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Remote Sensing and Image Processing: 8

Remote Sensing and Image Processing: 8. Dr. Mathias (Mat) Disney UCL Geography Office: 301, 3rd Floor, Chandler House Tel: 7670 4290 (x24290) Email: mdisney@geog.ucl.ac.uk www.geog.ucl.ac.uk/~mdisney. Recap. Last week introduced spatial and spectral resolution

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Remote Sensing and Image Processing: 8

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  1. Remote Sensing and Image Processing: 8 Dr. Mathias (Mat) Disney UCL Geography Office: 301, 3rd Floor, Chandler House Tel: 7670 4290 (x24290) Email: mdisney@geog.ucl.ac.uk www.geog.ucl.ac.uk/~mdisney

  2. Recap • Last week introduced • spatial and spectral resolution • narrow v broad band tradeoffs.... • This week • temporal and angular resolution • orbits and sensor swath

  3. Temporal • Single or multiple observations • How far apart are observations in time? • One-off, several or many? • Depends (as usual) on application • Is it dynamic? • If so, over what timescale? Useful link: http://www.earth.nasa.gov/science/index.html

  4. Temporal • Examples • Vegetation stress monitoring, weather, rainfall • hours to days • Terrestrial carbon, ocean surface temperature • days to months to years • Glacier dynamics, ice sheet mass balance • Months to decades Useful link: http://www.earth.nasa.gov/science/index.html

  5. What determines temporal sampling • Sensor orbit • geostationary orbit - over same spot • BUT distance means entire hemisphere is viewed e.g. METEOSAT • polar orbit can use Earth rotation to view entire surface • Sensor swath • Wide swath allows more rapid revisit • typical of moderate res. instruments for regional/global applications • Narrow swath == longer revisit times • typical of higher resolution for regional to local applications

  6. Orbits and swaths • Orbital characteristics • orbital mechanics developed by Johannes Kepler (1571-1630), German mathematician • Explained observations of Danish nobleman Tyco Brahe (1546-1601) • Kepler favoured elliptical orbits (from Copernicus’ solar-centric system) • Properties of ellipse?

  7. Kepler’s laws • Kepler’s Laws • deduced from Brahe’s data after his death • see nice Java applet http://www-groups.dcs.st-and.ac.uk/~history/Java/Ellipse.html • Kepler’s 1st law: • Orbits of planets are elliptical, with sun at one focus From:http://csep10.phys.utk.edu/astr161/lect/history/kepler.html

  8. Kepler’s laws • Kepler’s 2nd law • line joining planet to sun sweeps out equal areas in equal times From:http://csep10.phys.utk.edu/astr161/lect/history/kepler.html

  9. Kepler’s laws • Kepler’s 3rd law • ratio of the squares of the revolutionary periods for two planets (P1, P2) is equal to the ratio of the cubes of their semimajor axes (R1, R2) • P12/P22 = R13/R23 i.e. orbital period increases dramatically with R From:http://csep10.phys.utk.edu/astr161/lect/history/kepler.html

  10. Orbital pros and cons • Geostationary? • Circular orbit in the equatorial plane, altitude ~36,000km • Orbital period, T? • Advantages • See whole Earth disk at once due to large distance • See same spot on the surface all the time i.e. high temporal coverage • Big advantage for weather monitoring satellites - knowing atmos. dynamics critical to short-term forecasting and numerical weather prediction (NWP) • GOES (Geostationary Orbiting Environmental Satellites), operated by NOAA (US National Oceanic and Atmospheric Administration) • http://www.noaa.gov/ and http://www.goes.noaa.gov/

  11. GOES-E 75° W GOES-W 135° W METEOSAT 0° W IODC 63° E GMS 140° E Geostationary • Meteorological satellites - combination of GOES-E, GOES-W, METEOSAT (Eumetsat), GMS (NASDA), IODC (old Meteosat 5) • GOES 1st gen. (GOES-1 - ‘75  GOES-7 ‘95); 2nd gen. (GOES-8++ ‘94) From http://www.sat.dundee.ac.uk/pdusfaq.html

