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Orbits and Sensors Multispectral Sensors

Orbits and Sensors Multispectral Sensors. Outline for 15 October 2007. Orbits: geostationary, polar Cross-track scanners (whiskbroom sensor) Pushbroom sensors Field of View (“swath width”) Pixel size Multispectral sensors: Landsat. Satellite Orbits.

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Orbits and Sensors Multispectral Sensors

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  1. Orbits and SensorsMultispectral Sensors

  2. Outline for 15 October 2007 • Orbits: geostationary, polar • Cross-track scanners (whiskbroom sensor) • Pushbroom sensors • Field of View (“swath width”) • Pixel size • Multispectral sensors: • Landsat

  3. Satellite Orbits Orbital parameters can be tuned to produce particular, useful orbits • Geostationary • Sun synchronous (Polar, Low Earth Orbit) • Geosynchronous • Altimetric

  4. Geostationary Orbits • Geo orbit is stationary with respect to a location on the earth • Circular orbit around the equator (orbital inclination = zero) • Placed in high orbit (35,800 km) to match the angular velocity of Earth

  5. Uses of Geostationary Orbits • Weather satellites (GOES, METEOSAT) • Constant monitoring • Communication satellites Constant contact w/ground stations Limited spatial coverage • each satellite can only cover about 25-30% of the earth’s surface • coverage extends only to the mid-latitudes, no more than about 55o

  6. Sun-synchronous (Polar) Orbit • “Low Earth Orbit” (LEO) are typically about 700 km altitude • Precession of the satellite orbit is the same as the angular speed of rotation of the sun • Satellite crosses the equator at the same time each day • “Polar orbit” is very common • Orbital inclination is “retrograde” (typically ~98o) • Near circular orbits have period of about 98-102 minutes

  7. Animation of GEO and LEO orbits

  8. Polar Orbiting Satellite Tracks

  9. Uses of Sun-Synchronous Orbits • Equatorial crossing time depends on nature of application (low sun angle vs. high sun angle needs) • Earth monitoring -- global coverage • Good spatial resolution

  10. Terra satellite overpasses for today over North America See http://www.ssec.wisc.edu/datacenter/terra/

  11. Getting the Data to the Ground • On-board recording and pre-processing • Direct telemetry to ground stations • receive data transmissions from satellites • transmit commands to satellites (pointing, turning maneuvers, software updating • Indirect transmission through Tracking and Data Relay Satellites (TDRS)

  12. Imaging Systems • Cross-track scanning systems • “whiskbroom” • Along-track (non-scanning) system • “pushbroom”

  13. Cross-track Scanner • Single detector or a linear array of detectors • “Back and forth” motion of the scanner creates the orbital “swath” • Image is built up by movement of satellite along its orbital track Produces a wide field-of-view • Pixel resolution varies with scan angle

  14. Field of View (FOV) • FOV is the swath width of the instrument • It is the width of an orbital swath • Depends on the across track scan angle of the sensor (for whiskbroom) or the width of the linear detector array (for a pushbroom sensor)

  15. Along-track scanner (Pushbroom) • Linear array of detectors (aligned cross-track) • radiance passes through a lens and onto a line of detectors • Image is built up by movement of the satellite along its orbital track (no scanning mirror) • Multiple linear arrays are used for multi-spectral remote sensing • dispersion element splits light into different wavelengths and onto individual detectors

  16. Dwell Time • The amount of time a scanner has to collect photons from a ground resolution cell • Depends on: • satellite speed • width of scan line • time per scan line • time per pixel • Whiskbroom scanners have much shorter dwell time than do pushbroom scanners

  17. Wide swath width Complex mechanical system Simple optical system Shorter dwell time Pixel distortion Narrow swath width Simple mechanical system Complex optical system Longer dwell time No pixel distortion Whiskbroom vs. Pushbroom

  18. Signal Strength • Need enough photons incident on the detector to record a strong signal • Signal strength depends on • Energy flux from the surface • Altitude of the sensor • Location of the spectral bands (e.g. visible, NIR, thermal, etc.) • Spectral bandwidth of the detector • IFOV • Dwell time

  19. Calculating the Field of View (FOV) q H FOV = 2 H tan(scan angle) H = satellite altitude Example: SeaWIFS satellite altitude = 705 km Scan angle = 58.3o FOV = 1410 x tan(58.3o) = 2282 km FOV

  20. Cross-track pixel size x = H tan(q + b/2) x2 = H tan(q - b/2) x1 = x - x2 Pc = H tan(q + b/2) - H tan(q - b/2) q H H/cosq = Hsecq x1 x2 x

  21. History of the Landsat series Currently, Landsat 5 and Landsat 7 (ETM+) are in orbit

  22. Landsat MSS 1972-present

  23. Landsat orbits

  24. Landsat MSS Bands and their Uses • Band 4 (Green: 0.5 - 0.6 mm) • water features (large penetration depths) • sensitivity to turbidity (suspended sediments) • sensitivity to atmospheric haze (lack of tonal contrast) • Band 5 (Red: 0.6 - 0.7 mm) • chlorophyll absorption region • good contrast between vegetated and non-veg. areas • haze penetration better than Band 4 • Band 6 (NIR1: 0.7 - 0.8 mm) and Band 7 (NIR2: 0.8 - 1.1 mm) • similar for most surface features • good contrast between land and water (water is strong absorber in near IR) • both bands excellent haze penetration • Band 7 good for discrimination of snow and ice

  25. Landsat MSS Images of Mount St. Helens September 15, 1973 May 22, 1983 August 31, 1988

  26. Landsat Thematic TM 1982 - present

  27. Landsat Thematic Mapper Bands and their Uses • Band 1 (Blue: 0.45 - 0.52 mm) • good water penetration • differentiating soil and rock surfaces from vegsmoke plumes • most sensitive to atmospheric haze • Band 2 (Green: 0.52 - 0.60 mm) • water turbidity differences • sediment and pollution plumes • discrimination of broad classes of vegetation • Band 3 (Red: 0.63 - 0.69 mm) • strong chlorophyll absorption (veg. vs. soil) • urban vs. rural areas

  28. Landsat Thematic Mapper Bands and their Uses • Band 4 (NIR1: 0.76 - 0.90 mm) • different vegetation varieties and conditions • dry vs. moist soil • coastal wetland, swamps, flooded areas • Band 5 (NIR2: 1.55 - 1.75 mm) • leaf-tissue water content • soil moisture • snow vs cloud discrimination • Band 6 (Thermal: 10.4 - 12.5 mm) • heat mapping applications (coarse resolution) • radiant surface temperature range: -100oC to +150oC • Band 7 (NIR3: 2.08 - 2.35 mm) • absorption band by hydrous minerals (clay, mica) • lithologic mapping (clay zones)

  29. Landsat 7 Enhanced Thematic Mapper (ETM+) • 1999-present • 15m panchromatic band • on-board calibration

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