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: Phone: Phone: Photograph taken from ISS by Donald Pettit, Space Station Science Officer

Outline Lecture 1 Introduction & EM Radiation Bracher Lecture 2 EMR II & Radiative Transfer Bracher Lecture 3 Retrieval Techniques, Inverse Methods Palm Lecture 4 Satellite Remote Sensing (RS) Bracher Lecture 5 Spectroscopy Bracher Lecture 6 Infra-red Techniques Palm Lecture 7 UV-visible Atmospheric RS I Bracher Lecture 8 UV-visible Atmospheric RS II Bracher Lecture 9 Ocean Optics Bracher Lecture 10 Ocean Color Remote Sensing Bracher Lecture 11 Microwave RS Palm Lecture 12 Sea Ice Remote Sensing Melsheimer Lecture 13 Summary & Lab Tour Bracher/MW Group Exam: 11 July Remote Sensing I, Bracher Palm Melsheimer 2

Lecture 4: Satellite Remote Sensing Remote Sensing I, Bracher Palm Melsheimer 3

Principle of Satellite Remote Sensing Remote Sensing I, Bracher Palm Melsheimer 4

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) Disadvantages Low spatial resolution Remote Sensing I, Bracher Palm Melsheimer 6

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 Remote Sensing I, Bracher Palm Melsheimer 7

METEOSAT - whole earth disk every 15 mins Geostationary orbit Remote Sensing I, Bracher Palm Melsheimer 8

Orbital Disadvantages of GEO 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 Remote Sensing I, Bracher Palm Melsheimer 9

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) totalcoverage ground track repeat typically days Remote Sensing I, Bracher Palm Melsheimer 10

Lower Earth Orbit (LEO) Ground track of SCIAMACHY (on ENVISAT with 98° inclination and 780 km orbit height) nadir at 1 day Remote Sensing I, Bracher Palm Melsheimer 11

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) Remote Sensing I, Bracher Palm Melsheimer 12

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-synchronousview a place on Earth at varying times Remote Sensing I, Bracher Palm Melsheimer 13

Orbital Disadvantages for LEO 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) Remote Sensing I, Bracher Palm Melsheimer 14

Swath describes ground area imaged by instrument during overpass Instrument’s Swath Remote Sensing I, Bracher Palm Melsheimer 15

Lower Earth Orbit (LEO) Ground track of SCIAMACHY (on ENVISAT with 98° inclination and 780 km orbit height) nadir at 1 day Remote Sensing I, Bracher Palm Melsheimer 16

ENVISAT: 1 March Apr 2012) SCIAMACHY  UV/Vis/NIR grating spectrometers:  8 channels, 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 Remote Sensing I, Bracher Palm Melsheimer 17

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 Remote Sensing I, Bracher Palm Melsheimer 18

Swath describes ground area imaged by instrument during overpass Instrument’s Swath Remote Sensing I, Bracher Palm Melsheimer 19

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 ( m resolution) Hence, 1-2 day repeat cycle Remote Sensing I, Bracher Palm Melsheimer 20

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°) Remote Sensing I, Bracher Palm Melsheimer 21

Narrow Swath: IKONOS & QuickBird - very local view! Remote Sensing I, Bracher Palm Melsheimer 22

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? Remote Sensing I, Bracher Palm Melsheimer 23

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 Remote Sensing I, Bracher Palm Melsheimer 24

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 = 99mins BUT 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 Remote Sensing I, Bracher Palm Melsheimer 25

End of Lecture 4 Remote Sensing I, Bracher Palm Melsheimer 26