Remote Sensing and Image Processing 8 Dr Mathias

Remote Sensing and Image Processing: 8 Dr. Mathias (Mat) Disney UCL Geography Office: 301, 3 rd Floor, Chandler House Tel: 7670 4290 (x 24290) Email: mdisney@geog. ucl. ac. uk www. geog. ucl. ac. uk/~mdisney 1

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

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 3

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 4

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 5

Orbits and swaths • Orbital characteristics – orbital mechanics developed by Johannes Kepler (15711630), German mathematician – Explained observations of Danish nobleman Tyco Brahe (1546 -1601) – Kepler favoured elliptical orbits (from Copernicus’ solarcentric system) • Properties of ellipse? 6

Kepler’s laws • Kepler’s Laws – deduced from Brahe’s data after his death – see nice Java applet http: //www-groups. dcs. stand. ac. uk/~history/Java/Ellipse. html • Kepler’s 1 st law: – Orbits of planets are elliptical, with sun at one focus From: http: //csep 10. phys. utk. edu/astr 161/lect/history/kepler. html 7

Kepler’s laws • Kepler’s 2 nd law – line joining planet to sun sweeps out equal areas in equal times From: http: //csep 10. phys. utk. edu/astr 161/lect/history/kepler. html 8

Kepler’s laws • Kepler’s 3 rd 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 12/P 22 = R 13/R 23 i. e. orbital period increases dramatically with R From: http: //csep 10. phys. utk. edu/astr 161/lect/history/kepler. html 9

Orbital pros and cons • Geostationary? – Circular orbit in the equatorial plane, altitude ~36, 000 km – 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/ 10

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

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

Geostationary orbits • Disadvantages – typically low spatial resolution due to high altitude – e. g. METEOSAT 2 nd Generation (MSG) 1 x 1 km visible, 3 x 3 km IR (used to be 3 x 3 and 6 x 6 respectively) • MSG has SEVIRI and GERB instruments • http: //www. meteo. pt/landsaf/eumetsat_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 13

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 - 105 mins – near circular orbit between 300 km (low Earth orbit) and 1000 km – 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 / 14

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

Near-polar orbits: Landsat – inclination 98. 2 , T = 98. 8 mins – – 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_lecture_new. html & http: //eosims. cr. usgs. gov: 5725/DATASET_DOCS/landsat 7_dataset. html 16

(near) Polar orbits • Disadvantages – need to launch to precise altitude and orbital inclination – orbital decay • at LEOs (Low Earth Orbits) < 1000 km, drag from atmosphere • causes orbit to become more eccentric • Drag increases with increasing solar activity (sun spots) - during solar maximum (~11 yr cycle) drag height increased by 100 km! – Build your own orbit: http: //lectureonline. cl. msu. edu/~mmp/kap 7/orbiter/orbit. htm From http: //collections. ic. gc. ca/satellites/english/anatomy/orbit/ 17

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

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

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

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

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

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

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

Landsat: local view • 185 km swath width, hence 16 -day repeat cycle (and spatial res. 25 m) • 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/ 25

IKONOS & Quick. Bird: very local view! • IKONOS: 11 km swath at nadir, 1 m panchromatic, 4 m multispectral • Quick. Bird: 16. 5 km swath at nadir, 61 cm! panchromatic, 2. 44 m multispectral • http: //www. spaceimaging. com/ • http: //www. digitalglobe. com 26

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=705 km, T = 99 mins BUT swaths of 2400 km and 185 km 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 27