  12. Geostationary • METEOSAT - whole earth disk every 15 mins From http://www.goes.noaa.gov/f_meteo.html

  13. Geostationary orbits • Disadvantages • typically low spatial resolution due to high altitude • e.g. METEOSAT 2nd Generation (MSG) 1x1km visible, 3x3km IR (used to be 3x3 and 6x6 respectively) • MSG has SEVIRI and GERB instruments • http://www.meteo.pt/landsaf/eumetsat_sat_char.html • Cannot see poles very well (orbit over equator) • spatial resolution at 60-70° N several times lower • not much good beyond 60-70° • NB Geosynchronous orbit same period as Earth, but not equatorial From http://www.esa.int/SPECIALS/MSG/index.html

  14. Polar & near polar orbits • Advantages • full polar orbit inclined 90 to equator • typically few degrees off so poles not covered • orbital period, T, typically 90 - 105mins • near circular orbit between 300km (low Earth orbit) and 1000km • typically higher spatial resolution than geostationary • rotation of Earth under satellite gives (potential) total coverage • ground track repeat typically 14-16 days From http://collections.ic.gc.ca/satellites/english/anatomy/orbit/

  15. (near) Polar orbits: NASA Terra From http://visibleearth.nasa.gov/cgi-bin/viewrecord?134

  16. Near-polar orbits: Landsat • inclination 98.2, T = 98.8mins • http://www.cscrs.itu.edu.tr/page.en.php?id=51 • http://landsat.gsfc.nasa.gov/project/Comparison.html From http://www.iitap.iastate.edu/gccourse/satellite/satellite_lecture_new.html & http://eosims.cr.usgs.gov:5725/DATASET_DOCS/landsat7_dataset.html

  17. (near) Polar orbits • Disadvantages • need to launch to precise altitude and orbital inclination • orbital decay • at LEOs (Low Earth Orbits) < 1000km, drag from atmosphere • causes orbit to become more eccentric • Drag increases with increasing solar activity (sun spots) - during solar maximum (~11yr cycle) drag height increased by 100km! • Build your own orbit: http://lectureonline.cl.msu.edu/~mmp/kap7/orbiter/orbit.htm From http://collections.ic.gc.ca/satellites/english/anatomy/orbit/

  18. direction of travel satellite ground swath one sample two samples three samples Instrument swath • Swath describes ground area imaged by instrument during overpass

  19. MODIS on-board Terra From http://visibleearth.nasa.gov/cgi-bin/viewrecord?130

  20. Terra instrument swaths compared From http://visibleearth.nasa.gov/Sensors/Terra/

  21. Broad swath • MODIS, POLDER, AVHRR etc. • swaths typically several 1000s of km • lower spatial resolution • Wide area coverage • Large overlap obtains many more view and illumination angles (much better BRDF sampling) • Rapid repeat time

  22. MODIS: building global picture • Note across-track “whiskbroom” type scanning mechanism • swath width of 2330km (250-1000m resolution) • Hence, 1-2 day repeat cycle From http://visibleearth.nasa.gov/Sensors/Terra/

  23. MODIS: building global picture From http://visibleearth.nasa.gov/Sensors/Terra/

  24. Narrow swath • Landsat TM/MSS/ETM+, IKONOS, QuickBird etc. • swaths typically few 10s to 100skm • higher spatial resolution • local to regional coverage NOT global • far less overlap (particularly at lower latitudes) • May have to wait weeks/months for revisit

  25. Landsat: local view • 185km swath width, hence 16-day repeat cycle (and spatial res. 25m) • Contiguous swaths overlap (sidelap) by 7.3% at the equator • Much greater overlap at higher latitudes (80% at 84°) From http://visibleearth.nasa.gov/Sensors/Terra/

  26. QuickBird: 16.5km swath at nadir, 61cm! panchromatic, 2.44m multispectral • http://www.digitalglobe.com • IKONOS: 11km swath at nadir, 1m panchromatic, 4m multispectral • http://www.spaceimaging.com/ IKONOS & QuickBird: very local view!

  27. Summary: angular, temporal resolution • Coverage (hence angular &/or temporal sampling) due to combination of orbit and swath • Mostly swath - many orbits nearly same • MODIS and Landsat have identical orbital characteristics: inclination 98.2°, h=705km, T = 99mins BUT swaths of 2400km and 185km hence repeat of 1-2 days and 16 days respectively • Most EO satellites typically near-polar orbits with repeat tracks every 16 or so days • BUT wide swath instrument can view same spot much more frequently than narrow • Tradeoffs again, as a function of objectives

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