GEOG/GEOL 4093 Remote Sensing of the Environment Lecture 9 Outline of today’s lecture • Review • Imaging Systems- Passive Sensors • Whiskbroom vs. Pushbroom • Multi-spectral remote sensing: Landsat (MSS, TM, and ETM+)Aditive Substractive Processes in color formation Additive colors refer to the combination of light from multiple sources at specific wavelengths • used in remote sensing, and on computer monitors, etc.Landsat TM and Color Composites Band 2 Band 1 Band 3 Band 4 Band 5 Band 6 Band 7 0.450-0.515 µm (Blue) 0.525-0.605 µm (Green) 0.63-0.69 µm (Red) 0.75-0.90 µm (Near-Infrared) 1.55-1.75 µm (Middle-Infrared) 2.08-2.35 µm (Middle-Infrared) 10.40-12.50 µm (Thermal-Infrared)Spectral SignatureSpectral 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. Resolution: Four types of resolutions in remote sensing: (1) spatial: the smallest angular or linear separation between two objects that can be resolved by the sensor (IFOV). (2) Temporal: the repeat frequency of information gathered at a specific point. (3) spectral: the number and dimension of specific wavelength intervals in the EM spectrum to which the instrument is sensitive. (4) Radiometric: sensitivity of the sensor to different signal strengths.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 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(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 distortionWhiskbroom vs. Pushbroom Whiskbroom • Landsat Multispectral Scanner (MSS) • Landsat Thematic Mapper (TM) • Landsat 7 Enhancement Thematic Mapper (ETM+) • NOAA Advance Very High Resolution Radiometer (AVHRR) • Geostationary Operational Environmental Satellite (GOES) Pushbroom • SPOT S 1,2, and 3 High Resolution Visible (HRV) . SPOT 4 and 5 (HRVIR) and Vegetation sensors • Indian Remote Sensing System (IRS) • Terra Advance Spacebone Thermal Emission and Reflection Radiometer (ASTER) • Terra Multiangle Imaging Spectroradiometer (MISR) • IKONOS , QuickBirdSelected Remote Sensing System and Their characteristics Adapted from Jensen, 2007Multi-spectral remote sensing: Landsat (MSS, TM, and ETM+) • Sun-synchronous near polar orbits • Inclination 99° and 98.2° • 919 km altitude (landsat 1, 2, 3), 705 km for the others • Orbits the earth every 103 minutes (landsat 1, 2, 3) • Cross latitude at approximately the same local time (equator 9:30 to 10:00 am)Landsat 1: 1972–1978 Landsat 2: 1975–1982 Landsat 3: 1978–1983 Landsat 4: 1982–2001 Landsat 5: 1984– Landsat 6: failed launch, 1993 Landsat 7: 1999– LDCM: Scheduled to launch in December 2012 Multi-spectral remote sensing: Landsat (MSS, TM, and ETM+)Landsat-1 to 3 Landsat-4 and 5 Landsat-6 and 7.Landsats sensors • carried combinations of 5 types of sensors: – Return Beam Vidicon (RBV) camera systems • Imaged entire ground scene instantaneously • Improved cartographic fidelity • Only flew on Landsats 1-3 – Multispectral Scanner (MSS) systems – Thematic Mapper (TM) – Enhanced Thematic Mapper (ETM) – Enhanced Thematic Mapper Plus (ETM+)Landsat Ground Receiving Station http://landsat.usgs.gov/about_ground_stations.phpLandsat 4 & 5 coverage • Augmented by Tracking and Data Relay Satellite System (TDRS) - GeosynchronousMulti-spectral Scanner (MSS) IFOV at nadir 79 x 79 m for bands 4 to 7 240 x 240 m for band 8 Quantization levels 6 bit (values from 0 to 63) in 1970’s 8 bit (values from 0 to 255) in 1980’s Earth coverage 18