radiometric resolution

19 Dynamic Range Saturation Dark Current Signal Image Brightness Actual Sensor Response Scene Brightness Ideal Response

20 Temporal Resolution The frequency of data acquisition over an area Depend on: the orbital parameters of the satellite latitude of the target swath width of the sensor pointing ability of the sensor

21 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 to distinguish it from otherwise similar features

22 Breakup of the Larsen B Ice Shelf Courtesy of Ted Scambos, NSIDC MODIS imagery from January 31, March 6, 2002

23 Signal Strength Depends on Energy flux from the surface Altitude of the sensor Spectral bandwidth of the detector IFOV Dwell time

24 Signal-to-Noise Ratio (SNR) Sensor responds to a both target brightness (signal) and electronic errors from various sensor components (noise) SNR = signal to noise ratio signal noise signal = the actual energy reaching the detector noise = random error in the measurement (all systematic noise has been removed) To be effective, sensor must have high SNR

25 Noise = n  i=1 ( ) 2 DN i - m DN n -1

26 Mean DN = 201 Noise = SNR = 201/1.345 = %

27 Noise Equivalent Radiance or Reflectance A measure of the lowest signal that can be detected just before the signal falls below the level of the noise NEDL or NEDr = the standard deviation of the Mean (of a set of measurements) that produces a SNR of 1