remote sensing and image processing: 8 dr. hassan j. eghbali

Remote Sensing and Image Processing: 8

Dr. Hassan J. Eghbali

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• Last week introduced– spatial and spectral resolution

– narrow v broad band tradeoffs....

• This week– temporal and angular resolution

– orbits and sensor swath

Recap


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• 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?

Temporal


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• 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

Temporal


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• 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

What determines temporal sampling


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• 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?

Orbits and swaths


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• 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

Kepler’s laws


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• Kepler’s 2nd law – line joining planet to sun sweeps out equal areas in equal times

Kepler’s laws


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• 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/P2

2 = R13/R2

3 i.e. orbital period increases dramatically with R

Kepler’s laws


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• 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/

Orbital pros and cons


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• 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)

Geostationary

METEOSAT 0° WGOES-W 135° WGOES-E 75° W GMS 140° EIODC 63° E


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• METEOSAT - whole earth disk every 15 mins

Geostationary


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• 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

Geostationary orbits


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• 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

Polar & near polar orbits


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(near) Polar orbits: NASA Terra


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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


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• 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

(near) Polar orbits


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• Swath describes ground area imaged by instrument during overpass

Instrument swath

one sample

two samples

three samples

satellite ground swath

direction of travel


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MODIS on-board Terra


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Terra instrument swaths compared


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• 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

Broad swath


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MODIS: building global picture

• Note across-track “whiskbroom” type scanning mechanism

• swath width of 2330km (250-1000m resolution)

• Hence, 1-2 day repeat cycle


MODIS: building global picture


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• 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

Narrow swath


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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°)


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IKONOS & QuickBird: very local view!

•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/


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• 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

Summary: angular, temporal resolution


remote sensing and image processing: 8 dr. hassan j. eghbali

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