Remote Sensing and Image Processing: 8 Dr. Hassan J. Eghbali

Last week introduced –spatial and spectral resolution –narrow v broad band tradeoffs.... This week –temporal and angular resolution –orbits and sensor swath Recap Dr. Hassan J. Eghbali

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 Dr. Hassan J. Eghbali

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 Dr. Hassan J. Eghbali

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 Dr. Hassan J. Eghbali

Orbital characteristics –orbital mechanics developed by Johannes Kepler ( ), German mathematician –Explained observations of Danish nobleman Tyco Brahe ( ) –Kepler favoured elliptical orbits (from Copernicus’ solar- centric system) Properties of ellipse? Orbits and swaths Dr. Hassan J. Eghbali

Kepler’s Laws –deduced from Brahe’s data after his death –see nice Java applet and.ac.uk/~history/Java/Ellipse.html Kepler’s 1st law: –Orbits of planets are elliptical, with sun at one focus Kepler’s laws Dr. Hassan J. Eghbali

Kepler’s 2nd law –line joining planet to sun sweeps out equal areas in equal times Kepler’s laws Dr. Hassan J. Eghbali

Kepler’s 3rd law –ratio of the squares of the revolutionary periods for two planets (P 1, P 2 ) is equal to the ratio of the cubes of their semimajor axes (R 1, R 2 ) –P 1 2 /P 2 2 = R 1 3 /R 2 3 i.e. orbital period increases dramatically with R Kepler’s laws Dr. Hassan J. Eghbali

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) and Orbital pros and cons Dr. Hassan J. Eghbali

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° WGMS 140° EIODC 63° E Dr. Hassan J. Eghbali

METEOSAT - whole earth disk every 15 mins Geostationary Dr. Hassan J. Eghbali

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 –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 Dr. Hassan J. Eghbali

Advantages –full polar orbit inclined 90 to equator typically few degrees off so poles not covered orbital period, T, typically mins –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 days Polar & near polar orbits Dr. Hassan J. Eghbali

(near) Polar orbits: NASA Terra Dr. Hassan J. Eghbali

Near-polar orbits: Landsat –inclination 98.2 , T = 98.8mins – – Dr. Hassan J. Eghbali

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: (near) Polar orbits Dr. Hassan J. Eghbali

Swath describes ground area imaged by instrument during overpass Instrument swath one sample two samples three samples satellite ground swath direction of travel Dr. Hassan J. Eghbali

MODIS on-board Terra Dr. Hassan J. Eghbali

Terra instrument swaths compared Dr. Hassan J. Eghbali

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 Dr. Hassan J. Eghbali

MODIS: building global picture Note across-track “whiskbroom” type scanning mechanism swath width of 2330km ( m resolution) Hence, 1-2 day repeat cycle Dr. Hassan J. Eghbali

MODIS: building global picture Dr. Hassan J. Eghbali

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 Dr. Hassan J. Eghbali

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°) Dr. Hassan J. Eghbali

IKONOS & QuickBird: very local view! QuickBird: 16.5km swath at nadir, 61cm! panchromatic, 2.44m multispectral IKONOS: 11km swath at nadir, 1m panchromatic, 4m multispectral Dr. Hassan J. Eghbali

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 Dr. Hassan J. Eghbali