Remote Sensing I
Summer Term 2013
Lecturers:
Astrid Bracher,Mathias Palm and Christian Melsheimer
Contact:Prof. Dr. Astrid Bracher Dr. Mathias Palm Dr. Christian Melsheimer Office: U-3215 (NW 1) Office: U-3235 NW 1) Office: N-3371 (NW 1)Phone: 0421-218-62112 Phone: 0421-218-62179 Phone: 0421-218-62181Email: [email protected] [email protected] melsheimer@uni-
bremen.de
Photograph taken from ISS by Donald Pettit, Space Station Science Officer
OutlineLecture 1 Introduction & EM Radiation 04.04.2013 Bracher
Lecture 2 EMR II & Radiative Transfer 11.04.2013 Bracher
Lecture 3 Retrieval Techniques, Inverse Methods 18.04.2013 Palm
Lecture 4 Satellite Remote Sensing (RS) 25.04.2013 Bracher
Lecture 5 Spectroscopy 02.05.2013 Bracher
Lecture 6 Infra-red Techniques 16.05.2013 Palm
Lecture 7 UV-visible Atmospheric RS I 23.05.2013 Bracher
Lecture 8 UV-visible Atmospheric RS II 30.05.2013 Bracher
Lecture 9 Ocean Optics 06.06.2013 Bracher
Lecture 10 Ocean Color Remote Sensing 13.06.2013 Bracher
Lecture 11 Microwave RS 20.06.2013 Palm
Lecture 12 Sea Ice Remote Sensing 27.06.2013 Melsheimer
Lecture 13 Summary & Lab Tour 04.07.2013 Bracher/MW Group
Exam: 11 July 2013 10-12
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Lecture 4: Satellite Remote Sensing
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Principle of Satellite Remote Sensing
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ENVISAT: Launched 1 March 2002
Geostationary orbit• Circular orbit in the equatorial plane, altitude ~36,000km• Orbital period ~1 day , orbit matches Earth’s rotation
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 the
atmospheric dynamics is critical to short-term forecasting and numerical weather prediction - NWP)
DisadvantagesLow spatial resolution
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Meteorological satellites:A combination of OES-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 orbit
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• METEOSAT - whole earth disk every 15 mins
Geostationary orbit
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Orbital Disadvantages ofGEO
• typically low spatial resolution due to high altitude: e.g. METEOSAT 2nd Generation (MSG) 1 kmx1 km visible, 3 kmx3 km IR (used to be 3 x 3 & 6 x 6, respectively)
• spatial resolution at 60-70° several times lower• not much good beyond 70°- cannot see the poles very
well (orbit over equator)
Other geosynchronous orbits which are not GEO: same period as Earth, but not equatorial
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Lower Earth Orbit (LEO):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 – 110 min
– near circular orbit between 300 km and 1000 km (low Earth orbit) – typically higher spatial resolution than geostationary– rotation of Earth under satellite gives (potential) total coverage
• ground track repeat typically 14-16 days
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Lower Earth Orbit (LEO)Ground track of SCIAMACHY (on ENVISAT with 98° inclination and 780 km orbit height) nadir at 1 day
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Sun elevation at local noon
Sun elevation angle at local noon at the four seasons
21 Dec 21 Mar/22 Sep 21 Jun
Bremen, 53°N 14° 37.5° 61°
Delhi, 28°N 39° 63° 85°
Singapore, 1°N 65.5° 90° 67.5°
(over S horizon) (zenith) (over N horizon)
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Lower Earth Orbit (LEO):Inclination (tropical) orbits
• orbit inclined >0° to <90° to equator• Determined by the region of Earth that is of most interest (e.g. low
inclination angle for tropics) • Orbital altitude typically a few hundreds km• Orbital period around a few hours• These satellites are not sun-synchronous view a place on Earth
at varying times
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Orbital Disadvantages forLEO
• 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)– ~ solar maximum (~11yr cycle) drag height increased by 100km!
Lower Earth Orbit (LEO)
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Swath describes ground area imaged by instrument during overpass
Instrument’s Swath
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Lower Earth Orbit (LEO)Ground track of SCIAMACHY (on ENVISAT with 98° inclination and 780 km orbit height) nadir at 1 day
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ENVISAT: 1 March 2002 - 12 Apr 2012)SCIAMACHY UV/Vis/NIR grating spectrometers: 8 channels, 240 - 2380 nm Moderate spectral resolution: 0.2 –
1.5 nm Measurement Geometries :
nadir viewing+ limb + solar / lunar occultation
Polar, sun-synchronous orbit, 10:00 Global coverage in 6 days During eclipse calibration and limb
measurements Spectroscopy is used to derive trace
gas distributions in the troposphere, stratosphere and mesosphere
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Overview of satellite observations geometries
Measured signal:Reflected and scattered sunlight(Thermal emission from Earth)
Measured signal:Directly transmitted solar radiation(Thermal emission from Earth)
Measured signal:Scattered solar radiation(Thermal emission from Earth)
Overview of satellite observations geometries
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Swath describes ground area imaged by instrument during overpass
Instrument’s Swath
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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
MODIS:• Note across-track “whiskbroom” type
scanning mechanism• swath width of 2330 km (250-1000m
resolution)• Hence, 1-2 day repeat cycle
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Narrow Swath• Landsat TM/MSS/ETM+, IKONOS, QuickBird etc.
– swaths typically few 10s to 100s km– higher spatial resolution– local to regional coverage NOT global– far less overlap (particularly at lower latitudes)– May have to wait weeks/months for revisit
Landsat:• 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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Narrow Swath: IKONOS & QuickBird - very local view!
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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?
• 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
What temporal resolution chosen for measurements?
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• Sensor orbit– geostationary orbit – good temporal sampling over
same spot: BUT due to large orbit height nearly the entire hemisphere can be viewed (e.g. METEOSAT)
– Near-polar orbit – less temporal sampling, but can use Earth rotation to view entire surface
• Sensor swath– Wide swath allows more rapid revisit
• typical are moderate resolution instruments for regional/global applications
– Narrow swath == longer revisit times• typical of higher resolution for regional to local applications
What determines the temporal sampling
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• Coverage (hence spatial and/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 = 99minsBUT swaths of 2400 km and 185 km, 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: spatial and temporal resolution
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End of Lecture 4
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