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CU-Boulder GEOG 5093 - Lecture Notes

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Review • Single Lens Camera • Multi-Lens Camera • Focal Plane and Focal Length • Aperture or F-Stop • Shutter Speed • Films: panchromatic, Infrared enhancedColor as seen in white light Colors of light absorbed Blue Green & red Green Blue & red Red Blue & green Yellow Blue Magenta green Cyan Red Black Blue, green, red White ? Gray ?Imaging Systems- Passive Sensors1. Basic terms Pixel: The smallest, non-divisible element in an image. The spatial resolution of a sensor is usually represented by a single pixel. Bits: Each digital value is recorded as a series of binary digits known as bits. Brightness or Intensity Level: The number of discrete levels for describing the radiance level of a pixel, scaled from zero brightness to the highest brightness that would be expected. The more discrete levels (or bits of information), the more precisely the radiance of the scene can be measured. We typically deal with 8-bit brightness measurements or 256 levels, also referred to as dynamic range.1. Basic terms Pixel: The smallest, non-divisible element in an image. The spatial resolution of a sensor is usually represented by a single pixel. Bits: Each digital value is recorded as a series of binary digits known as bits. Brightness or Intensity Level: The number of discrete levels for describing the radiance level of a pixel, scaled from zero brightness to the highest brightness that would be expected. The more discrete levels (or bits of information), the more precisely the radiance of the scene can be measured. We typically deal with 8-bit brightness measurements or 256 levels, also referred to as dynamic range. Possible Combinations with: 1 bit: 0 or 1 = 2 or 21 2 bits: 0,0, 0,1 1,0 1,1 = 4 or 22 3 bits: 0,0,0 0,0,1 0,1,0 0,1,1 = 8 or 23 1,0,0 1,0,1 1,1,0 1,1,1 N bits: = 2N1. Basic terms Spectral Band: A spectral band in a digital image represents a narrow slice of radiance in a given wavelength range. The brightness level in a given spectral band is measured using a sensor that is responsive only in that band or by placing a filter in front of a broad band sensor. Instantaneous Field of View (IFOV) : The ground area covered by a solid angle of a satellite sensor. Resolution: Four types of resolutions in remote sensing: (1) spectral: the number and dimension of specific wavelength intervals in the EM spectrum to which the instrument is sensitive. (2) spatial: the smallest angular or linear separation between two objects that can be resolved by the sensor (IFOV).1. Basic terms (3) Temporal: the repeat frequency of information gathered at a specific point. (4) Radiometric: sensitivity of the sensor to different signal strengths in radiant flux.2. Imaging Systems Many electronic (as opposed to photographic) remote sensors acquire data using scanning systems, which employ a sensor with a narrow field of view (i.e. IFOV) that sweeps over the terrain to build up and produce a two-dimensional image of the surface. There are two main modes or methods of scanning employed to acquire multispectral image data - across-track scanning, and along-track scanning.Imaging Systems: Whiskbroom Scanners Artists impression of Spot 5 Across-track scanners scan the Earth in a series of lines. The lines are oriented perpendicular to the direction of motion of the sensor platform (i.e. across the swath). Each line is scanned from one side of the sensor to the other, using a rotating mirror.Imaging Systems: Whiskbroom Scanners • The IFOV (C) of the sensor and the altitude of the platform determine the ground resolution cell viewed (D), and thus the spatial resolution. The angular field of view (E) is the sweep of the mirror, measured in degrees, used to record a scan line, and determines the width of the imaged swath (F). • Because the distance from the sensor to the target increases towards the edges of the swath, the ground resolution cells also become larger and introduce geometric distortions to the images.Whiskbroom Scanners: Dwell Time • The amount of time a scanner has to collect photons from a ground resolution cell: (scan time per line)/(#cells per line) depends on: – satellite speed – width of scan line – time per scan line – time per pixel(down track pixel size / orbital velocity) (cross-track line width / cross-track pixel size) dwell time = [(30m / 7500 m/s)/(185000m / 30m)] =6.5 x 10-7 seconds/pixel This is a very short time per pixel Dwell Time Example: Landsat TMImaging Systems: Pushbroom Scanners • Pushbroom scanners use a linear array of detectors (A) located at the focal plane of the image (B) formed by lens systems (C), which are "pushed" along in the flight track direction (i.e. along track). • Each individual detector measures the energy for a single ground resolution cell (D) and thus the size and IFOV of the detectors determines the spatial resolution of the system.Imaging Systems: Pushbroom Scanners Artists impression of Spot 5(down track pixel size / orbital velocity) (cross-track line width / cross-track pixel size) • denominator = 1.0 • dwell time is longer than that of whiskbroom • but different response sensitivities in each detector can cause striping in the image Dwell Time Example: Pushbroom ScannerWhiskbroom vs. Pushbroom  Wide swath width  Complex mechanical system  Simple optical system  Filters and sensors  Shorter dwell time  Pixel distortion  Narrow swath width  Simple mechanical system  Complex optical system  Dispersion grating and CCDs  Longer dwell time  Less pixel distortionEffect of Scan AngleComputing pixel size tan(angle)= opposite/adjacent β = IFOV Pn = pixel size at nadir H = altitude of satellite tan(β/2) = (Pn/2) / H Pn= 2 H tan(β/2) β"H!Pn!x2!x1!x!x = H tan(θ + β/2)!x2 = H tan(θ - β/2)!x1 = x - x2!Pc = H tan(θ + β/2) - H tan(θ - β/2)!H!H/cosθ =


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